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\chapter{Machine-Level ISA, Version 1.12} \label{machine} This chapter describes the machine-level operations available in machine-mode (M-mode), which is the highest privilege mode in a RISC-V system. M-mode is used for low-level access to a hardware platform and is the first mode entered at reset. M-mode can also be used to implement features that are too difficult or expensive to implement in hardware directly. The RISC-V machine-level ISA contains a common core that is extended depending on which other privilege levels are supported and other details of the hardware implementation. \section{Machine-Level CSRs} In addition to the machine-level CSRs described in this section, M-mode code can access all CSRs at lower privilege levels. \subsection{Machine ISA Register {\tt misa}} \label{sec:misa} The {\tt misa} CSR is a \warl\ read-write register reporting the ISA supported by the hart. This register must be readable in any implementation, but a value of zero can be returned to indicate the {\tt misa} register has not been implemented, requiring that CPU capabilities be determined through a separate non-standard mechanism. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{c@{}c@{}L} \instbitrange{MXLEN-1}{MXLEN-2} & \instbitrange{MXLEN-3}{26} & \instbitrange{25}{0} \\ \hline \multicolumn{1}{|c|}{MXL[1:0] (\warl)} & \multicolumn{1}{c|}{0 (\warl)} & \multicolumn{1}{c|}{Extensions[25:0] (\warl)} \\ \hline 2 & MXLEN-28 & 26 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine ISA register ({\tt misa}).} \label{misareg} \end{figure*} The MXL (Machine XLEN) field encodes the native base integer ISA width as shown in Table~\ref{misabase}. The MXL field may be writable in implementations that support multiple base ISAs. The effective XLEN in M-mode, {\em MXLEN}, is given by the setting of MXL, or has a fixed value if {\tt misa} is zero. The MXL field is always set to the widest supported ISA variant at reset. \begin{table*}[h!] \begin{center} \begin{tabular}{|r|r|} \hline MXL & XLEN \\ \hline 1 & 32 \\ 2 & 64 \\ 3 & 128 \\ \hline \end{tabular} \end{center} \caption{Encoding of MXL field in {\tt misa}} \label{misabase} \end{table*} The {\tt misa} CSR is MXLEN bits wide. If the value read from {\tt misa} is nonzero, field MXL of that value always denotes the current MXLEN. If a write to {\tt misa} causes MXLEN to change, the position of MXL moves to the most-significant two bits of {\tt misa} at the new width. \begin{commentary} The base width can be quickly ascertained using branches on the sign of the returned {\tt misa} value, and possibly a shift left by one and a second branch on the sign. These checks can be written in assembly code without knowing the register width (XLEN) of the machine. The base width is given by $\mbox{XLEN}=2^{\mbox{MXL+4}}$. The base width can also be found if {\tt misa} is zero, by placing the immediate 4 in a register then shifting the register left by 31 bits at a time. If zero after one shift, then the machine is RV32. If zero after two shifts, then the machine is RV64, else RV128. \end{commentary} The Extensions field encodes the presence of the standard extensions, with a single bit per letter of the alphabet (bit 0 encodes presence of extension ``A'' , bit 1 encodes presence of extension ``B'', through to bit 25 which encodes ``Z''). The ``I'' bit will be set for RV32I, RV64I, RV128I base ISAs, and the ``E'' bit will be set for RV32E. The Extensions field is a \warl\ field that can contain writable bits where the implementation allows the supported ISA to be modified. At reset, the Extensions field shall contain the maximal set of supported extensions, and I shall be selected over E if both are available. When a standard extension is disabled by clearing its bit in {\tt misa}, the instructions and CSRs defined or modified by the extension revert to their defined or reserved behaviors as if the extension is not implemented. The RV128I base ISA is not yet frozen, and while much of the remainder of this specification is expected to apply to RV128, this version of the document focuses only on RV32 and RV64. The ``U'' and ``S'' bits will be set if there is support for user and supervisor modes respectively. The ``X'' bit will be set if there are any non-standard extensions. \begin{table*} \begin{center} \begin{tabular}{|r|r|l|} \hline Bit & Character & Description \\ \hline 0 & A & Atomic extension \\ 1 & B & {\em Tentatively reserved for Bit-Manipulation extension} \\ 2 & C & Compressed extension \\ 3 & D & Double-precision floating-point extension \\ 4 & E & RV32E base ISA \\ 5 & F & Single-precision floating-point extension \\ 6 & G & {\em Reserved} \\ 7 & H & Hypervisor extension \\ 8 & I & RV32I/64I/128I base ISA \\ 9 & J & {\em Tentatively reserved for Dynamically Translated Languages extension} \\ 10 & K & {\em Reserved} \\ 11 & L & {\em Reserved} \\ 12 & M & Integer Multiply/Divide extension \\ 13 & N & {\em Tentatively reserved for User-Level Interrupts extension} \\ 14 & O & {\em Reserved} \\ 15 & P & {\em Tentatively reserved for Packed-SIMD extension} \\ 16 & Q & Quad-precision floating-point extension \\ 17 & R & {\em Reserved} \\ 18 & S & Supervisor mode implemented \\ 19 & T & {\em Reserved} \\ 20 & U & User mode implemented \\ 21 & V & {\em Tentatively reserved for Vector extension} \\ 22 & W & {\em Reserved} \\ 23 & X & Non-standard extensions present \\ 24 & Y & {\em Reserved} \\ 25 & Z & {\em Reserved} \\ \hline \end{tabular} \end{center} \caption{Encoding of Extensions field in {\tt misa}. All bits that are reserved for future use must return zero when read.} \label{misaletters} \end{table*} \begin{commentary} The {\tt misa} CSR exposes a rudimentary catalog of CPU features to machine-mode code. More extensive information can be obtained in machine mode by probing other machine registers, and examining other ROM storage in the system as part of the boot process. We require that lower privilege levels execute environment calls instead of reading CPU registers to determine features available at each privilege level. This enables virtualization layers to alter the ISA observed at any level, and supports a much richer command interface without burdening hardware designs. \end{commentary} The ``E'' bit is read-only. Unless {\tt misa} is hardwired to zero, the ``E'' bit always reads as the complement of the ``I'' bit. An implementation that supports both RV32E and RV32I can select RV32E by clearing the ``I'' bit. If an ISA feature {\em x} depends on an ISA feature {\em y}, then attempting to enable feature {\em x} but disable feature {\em y} results in both features being disabled. For example, setting ``F''=0 and ``D''=1 results in both ``F'' and ``D'' being cleared. An implementation may impose additional constraints on the collective setting of two or more {\tt misa} fields, in which case they function collectively as a single \warl\ field. An attempt to write an unsupported combination causes those bits to be set to some supported combination. Writing {\tt misa} may increase IALIGN, e.g., by disabling the ``C'' extension. If an instruction that would write {\tt misa} increases IALIGN, and the subsequent instruction's address is not IALIGN-bit aligned, the write to {\tt misa} is suppressed, leaving {\tt misa} unchanged. When software enables an extension that was previously disabled, then all state uniquely associated with that extension is \unspecified, unless otherwise specified by that extension. \subsection{Machine Vendor ID Register {\tt mvendorid}} The {\tt mvendorid} CSR is a 32-bit read-only register providing the JEDEC manufacturer ID of the provider of the core. This register must be readable in any implementation, but a value of 0 can be returned to indicate the field is not implemented or that this is a non-commercial implementation. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{JS} \instbitrange{31}{7} & \instbitrange{6}{0} \\ \hline \multicolumn{1}{|c|}{Bank} & \multicolumn{1}{c|}{Offset} \\ \hline 25 & 7 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Vendor ID register ({\tt mvendorid}).} \label{mvendorreg} \end{figure*} JEDEC manufacturer IDs are ordinarily encoded as a sequence of one-byte continuation codes {\tt 0x7f}, terminated by a one-byte ID not equal to {\tt 0x7f}, with an odd parity bit in the most-significant bit of each byte. {\tt mvendorid} encodes the number of one-byte continuation codes in the Bank field, and encodes the final byte in the Offset field, discarding the parity bit. For example, the JEDEC manufacturer ID {\tt 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x7f 0x8a} (twelve continuation codes followed by {\tt 0x8a}) would be encoded in the {\tt mvendorid} CSR as {\tt 0x60a}. \begin{commentary} In JEDEC's parlance, the bank number is one greater than the number of continuation codes; hence, the {\tt mvendorid} Bank field encodes a value that is one less than the JEDEC bank number. \end{commentary} \begin{commentary} Previously the vendor ID was to be a number allocated by the RISC-V Foundation, but this duplicates the work of JEDEC in maintaining a manufacturer ID standard. At time of writing, registering a manufacturer ID with JEDEC has a one-time cost of \$500. \end{commentary} \subsection{Machine Architecture ID Register {\tt marchid}} The {\tt marchid} CSR is an MXLEN-bit read-only register encoding the base microarchitecture of the hart. This register must be readable in any implementation, but a value of 0 can be returned to indicate the field is not implemented. The combination of {\tt mvendorid} and {\tt marchid} should uniquely identify the type of hart microarchitecture that is implemented. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{J} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Architecture ID} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Architecture ID register ({\tt marchid}).} \label{marchreg} \end{figure*} Open-source project architecture IDs are allocated globally by the RISC-V Foundation, and have non-zero architecture IDs with a zero most-significant-bit (MSB). Commercial architecture IDs are allocated by each commercial vendor independently, but must have the MSB set and cannot contain zero in the remaining MXLEN-1 bits. \begin{commentary} The intent is for the architecture ID to represent the microarchitecture associated with the repo around which development occurs rather than a particular organization. Commercial fabrications of open-source designs should (and might be required by the license to) retain the original architecture ID. This will aid in reducing fragmentation and tool support costs, as well as provide attribution. Open-source architecture IDs should be administered by the Foundation and should only be allocated to released, functioning open-source projects. Commercial architecture IDs can be managed independently by any registered vendor but are required to have IDs disjoint from the open-source architecture IDs (MSB set) to prevent collisions if a vendor wishes to use both closed-source and open-source microarchitectures. The convention adopted within the following Implementation field can be used to segregate branches of the same architecture design, including by organization. The {\tt misa} register also helps distinguish different variants of a design. \end{commentary} \subsection{Machine Implementation ID Register {\tt mimpid}} The {\tt mimpid} CSR provides a unique encoding of the version of the processor implementation. This register must be readable in any implementation, but a value of 0 can be returned to indicate that the field is not implemented. The Implementation value should reflect the design of the RISC-V processor itself and not any surrounding system. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{J} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Implementation} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Implementation ID register ({\tt mimpid}).} \label{mimpidreg} \end{figure*} \begin{commentary} The format of this field is left to the provider of the architecture source code, but will often be printed by standard tools as a hexadecimal string without any leading or trailing zeros, so the Implementation value can be left-justified (i.e., filled in from most-significant nibble down) with subfields aligned on nibble boundaries to ease human readability. \end{commentary} \subsection{Hart ID Register {\tt mhartid}} The {\tt mhartid} CSR is an MXLEN-bit read-only register containing the integer ID of the hardware thread running the code. This register must be readable in any implementation. Hart IDs might not necessarily be numbered contiguously in a multiprocessor system, but at least one hart must have a hart ID of zero. Hart IDs must be unique within the execution environment. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{J} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Hart ID}\\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Hart ID register ({\tt mhartid}).} \label{mhartidreg} \end{figure*} \begin{commentary} In certain cases, we must ensure exactly one hart runs some code (e.g., at reset), and so require one hart to have a known hart ID of zero. For efficiency, system implementers should aim to reduce the magnitude of the largest hart ID used in a system. \end{commentary} \subsection{Machine Status Registers ({\tt mstatus} and {\tt mstatush})} The {\tt mstatus} register is an MXLEN-bit read/write register formatted as shown in Figure~\ref{mstatusreg-rv32} for RV32 and Figure~\ref{mstatusreg} for RV64. The {\tt mstatus} register keeps track of and controls the hart's current operating state. A restricted view of {\tt mstatus} appears as the {\tt sstatus} register in the S-level ISA. \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{cKccccccc} \\ \instbit{31} & \instbitrange{30}{23} & \instbit{22} & \instbit{21} & \instbit{20} & \instbit{19} & \instbit{18} & \instbit{17} & \\ \hline \multicolumn{1}{|c|}{SD} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{TSR} & \multicolumn{1}{c|}{TW} & \multicolumn{1}{c|}{TVM} & \multicolumn{1}{c|}{MXR} & \multicolumn{1}{c|}{SUM} & \multicolumn{1}{c|}{MPRV} & \\ \hline 1 & 8 & 1 & 1 & 1 & 1 & 1 & 1 & \\ \end{tabular} \begin{tabular}{cWWcWccccccccc} \\ & \instbitrange{16}{15} & \instbitrange{14}{13} & \instbitrange{12}{11} & \instbitrange{10}{9} & \instbit{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline & \multicolumn{1}{|c|}{XS[1:0]} & \multicolumn{1}{c|}{FS[1:0]} & \multicolumn{1}{c|}{MPP[1:0]} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{SPP} & \multicolumn{1}{c|}{MPIE} & \multicolumn{1}{c|}{UBE} & \multicolumn{1}{c|}{SPIE} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{MIE} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{SIE} & \multicolumn{1}{c|}{\wpri} \\ \hline & 2 & 2 & 2 & 2 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine-mode status register ({\tt mstatus}) for RV32.} \label{mstatusreg-rv32} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \scalebox{0.95}{ \begin{tabular}{cRccccYcccccc} \\ \instbit{63} & \instbitrange{62}{38} & \instbit{37} & \instbit{36} & \instbitrange{35}{34} & \instbitrange{33}{32} & \instbitrange{31}{23} & \instbit{22} & \instbit{21} & \instbit{20} & \instbit{19} & \instbit{18} & \\ \hline \multicolumn{1}{|c|}{SD} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{MBE} & \multicolumn{1}{c|}{SBE} & \multicolumn{1}{c|}{SXL[1:0]} & \multicolumn{1}{c|}{UXL[1:0]} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{TSR} & \multicolumn{1}{c|}{TW} & \multicolumn{1}{c|}{TVM} & \multicolumn{1}{c|}{MXR} & \multicolumn{1}{c|}{SUM} & \\ \hline 1 & 25 & 1 & 1 & 2 & 2 & 9 & 1 & 1 & 1 & 1 & 1 & \\ \end{tabular}} \scalebox{0.95}{ \begin{tabular}{ccWWcWccccccccc} \\ & \instbit{17} & \instbitrange{16}{15} & \instbitrange{14}{13} & \instbitrange{12}{11} & \instbitrange{10}{9} & \instbit{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline & \multicolumn{1}{|c|}{MPRV} & \multicolumn{1}{c|}{XS[1:0]} & \multicolumn{1}{c|}{FS[1:0]} & \multicolumn{1}{c|}{MPP[1:0]} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{SPP} & \multicolumn{1}{c|}{MPIE} & \multicolumn{1}{c|}{UBE} & \multicolumn{1}{c|}{SPIE} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{MIE} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{SIE} & \multicolumn{1}{c|}{\wpri} \\ \hline & 1 & 2 & 2 & 2 & 2 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\ \end{tabular}} \end{center} } \vspace{-0.1in} \caption{Machine-mode status register ({\tt mstatus}) for RV64.} \label{mstatusreg} \end{figure*} For RV32 only, {\tt mstatush} is a 32-bit read/write register formatted as shown in Figure~\ref{mstatushreg}. Bits 30:4 of {\tt mstatush} generally contain the same fields found in bits 62:36 of {\tt mstatus} for RV64. Fields SD, SXL, and UXL do not exist in {\tt mstatush}. \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{JccF} \\ \instbitrange{31}{6} & \instbit{5} & \instbit{4} & \instbitrange{3}{0} \\ \hline \multicolumn{1}{|c|}{\wpri} & \multicolumn{1}{c|}{MBE} & \multicolumn{1}{c|}{SBE} & \multicolumn{1}{c|}{\wpri} \\ \hline 26 & 1 & 1 & 4 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Additional machine-mode status register ({\tt mstatush}) for RV32.} \label{mstatushreg} \end{figure*} \subsubsection{Privilege and Global Interrupt-Enable Stack in {\tt mstatus} register} \label{privstack} Global interrupt-enable bits, MIE and SIE, are provided for M-mode and S-mode respectively. These bits are primarily used to guarantee atomicity with respect to interrupt handlers in the current privilege mode. \begin{commentary} The global {\em x}\/IE bits are located in the low-order bits of {\tt mstatus}, allowing them to be atomically set or cleared with a single CSR instruction. \end{commentary} When a hart is executing in privilege mode {\em x}, interrupts are globally enabled when {\em x}\/IE=1 and globally disabled when {\em x}\/IE=0. Interrupts for lower-privilege modes, {\em w}$<${\em x}, are always globally disabled regardless of the setting of any global {\em w}\/IE bit for the lower-privilege mode. Interrupts for higher-privilege modes, {\em y}$>${\em x}, are always globally enabled regardless of the setting of the global {\em y}\/IE bit for the higher-privilege mode. Higher-privilege-level code can use separate per-interrupt enable bits to disable selected higher-privilege-mode interrupts before ceding control to a lower-privilege mode. \begin{commentary} A higher-privilege mode {\em y} could disable all of its interrupts before ceding control to a lower-privilege mode but this would be unusual as it would leave only a synchronous trap, non-maskable interrupt, or reset as means to regain control of the hart. \end{commentary} To support nested traps, each privilege mode {\em x} that can respond to interrupts has a two-level stack of interrupt-enable bits and privilege modes. {\em x}\/PIE holds the value of the interrupt-enable bit active prior to the trap, and {\em x}\/PP holds the previous privilege mode. The {\em x}\/PP fields can only hold privilege modes up to {\em x}, so MPP is two bits wide and SPP is one bit wide. When a trap is taken from privilege mode {\em y} into privilege mode {\em x}, {\em x}\/PIE is set to the value of {\em x}\/IE; {\em x}\/IE is set to 0; and {\em x}\/PP is set to {\em y}. \begin{commentary} For lower privilege modes, any trap (synchronous or asynchronous) is usually taken at a higher privilege mode with interrupts disabled upon entry. The higher-level trap handler will either service the trap and return using the stacked information, or, if not returning immediately to the interrupted context, will save the privilege stack before re-enabling interrupts, so only one entry per stack is required. \end{commentary} An MRET or SRET instruction is used to return from a trap in M-mode or S-mode respectively. When executing an {\em x}\/RET instruction, supposing {\em x}\/PP holds the value {\em y}, {\em x}\/IE is set to {\em x}\/PIE; the privilege mode is changed to {\em y}; {\em x}\/PIE is set to 1; and {\em x}\/PP is set to the least-privileged supported mode (U if U-mode is implemented, else M). If {\em x}\/PP$\neq$M, {\em x}\/RET also sets MPRV=0. \begin{commentary} Setting {\em x}\/PP to the least-privileged supported mode on an {\em x}\/RET helps identify software bugs in the management of the two-level privilege-mode stack. \end{commentary} {\em x}\/PP fields are \warl\ fields that can hold only privilege mode {\em x} and any implemented privilege mode lower than {\em x}. If privilege mode {\em x} is not implemented, then {\em x}\/PP must be hardwired to 0. \begin{commentary} M-mode software can determine whether a privilege mode is implemented by writing that mode to MPP then reading it back. If the machine provides only U and M modes, then only a single hardware storage bit is required to represent either 00 or 11 in MPP. \end{commentary} \subsubsection{Base ISA Control in {\tt mstatus} Register} \label{xlen-control} For RV64 systems, the SXL and UXL fields are \warl\ fields that control the value of XLEN for S-mode and U-mode, respectively. The encoding of these fields is the same as the MXL field of {\tt misa}, shown in Table~\ref{misabase}. The effective XLEN in S-mode and U-mode are termed {\em SXLEN} and {\em UXLEN}, respectively. For RV32 systems, the SXL and UXL fields do not exist, and SXLEN=32 and UXLEN=32. For RV64 systems, if S-mode is not supported, then SXL is hardwired to zero. Otherwise, it is a \warl\ field that encodes the current value of SXLEN. In particular, an implementation may make SXL be a read-only field whose value always ensures that SXLEN=MXLEN. For RV64 systems, if U-mode is not supported, then UXL is hardwired to zero. Otherwise, it is a \warl\ field that encodes the current value of UXLEN. In particular, an implementation may make UXL be a read-only field whose value always ensures that UXLEN=MXLEN or UXLEN=SXLEN. Whenever XLEN in any mode is set to a value less than the widest supported XLEN, all operations must ignore source operand register bits above the configured XLEN, and must sign-extend results to fill the entire widest supported XLEN in the destination register. Similarly, {\tt pc} bits above XLEN are ignored, and when the {\tt pc} is written, it is sign-extended to fill the widest supported XLEN. \begin{commentary} We require that operations always fill the entire underlying hardware registers with defined values to avoid implementation-defined behavior. To reduce hardware complexity, the architecture imposes no checks that lower-privilege modes have XLEN settings less than or equal to the next-higher privilege mode. In practice, such settings would almost always be a software bug, but machine operation is well-defined even in this case. \end{commentary} If MXLEN is changed from 32 to a wider width, each of {\tt mstatus} fields SXL and UXL, if not restricted to a single value, gets the value corresponding to the widest supported width not wider than the new MXLEN. \subsubsection{Memory Privilege in {\tt mstatus} Register} The MPRV (Modify PRiVilege) bit modifies the privilege level at which loads and stores execute. When MPRV=0, loads and stores behave as normal, using the translation and protection mechanisms of the current privilege mode. When MPRV=1, load and store memory addresses are translated and protected, and endianness is applied, as though the current privilege mode were set to MPP. Instruction address-translation and protection are unaffected by the setting of MPRV. MPRV is hardwired to 0 if U-mode is not supported. An MRET or SRET instruction that changes the privilege mode to a mode less privileged than M also sets MPRV=0. The MXR (Make eXecutable Readable) bit modifies the privilege with which loads access virtual memory. When MXR=0, only loads from pages marked readable (R=1 in Figure~\ref{sv32pte}) will succeed. When MXR=1, loads from pages marked either readable or executable (R=1 or X=1) will succeed. MXR has no effect when page-based virtual memory is not in effect. MXR is hardwired to 0 if S-mode is not supported. \begin{commentary} The MPRV and MXR mechanisms were conceived to improve the efficiency of M-mode routines that emulate missing hardware features, e.g., misaligned loads and stores. MPRV obviates the need to perform address translation in software. MXR allows instruction words to be loaded from pages marked execute-only. The current privilege mode and the privilege mode specified by MPP might have different XLEN settings. When MPRV=1, load and store memory addresses are treated as though the current XLEN were set to MPP's XLEN, following the rules in Section~\ref{xlen-control}. \end{commentary} The SUM (permit Supervisor User Memory access) bit modifies the privilege with which S-mode loads and stores access virtual memory. When SUM=0, S-mode memory accesses to pages that are accessible by U-mode (U=1 in Figure~\ref{sv32pte}) will fault. When SUM=1, these accesses are permitted. SUM has no effect when page-based virtual memory is not in effect. Note that, while SUM is ordinarily ignored when not executing in S-mode, it {\em is} in effect when MPRV=1 and MPP=S. SUM is hardwired to 0 if S-mode is not supported or if {\tt satp}.MODE is hardwired to 0. The MXR and SUM mechanisms only affect the interpretation of permissions encoded in page-table entries. In particular, they have no impact on whether access-fault exceptions are raised due to PMAs or PMP. \subsubsection{Endianness Control in {\tt mstatus} and {\tt mstatush} Registers} The MBE, SBE, and UBE bits in {\tt mstatus} and {\tt mstatush} are \warl\ fields that control the endianness of memory accesses other than instruction fetches. Instruction fetches are always little-endian. MBE controls whether non-instruction-fetch memory accesses made from M-mode (assuming {\tt mstatus}.MPRV=0) are little-endian (MBE=0) or big-endian (MBE=1). If S-mode is not supported, SBE is hardwired to~0. Otherwise, SBE controls whether explicit load and store memory accesses made from S-mode are little-endian (SBE=0) or big-endian (SBE=1). If U-mode is not supported, UBE is hardwired to~0. Otherwise, UBE controls whether explicit load and store memory accesses made from U-mode are little-endian (UBE=0) or big-endian (UBE=1). For {\em implicit} accesses to supervisor-level memory management data structures, such as page tables, endianness is always controlled by SBE. Since changing SBE alters the implementation's interpretation of these data structures, if any such data structures remain in use across a change to SBE, M-mode software must follow such a change to SBE by executing an SFENCE.VMA instruction with {\em rs1}={\tt x0} and {\em rs2}={\tt x0}. \begin{commentary} Only in contrived scenarios will a given memory-management data structure be interpreted as both little-endian and big-endian. In practice, SBE will only be changed at runtime on world switches, in which case neither the old nor new memory-management data structure will be reinterpreted in a different endianness. In this case, no additional SFENCE.VMA is necessary, beyond what would ordinarily be required for a world switch. \end{commentary} If S-mode is supported, an implementation may make SBE be a read-only copy of MBE. If U-mode is supported, an implementation may make UBE be a read-only copy of either MBE or SBE. \begin{commentary} An implementation supports only little-endian memory accesses if fields MBE, SBE, and UBE are all hardwired to~0. An implementation supports only big-endian memory accesses (aside from instruction fetches) if MBE is hardwired to 1 and SBE and UBE are each hardwired to 1 when S-mode and U-mode are supported. \end{commentary} \begin{commentary} Volume I defines a hart's address space as a circular sequence of $2^{\text{XLEN}}$ bytes at consecutive addresses. The correspondence between addresses and byte locations is fixed and not affected by any endianness mode. Rather, the applicable endianness mode determines the order of mapping between memory bytes and a multibyte quantity (halfword, word, etc.). \end{commentary} \begin{commentary} Standard RISC-V ABIs are expected to be purely little-endian-only or big-endian-only, with no accommodation for mixing endianness. Nevertheless, endianness control has been defined so as to permit, for instance, an OS of one endianness to execute user-mode programs of the opposite endianness. Consideration has been given also to the possibility of nonstandard usages whereby software flips the endianness of memory accesses as needed. \end{commentary} \begin{commentary} RISC-V instructions are uniformly little-endian to decouple instruction encoding from the current endianness settings, for the benefit of both hardware and software. Otherwise, for instance, a RISC-V assembler or disassembler would always need to know the intended active endianness, despite that the endianness mode might change dynamically during execution. In contrast, by giving instructions a fixed endianness, it is sometimes possible for carefully written software to be endianness-agnostic even in binary form, much like position-independent code. The choice to have instructions be only little-endian does have consequences, however, for RISC-V software that encodes or decodes machine instructions. In big-endian mode, such software must account for the fact that explicit loads and stores have endianness opposite that of instructions, for example by swapping byte order after loads and before stores. \end{commentary} \subsubsection{Virtualization Support in {\tt mstatus} Register} \label{virt-control} The TVM (Trap Virtual Memory) bit is a \warl\ field that supports intercepting supervisor virtual-memory management operations. When TVM=1, attempts to read or write the {\tt satp} CSR or execute the SFENCE.VMA instruction while executing in S-mode will raise an illegal instruction exception. When TVM=0, these operations are permitted in S-mode. TVM is hard-wired to 0 when S-mode is not supported. \begin{commentary} The TVM mechanism improves virtualization efficiency by permitting guest operating systems to execute in S-mode, rather than classically virtualizing them in U-mode. This approach obviates the need to trap accesses to most S-mode CSRs. Trapping {\tt satp} accesses and the SFENCE.VMA instruction provides the hooks necessary to lazily populate shadow page tables. \end{commentary} The TW (Timeout Wait) bit is a \warl\ field that supports intercepting the WFI instruction (see Section~\ref{wfi}). When TW=0, the WFI instruction may execute in lower privilege modes when not prevented for some other reason. When TW=1, then if WFI is executed in any less-privileged mode, and it does not complete within an implementation-specific, bounded time limit, the WFI instruction causes an illegal instruction exception. The time limit may always be 0, in which case WFI always causes an illegal instruction exception in less-privileged modes when TW=1. TW is hard-wired to 0 when there are no modes less privileged than M. \begin{commentary} Trapping the WFI instruction can trigger a world switch to another guest OS, rather than wastefully idling in the current guest. \end{commentary} When S-mode is implemented, then executing WFI in U-mode causes an illegal instruction exception, unless it completes within an implementation-specific, bounded time limit. A future revision of this specification might add a feature that allows S-mode to selectively permit WFI in U-mode. Such a feature would only be active when TW=0. The TSR (Trap SRET) bit is a \warl\ field that supports intercepting the supervisor exception return instruction, SRET. When TSR=1, attempts to execute SRET while executing in S-mode will raise an illegal instruction exception. When TSR=0, this operation is permitted in S-mode. TSR is hard-wired to 0 when S-mode is not supported. \begin{commentary} Trapping SRET is necessary to emulate the hypervisor extension (see Chapter~\ref{hypervisor}) on implementations that do not provide it. \end{commentary} \subsubsection{Extension Context Status in {\tt mstatus} Register} Supporting substantial extensions is one of the primary goals of RISC-V, and hence we define a standard interface to allow unchanged privileged-mode code, particularly a supervisor-level OS, to support arbitrary user-mode state extensions. \begin{commentary} To date, the V extension is the only standard extension that defines additional state beyond the floating-point CSR and data registers. \end{commentary} The FS[1:0] \warl\ field and the XS[1:0] read-only field are used to reduce the cost of context save and restore by setting and tracking the current state of the floating-point unit and any other user-mode extensions respectively. The FS field encodes the status of the floating-point unit, including the CSR {\tt fcsr} and floating-point data registers {\tt f0}--{\tt f31}, while the XS field encodes the status of additional user-mode extensions and associated state. These fields can be checked by a context switch routine to quickly determine whether a state save or restore is required. If a save or restore is required, additional instructions and CSRs are typically required to effect and optimize the process. \begin{commentary} The design anticipates that most context switches will not need to save/restore state in either or both of the floating-point unit or other extensions, so provides a fast check via the SD bit. \end{commentary} The FS and XS fields use the same status encoding as shown in Table~\ref{fsxsencoding}, with the four possible status values being Off, Initial, Clean, and Dirty. \begin{table*}[h!] \begin{center} \begin{tabular}{|r|l|l|} \hline Status & FS Meaning & XS Meaning\\ \hline 0 & Off & All off \\ 1 & Initial & None dirty or clean, some on\\ 2 & Clean & None dirty, some clean \\ 3 & Dirty & Some dirty \\ \hline \end{tabular} \end{center} \caption{Encoding of FS[1:0] and XS[1:0] status fields.} \label{fsxsencoding} \end{table*} If the F extension is implemented, the FS field shall not be hardwired to zero. If neither the F extension nor S-mode is implemented, then FS is hardwired to zero. If S-mode is implemented but the F extension is not, FS may optionally be hardwired to zero. \begin{commentary} Implementations with S-mode but without the F extension are permitted, but not required, to hardwire the FS field to zero. Some such implementations will choose {\em not} to hardwire the FS field to zero, so as to enable emulation of the F extension for both S-mode and U-mode via invisible traps into M-mode. \end{commentary} In systems without additional user extensions requiring new state, the XS field is hardwired to zero. Every additional extension with state provides a CSR field that encodes the equivalent of the XS states. The XS field represents a summary of all extensions' status as shown in Table~\ref{fsxsencoding}. \begin{commentary} The XS field effectively reports the maximum status value across all user-extension status fields, though individual extensions can use a different encoding than XS. \end{commentary} The SD bit is a read-only bit that summarizes whether either the FS field or XS field signals the presence of some dirty state that will require saving extended user context to memory. If both XS and FS are hardwired to zero, then SD is also always zero. When an extension's status is set to Off, any instruction that attempts to read or write the corresponding state will cause an illegal instruction exception. When the status is Initial, the corresponding state should have an initial constant value. When the status is Clean, the corresponding state is potentially different from the initial value, but matches the last value stored on a context swap. When the status is Dirty, the corresponding state has potentially been modified since the last context save. During a context save, the responsible privileged code need only write out the corresponding state if its status is Dirty, and can then reset the extension's status to Clean. During a context restore, the context need only be loaded from memory if the status is Clean (it should never be Dirty at restore). If the status is Initial, the context must be set to an initial constant value on context restore to avoid a security hole, but this can be done without accessing memory. For example, the floating-point registers can all be initialized to the immediate value 0. The FS and XS fields are read by the privileged code before saving the context. The FS field is set directly by privileged code when resuming a user context, while the XS field is set indirectly by writing to the status register of the individual extensions. The status fields will also be updated during execution of instructions, regardless of privilege mode. Extensions to the user-mode ISA often include additional user-mode state, and this state can be considerably larger than the base integer registers. The extensions might only be used for some applications, or might only be needed for short phases within a single application. To improve performance, the user-mode extension can define additional instructions to allow user-mode software to return the unit to an initial state or even to turn off the unit. For example, a coprocessor might require to be configured before use and can be ``unconfigured'' after use. The unconfigured state would be represented as the Initial state for context save. If the same application remains running between the unconfigure and the next configure (which would set status to Dirty), there is no need to actually reinitialize the state at the unconfigure instruction, as all state is local to the user process, i.e., the Initial state may only cause the coprocessor state to be initialized to a constant value at context restore, not at every unconfigure. Executing a user-mode instruction to disable a unit and place it into the Off state will cause an illegal instruction exception to be raised if any subsequent instruction tries to use the unit before it is turned back on. A user-mode instruction to turn a unit on must also ensure the unit's state is properly initialized, as the unit might have been used by another context meantime. Changing the setting of FS has no effect on the contents of the floating-point register state. In particular, setting FS=Off does not destroy the state, nor does setting FS=Initial clear the contents. Other extensions might not preserve state when set to Off. Implementations may choose to track the dirtiness of the floating-point register state imprecisely by reporting the state to be dirty even when it has not been modified. On some implementations, some instructions that do not mutate the floating-point state may cause the state to transition from Initial or Clean to Dirty. On other implementations, dirtiness might not be tracked at all, in which case the valid FS states are Off and Dirty, and an attempt to set FS to Initial or Clean causes it to be set to Dirty. \begin{commentary} This definition of FS does not disallow setting FS to Dirty as a result of errant speculation. Some platforms may choose to disallow speculatively writing FS to close a potential side channel. \end{commentary} If an instruction explicitly or implicitly writes a floating-point register or the {\tt fcsr} but does not alter its contents, and FS=Initial or FS=Clean, it is implementation-defined whether FS transitions to Dirty. Table~\ref{fsxsstates} shows all the possible state transitions for the FS or XS status bits. Note that the standard floating-point extensions do not support user-mode unconfigure or disable/enable instructions. \begin{table*}[h!] \begin{center} \begin{tabular}{|l|l|l|l|l|} \hline \multicolumn{1}{|r|}{Current State} & Off & Initial & Clean & Dirty \\ Action & & & &\\ \hline \hline \multicolumn{5}{|c|}{At context save in privileged code}\\ \hline Save state? & No & No & No & Yes \\ Next state & Off & Initial & Clean & Clean \\ \hline \hline \multicolumn{5}{|c|}{At context restore in privileged code}\\ \hline Restore state? & No & Yes, to initial & Yes, from memory & N/A \\ Next state & Off & Initial & Clean & N/A \\ \hline \hline \multicolumn{5}{|c|}{Execute instruction to read state}\\ \hline Action? & Exception & Execute & Execute & Execute \\ Next state & Off & Initial & Clean & Dirty \\ \hline \hline \multicolumn{5}{|c|}{Execute instruction that possibly modifies state, including configuration}\\ \hline Action? & Exception & Execute & Execute & Execute \\ Next state & Off & Dirty & Dirty & Dirty \\ \hline \hline \multicolumn{5}{|c|}{Execute instruction to unconfigure unit}\\ \hline Action? & Exception & Execute & Execute & Execute \\ Next state & Off & Initial & Initial & Initial \\ \hline \hline \multicolumn{5}{|c|}{Execute instruction to disable unit}\\ \hline Action? & Execute & Execute & Execute & Execute \\ Next state & Off & Off & Off & Off \\ \hline \hline \multicolumn{5}{|c|}{Execute instruction to enable unit}\\ \hline Action? & Execute & Execute & Execute & Execute \\ Next state & Initial & Initial & Initial & Initial \\ \hline \end{tabular} \end{center} \caption{FS and XS state transitions.} \label{fsxsstates} \end{table*} Standard privileged instructions to initialize, save, and restore extension state are provided to insulate privileged code from details of the added extension state by treating the state as an opaque object. \begin{commentary} Many coprocessor extensions are only used in limited contexts that allows software to safely unconfigure or even disable units when done. This reduces the context-switch overhead of large stateful coprocessors. We separate out floating-point state from other extension state, as when a floating-point unit is present the floating-point registers are part of the standard calling convention, and so user-mode software cannot know when it is safe to disable the floating-point unit. \end{commentary} The XS field provides a summary of all added extension state, but additional microarchitectural bits might be maintained in the extension to further reduce context save and restore overhead. The SD bit is read-only and is set when either the FS or XS bits encode a Dirty state (i.e., SD=((FS==11) OR (XS==11))). This allows privileged code to quickly determine when no additional context save is required beyond the integer register set and PC. The floating-point unit state is always initialized, saved, and restored using standard instructions (F, D, and/or Q), and privileged code must be aware of FLEN to determine the appropriate space to reserve for each {\tt f} register. All privileged modes share a single copy of the FS and XS bits. In a system with more than one privileged mode, supervisor mode would normally use the FS and XS bits directly to record the status with respect to the supervisor-level saved context. Other more-privileged active modes must be more conservative in saving and restoring the extension state in their corresponding version of the context. \begin{commentary} In any reasonable use case, the number of context switches between user and supervisor level should far outweigh the number of context switches to other privilege levels. Note that coprocessors should not require their context to be saved and restored to service asynchronous interrupts, unless the interrupt results in a user-level context swap. \end{commentary} \subsection{Machine Trap-Vector Base-Address Register ({\tt mtvec})} The {\tt mtvec} register is an MXLEN-bit \warl\ read/write register that holds trap vector configuration, consisting of a vector base address (BASE) and a vector mode (MODE). \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{J@{}S} \instbitrange{MXLEN-1}{2} & \instbitrange{1}{0} \\ \hline \multicolumn{1}{|c|}{BASE[MXLEN-1:2] (\warl)} & \multicolumn{1}{c|}{MODE (\warl)} \\ \hline MXLEN-2 & 2 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine trap-vector base-address register ({\tt mtvec}).} \label{mtvecreg} \end{figure*} The {\tt mtvec} register must always be implemented, but can contain a hardwired read-only value. If {\tt mtvec} is writable, the set of values the register may hold can vary by implementation. The value in the BASE field must always be aligned on a 4-byte boundary, and the MODE setting may impose additional alignment constraints on the value in the BASE field. \begin{commentary} We allow for considerable flexibility in implementation of the trap vector base address. On the one hand, we do not wish to burden low-end implementations with a large number of state bits, but on the other hand, we wish to allow flexibility for larger systems. \end{commentary} \begin{table*}[h!] \begin{center} \begin{tabular}{|r|c|l|} \hline Value & Name & Description \\ \hline 0 & Direct & All exceptions set {\tt pc} to BASE. \\ 1 & Vectored & Asynchronous interrupts set {\tt pc} to BASE+4$\times$cause. \\ $\ge$2 & --- & {\em Reserved} \\ \hline \end{tabular} \end{center} \caption{Encoding of {\tt mtvec} MODE field.} \label{mtvec-mode} \end{table*} The encoding of the MODE field is shown in Table~\ref{mtvec-mode}. When MODE=Direct, all traps into machine mode cause the {\tt pc} to be set to the address in the BASE field. When MODE=Vectored, all synchronous exceptions into machine mode cause the {\tt pc} to be set to the address in the BASE field, whereas interrupts cause the {\tt pc} to be set to the address in the BASE field plus four times the interrupt cause number. For example, a machine-mode timer interrupt (see Table~\ref{mcauses} on page~\pageref{mcauses}) causes the {\tt pc} to be set to BASE+{\tt 0x1c}. \begin{commentary} When vectored interrupts are enabled, interrupt cause 0, which corresponds to user-mode software interrupts, are vectored to the same location as synchronous exceptions. This ambiguity does not arise in practice, since user-mode software interrupts are either disabled or delegated to user mode. \end{commentary} An implementation may have different alignment constraints for different modes. In particular, MODE=Vectored may have stricter alignment constraints than MODE=Direct. \begin{commentary} Allowing coarser alignments in Vectored mode enables vectoring to be implemented without a hardware adder circuit. \end{commentary} \begin{commentary} Reset and NMI vector locations are given in a platform specification. \end{commentary} \subsection{Machine Trap Delegation Registers ({\tt medeleg} and {\tt mideleg})} By default, all traps at any privilege level are handled in machine mode, though a machine-mode handler can redirect traps back to the appropriate level with the MRET instruction (Section~\ref{otherpriv}). To increase performance, implementations can provide individual read/write bits within {\tt medeleg} and {\tt mideleg} to indicate that certain exceptions and interrupts should be processed directly by a lower privilege level. The machine exception delegation register ({\tt medeleg}) and machine interrupt delegation register ({\tt mideleg}) are MXLEN-bit read/write registers. In systems with S-mode, the {\tt medeleg} and {\tt mideleg} registers must exist, and setting a bit in {\tt medeleg} or {\tt mideleg} will delegate the corresponding trap, when occurring in S-mode or U-mode, to the S-mode trap handler. In systems without S-mode, the {\tt medeleg} and {\tt mideleg} registers should not exist. \begin{commentary} In versions 1.9.1 and earlier , these registers existed but were hardwired to zero in M-mode only, or M/U without N systems. There is no reason to require they return zero in those cases, as the {\tt misa} register indicates whether they exist. \end{commentary} When a trap is delegated to S-mode, the {\tt scause} register is written with the trap cause; the {\tt sepc} register is written with the virtual address of the instruction that took the trap; the {\tt stval} register is written with an exception-specific datum; the SPP field of {\tt mstatus} is written with the active privilege mode at the time of the trap; the SPIE field of {\tt mstatus} is written with the value of the SIE field at the time of the trap; and the SIE field of {\tt mstatus} is cleared. The {\tt mcause}, {\tt mepc}, and {\tt mtval} registers and the MPP and MPIE fields of {\tt mstatus} are not written. An implementation can choose to subset the delegatable traps, with the supported delegatable bits found by writing one to every bit location, then reading back the value in {\tt medeleg} or {\tt mideleg} to see which bit positions hold a one. An implementation shall not hardwire any bits of {\tt medeleg} to one, i.e., any synchronous trap that can be delegated must support not being delegated. Similarly, an implementation shall not hardwire to one any bits of {\tt mideleg} corresponding to machine-level interrupts (but may do so for lower-level interrupts). \begin{commentary} Version 1.11 and earlier prohibited hardwiring any bits of {\tt mideleg} to one. Platform standards may always add such restrictions. \end{commentary} Traps never transition from a more-privileged mode to a less-privileged mode. For example, if M-mode has delegated illegal instruction exceptions to S-mode, and M-mode software later executes an illegal instruction, the trap is taken in M-mode, rather than being delegated to S-mode. By contrast, traps may be taken horizontally. Using the same example, if M-mode has delegated illegal instruction exceptions to S-mode, and S-mode software later executes an illegal instruction, the trap is taken in S-mode. Delegated interrupts result in the interrupt being masked at the delegator privilege level. For example, if the supervisor timer interrupt (STI) is delegated to S-mode by setting {\tt mideleg}[5], STIs will not be taken when executing in M-mode. By contrast, if {\tt mideleg}[5] is clear, STIs can be taken in any mode and regardless of current mode will transfer control to M-mode. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}U} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Synchronous Exceptions (\warl)} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Exception Delegation Register {\tt medeleg}.} \label{medelegreg} \end{figure} {\tt medeleg} has a bit position allocated for every synchronous exception shown in Table~\ref{mcauses} on page~\pageref{mcauses}, with the index of the bit position equal to the value returned in the {\tt mcause} register (i.e., setting bit 8 allows user-mode environment calls to be delegated to a lower-privilege trap handler). \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}U} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Interrupts (\warl)} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Interrupt Delegation Register {\tt mideleg}.} \label{midelegreg} \end{figure} {\tt mideleg} holds trap delegation bits for individual interrupts, with the layout of bits matching those in the {\tt mip} register (i.e., STIP interrupt delegation control is located in bit 5). For exceptions that cannot occur in less privileged modes, the corresponding {\tt medeleg} bits should be hardwired to zero. In particular, {\tt medeleg}[11] is hardwired to zero. \subsection{Machine Interrupt Registers ({\tt mip} and {\tt mie})} The {\tt mip} register is an MXLEN-bit read/write register containing information on pending interrupts, while {\tt mie} is the corresponding MXLEN-bit read/write register containing interrupt enable bits. Interrupt cause number \textit{i} (as reported in CSR {\tt mcause}, Section~\ref{sec:mcause}) corresponds with bit~\textit{i} in both {\tt mip} and {\tt mie}. Bits 15:0 are allocated to standard interrupt causes only, while bits 16 and above are designated for platform or custom use. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}U} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Interrupts (\warl)} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Interrupt-Pending Register ({\tt mip}).} \label{mipreg} \end{figure} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}U} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{Interrupts (\warl)} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Interrupt-Enable Register ({\tt mie}).} \label{miereg} \end{figure} An interrupt~\textit{i} will trap to M-mode (causing the privilege mode to change to M-mode) if all of the following are true: (a)~either the current privilege mode is M and the MIE bit in the {\tt mstatus} register is set, or the current privilege mode has less privilege than M-mode; (b)~bit~\textit{i} is set in both {\tt mip} and {\tt mie}; and (c)~if register {\tt mideleg} exists, bit~\textit{i} is not set in {\tt mideleg}. These conditions for an interrupt trap to occur must be evaluated in a bounded amount of time from when an interrupt becomes, or ceases to be, pending in {\tt mip}, and must also be evaluated immediately following the execution of an {\em x}\/RET instruction or an explicit write to a CSR on which these interrupt trap conditions expressly depend (including {\tt mip}, {\tt mie}, {\tt mstatus}, and {\tt mideleg}). Interrupts to M-mode take priority over any interrupts to lower privilege modes. Each individual bit in register {\tt mip} may be writable or may be read-only. When bit~\textit{i} in {\tt mip} is writable, a pending interrupt \textit{i} can be cleared by writing 0 to this bit. If interrupt \textit{i} can become pending but bit~\textit{i} in {\tt mip} is read-only, the implementation must provide some other mechanism for clearing the pending interrupt. A bit in {\tt mie} must be writable if the corresponding interrupt can ever become pending. Bits of {\tt mie} that are not writable must be hardwired to zero. The standard portions (bits 15:0) of registers {\tt mip} and {\tt mie} are formatted as shown in Figures \ref{mipreg-standard} and \ref{miereg-standard} respectively. \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{Rcccccccccccc} \instbitrange{15}{12} & \instbit{11} & \instbit{10} & \instbit{9} & \instbit{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{0} & \multicolumn{1}{c|}{MEIP} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{SEIP} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{MTIP} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{STIP} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{MSIP} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{SSIP} & \multicolumn{1}{c|}{0} \\ \hline 4 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Standard portion (bits 15:0) of {\tt mip}.} \label{mipreg-standard} \end{figure*} \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{Rcccccccccccc} \instbitrange{15}{12} & \instbit{11} & \instbit{10} & \instbit{9} & \instbit{8} & \instbit{7} & \instbit{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{0} & \multicolumn{1}{c|}{MEIE} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{SEIE} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{MTIE} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{STIE} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{MSIE} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{SSIE} & \multicolumn{1}{c|}{0} \\ \hline 4 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Standard portion (bits 15:0) of {\tt mie}.} \label{miereg-standard} \end{figure*} \begin{commentary} The machine-level interrupt registers handle a few root interrupt sources which are assigned a fixed service priority for simplicity, while separate external interrupt controllers can implement a more complex prioritization scheme over a much larger set of interrupts that are then muxed into the machine-level interrupt sources. \end{commentary} \begin{commentary} The non-maskable interrupt is not made visible via the {\tt mip} register as its presence is implicitly known when executing the NMI trap handler. \end{commentary} Bits {\tt mip}.MEIP and {\tt mie}.MEIE are the interrupt-pending and interrupt-enable bits for machine-level external interrupts. MEIP is read-only in {\tt mip}, and is set and cleared by a platform-specific interrupt controller. Bits {\tt mip}.MTIP and {\tt mie}.MTIE are the interrupt-pending and interrupt-enable bits for machine timer interrupts. MTIP is read-only in {\tt mip}, and is cleared by writing to the memory-mapped machine-mode timer compare register. Bits {\tt mip}.MSIP and {\tt mie}.MSIE are the interrupt-pending and interrupt-enable bits for machine-level software interrupts. MSIP is read-only in {\tt mip}, and is written by accesses to memory-mapped control registers, which are used by remote harts to provide machine-level interprocessor interrupts. A hart can write its own MSIP bit using the same memory-mapped control register. If supervisor mode is not implemented, bits SEIP, STIP, and SSIP of {\tt mip} and SEIE, STIE, and SSIE of {\tt mie} are hardwired to zeros. If supervisor mode is implemented, bits {\tt mip}.SEIP and {\tt mie}.SEIE are the interrupt-pending and interrupt-enable bits for supervisor-level external interrupts. SEIP is writable in {\tt mip}, and may be written by M-mode software to indicate to S-mode that an external interrupt is pending. Additionally, the platform-level interrupt controller may generate supervisor-level external interrupts. Supervisor-level external interrupts are made pending based on the logical-OR of the software-writable SEIP bit and the signal from the external interrupt controller. When {\tt mip} is read with a CSR instruction, the value of the SEIP bit returned in the {\tt rd} destination register is the logical-OR of the software-writable bit and the interrupt signal from the interrupt controller, but the signal from the interrupt controller is not used to calculate the value written to SEIP. Only the software-writable SEIP bit participates in the read-modify-write sequence of a CSRRS or CSRRC instruction. \begin{commentary} The SEIP field behavior is designed to allow a higher privilege layer to mimic external interrupts cleanly, without losing any real external interrupts. The behavior of the CSR instructions is slightly modified from regular CSR accesses as a result. \end{commentary} If supervisor mode is implemented, bits {\tt mip}.STIP and {\tt mie}.STIE are the interrupt-pending and interrupt-enable bits for supervisor-level timer interrupts. STIP is writable in {\tt mip}, and may be written by M-mode software to deliver timer interrupts to S-mode. If supervisor mode is implemented, bits {\tt mip}.SSIP and {\tt mie}.SSIE are the interrupt-pending and interrupt-enable bits for supervisor-level software interrupts. SSIP is writable in {\tt mip} and may also be set to 1 by a platform-specific interrupt controller. Multiple simultaneous interrupts destined for M-mode are handled in the following decreasing priority order: MEI, MSI, MTI, SEI, SSI, STI. \begin{commentary} The machine-level interrupt fixed-priority ordering rules were developed with the following rationale. Interrupts for higher privilege modes must be serviced before interrupts for lower privilege modes to support preemption. The platform-specific machine-level interrupt sources in bits 16 and above have platform-specific priority, but are typically chosen to have the highest service priority to support very fast local vectored interrupts. External interrupts are handled before internal (timer/software) interrupts as external interrupts are usually generated by devices that might require low interrupt service times. Software interrupts are handled before internal timer interrupts, because internal timer interrupts are usually intended for time slicing, where time precision is less important, whereas software interrupts are used for inter-processor messaging. Software interrupts can be avoided when high-precision timing is required, or high-precision timer interrupts can be routed via a different interrupt path. Software interrupts are located in the lowest four bits of {\tt mip} as these are often written by software, and this position allows the use of a single CSR instruction with a five-bit immediate. \end{commentary} Restricted views of the {\tt mip} and {\tt mie} registers appear as the {\tt sip} and {\tt sie} registers for supervisor level. If an interrupt is delegated to S-mode by setting a bit in the {\tt mideleg} register, it becomes visible in the {\tt sip} register and is maskable using the {\tt sie} register. Otherwise, the corresponding bits in {\tt sip} and {\tt sie} appear to be hardwired to zero. \subsection{Hardware Performance Monitor} M-mode includes a basic hardware performance-monitoring facility. The {\tt mcycle} CSR counts the number of clock cycles executed by the processor core on which the hart is running. The {\tt minstret} CSR counts the number of instructions the hart has retired. The {\tt mcycle} and {\tt minstret} registers have 64-bit precision on all RV32 and RV64 systems. The counter registers have an arbitrary value after the hart is reset, and can be written with a given value. Any CSR write takes effect after the writing instruction has otherwise completed. The {\tt mcycle} CSR may be shared between harts on the same core, in which case writes to {\tt mcycle} will be visible to those harts. The platform should provide a mechanism to indicate which harts share an {\tt mcycle} CSR. The hardware performance monitor includes 29 additional 64-bit event counters, {\tt mhpmcounter3}--{\tt mhpmcounter31}. The event selector CSRs, {\tt mhpmevent3}--{\tt mhpmevent31}, are MXLEN-bit \warl\ registers that control which event causes the corresponding counter to increment. The meaning of these events is defined by the platform, but event 0 is defined to mean ``no event.'' All counters should be implemented, but a legal implementation is to hard-wire both the counter and its corresponding event selector to 0. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}K@{}W@{}K} \instbitrange{63}{0} \\ \cline{1-1} \multicolumn{1}{|c|}{\tt mcycle} \\ \cline{1-1} \multicolumn{1}{|c|}{\tt minstret} \\ \cline{1-1} & & \instbitrange{MXLEN-1}{0} \\ \cline{1-1}\cline{3-3} \multicolumn{1}{|c|}{\tt mhpmcounter3} & & \multicolumn{1}{|c|}{\tt mhpmevent3} \\ \cline{1-1}\cline{3-3} \multicolumn{1}{|c|}{\tt mhpmcounter4} & & \multicolumn{1}{|c|}{\tt mhpmevent4} \\ \cline{1-1}\cline{3-3} \multicolumn{1}{c}{\vdots} & & \multicolumn{1}{c}{\vdots} \\ \cline{1-1}\cline{3-3} \multicolumn{1}{|c|}{\tt mhpmcounter30} & & \multicolumn{1}{|c|}{\tt mhpmevent30} \\ \cline{1-1}\cline{3-3} \multicolumn{1}{|c|}{\tt mhpmcounter31} & & \multicolumn{1}{|c|}{\tt mhpmevent31} \\ \cline{1-1}\cline{3-3} 64 & & MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Hardware performance monitor counters.} \end{figure} The {\tt mhpmcounter}s are \warl\ registers that support up to 64 bits of precision on RV32 and RV64. \begin{commentary} A future revision of this specification will define a mechanism to generate an interrupt when a hardware performance monitor counter overflows. \end{commentary} When MXLEN=32, reads of the {\tt mcycle}, {\tt minstret}, and {\tt mhpmcounter{\em n}} CSRs return bits 31--0 of the corresponding counter, and writes change only bits 31--0; reads of the {\tt mcycleh}, {\tt minstreth}, and {\tt mhpmcounter{\em n}h} CSRs return bits 63--32 of the corresponding counter, and writes change only bits 63--32. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}K} \instbitrange{31}{0} \\ \hline \multicolumn{1}{|c|}{\tt mcycleh} \\ \hline \multicolumn{1}{|c|}{\tt minstreth} \\ \hline \multicolumn{1}{|c|}{\tt mhpmcounter3h} \\ \hline \multicolumn{1}{|c|}{\tt mhpmcounter4h} \\ \hline \multicolumn{1}{c}{\vdots} \\ \hline \multicolumn{1}{|c|}{\tt mhpmcounter30h} \\ \hline \multicolumn{1}{|c|}{\tt mhpmcounter31h} \\ \hline 32 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Upper 32 bits of hardware performance monitor counters, RV32 only.} \end{figure} %On RV128 systems, the 64-bit values in {\tt mcycle}, {\tt minstret}, and %{\tt mhpmcounter{\em n}} are sign-extended to 128-bits when read. %\begin{commentary} %On RV128 systems, both signed and unsigned 64-bit values are held in a %canonical form with bit 63 repeated in all higher bit positions. The %counters are 64-bit values even in RV128, and so the counter CSR reads %preserve the sign-extension invariant. Implementations may choose to %implement fewer bits of the counters, provided software would be unlikely %to experience wraparound (e.g., $2^{63}$ instructions executed) %and thereby avoid having to actually implement the sign-extension %circuitry. %\end{commentary} \subsection{Machine Counter-Enable Register ({\tt mcounteren})} \label{sec:mcounteren} The counter-enable register {\tt mcounteren} is a 32-bit register that controls the availability of the hardware performance-monitoring counters to the next-lowest privileged mode. \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{cccMcccccc} \instbit{31} & \instbit{30} & \instbit{29} & \instbitrange{28}{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{HPM31} & \multicolumn{1}{c|}{HPM30} & \multicolumn{1}{c|}{HPM29} & \multicolumn{1}{c|}{...} & \multicolumn{1}{c|}{HPM5} & \multicolumn{1}{c|}{HPM4} & \multicolumn{1}{c|}{HPM3} & \multicolumn{1}{c|}{IR} & \multicolumn{1}{c|}{TM} & \multicolumn{1}{c|}{CY} \\ \hline 1 & 1 & 1 & 23 & 1 & 1 & 1 & 1 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Counter-enable register ({\tt mcounteren}).} \label{mcounteren} \end{figure*} The settings in this register only control accessibility. The act of reading or writing this register does not affect the underlying counters, which continue to increment even when not accessible. When the CY, TM, IR, or HPM{\em n} bit in the {\tt mcounteren} register is clear, attempts to read the {\tt cycle}, {\tt time}, {\tt instret}, or {\tt hpmcounter{\em n}} register while executing in S-mode or U-mode will cause an illegal instruction exception. When one of these bits is set, access to the corresponding register is permitted in the next implemented privilege mode (S-mode if implemented, otherwise U-mode). \begin{commentary} The counter-enable bits support two common use cases with minimal hardware. For systems that do not need high-performance timers and counters, machine-mode software can trap accesses and implement all features in software. For systems that need high-performance timers and counters but are not concerned with obfuscating the underlying hardware counters, the counters can be directly exposed to lower privilege modes. \end{commentary} The {\tt cycle}, {\tt instret}, and {\tt hpmcounter{\em n}} CSRs are read-only shadows of {\tt mcycle}, {\tt minstret}, and {\tt mhpmcounter{\em n}}, respectively. The {\tt time} CSR is a read-only shadow of the memory-mapped {\tt mtime} register. Analogously, on RV32I the {\tt cycleh}, {\tt instreth} and {\tt hpmcounter{\em n}h} CSRs are read-only shadows of {\tt mcycleh}, {\tt minstreth} and {\tt mhpmcounter{\em n}h}, respectively. On RV32I the {\tt timeh} CSR is a read-only shadow of the upper 32 bits of the memory-mapped {\tt mtime} register, while {\tt time} shadows only the lower 32 bits of {\tt mtime}. \begin{commentary} Implementations can convert reads of the {\tt time} and {\tt timeh} CSRs into loads to the memory-mapped {\tt mtime} register, or emulate this functionality in M-mode software. \end{commentary} In systems with U-mode, the {\tt mcounteren} must be implemented, but all fields are \warl\ and may be hardwired to zero, indicating reads to the corresponding counter will cause an illegal instruction exception when executing in a less-privileged mode. In systems without U-mode, the {\tt mcounteren} register should not exist. \subsection{Machine Counter-Inhibit CSR ({\tt mcountinhibit})} \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{cccMcccccc} \instbit{31} & \instbit{30} & \instbit{29} & \instbitrange{28}{6} & \instbit{5} & \instbit{4} & \instbit{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{HPM31} & \multicolumn{1}{c|}{HPM30} & \multicolumn{1}{c|}{HPM29} & \multicolumn{1}{c|}{...} & \multicolumn{1}{c|}{HPM5} & \multicolumn{1}{c|}{HPM4} & \multicolumn{1}{c|}{HPM3} & \multicolumn{1}{c|}{IR} & \multicolumn{1}{c|}{0} & \multicolumn{1}{c|}{CY} \\ \hline 1 & 1 & 1 & 23 & 1 & 1 & 1 & 1 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Counter-inhibit register {\tt mcountinhibit}.} \label{mcountinhibit} \end{figure*} The counter-inhibit register {\tt mcountinhibit} is a 32-bit \warl\ register that controls which of the hardware performance-monitoring counters increment. The settings in this register only control whether the counters increment; their accessibility is not affected by the setting of this register. When the CY, IR, or HPM{\em n} bit in the {\tt mcountinhibit} register is clear, the {\tt cycle}, {\tt instret}, or {\tt hpmcounter{\em n}} register increments as usual. When the CY, IR, or HPM{\em n} bit is set, the corresponding counter does not increment. The {\tt mcycle} CSR may be shared between harts on the same core, in which case the {\tt mcountinhibit}.CY field is also shared between those harts, and so writes to {\tt mcountinhibit}.CY will be visible to those harts. If the {\tt mcountinhibit} register is not implemented, the implementation behaves as though the register were set to zero. \begin{commentary} When the {\tt cycle} and {\tt instret} counters are not needed, it is desirable to conditionally inhibit them to reduce energy consumption. Providing a single CSR to inhibit all counters also allows the counters to be atomically sampled. Because the {\tt time} counter can be shared between multiple cores, it cannot be inhibited with the {\tt mcountinhibit} mechanism. \end{commentary} \subsection{Machine Scratch Register ({\tt mscratch})} The {\tt mscratch} register is an MXLEN-bit read/write register dedicated for use by machine mode. Typically, it is used to hold a pointer to a machine-mode hart-local context space and swapped with a user register upon entry to an M-mode trap handler. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{\tt mscratch} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine-mode scratch register.} \label{mscratchreg} \end{figure} \begin{commentary} The MIPS ISA allocated two user registers ({\tt k0}/{\tt k1}) for use by the operating system. Although the MIPS scheme provides a fast and simple implementation, it also reduces available user registers, and does not scale to further privilege levels, or nested traps. It can also require both registers are cleared before returning to user level to avoid a potential security hole and to provide deterministic debugging behavior. The RISC-V user ISA was designed to support many possible privileged system environments and so we did not want to infect the user-level ISA with any OS-dependent features. The RISC-V CSR swap instructions can quickly save/restore values to the {\tt mscratch} register. Unlike the MIPS design, the OS can rely on holding a value in the {\tt mscratch} register while the user context is running. \end{commentary} \subsection{Machine Exception Program Counter ({\tt mepc})} {\tt mepc} is an MXLEN-bit read/write register formatted as shown in Figure~\ref{mepcreg}. The low bit of {\tt mepc} ({\tt mepc[0]}) is always zero. On implementations that support only IALIGN=32, the two low bits ({\tt mepc[1:0]}) are always zero. If an implementation allows IALIGN to be either 16 or 32 (by changing CSR {\tt misa}, for example), then, whenever IALIGN=32, bit {\tt mepc[1]} is masked on reads so that it appears to be 0. This masking occurs also for the implicit read by the MRET instruction. Though masked, {\tt mepc[1]} remains writable when IALIGN=32. {\tt mepc} is a \warl\ register that must be able to hold all valid virtual addresses. It need not be capable of holding all possible invalid addresses. Prior to writing {\tt mepc}, implementations may convert an invalid address into some other invalid address that {\tt mepc} is capable of holding. \begin{commentary} When address translation is not in effect, virtual addresses and physical addresses are equal. Hence, the set of addresses {\tt mepc} must be able to represent includes the set of physical addresses that can be used as a valid {\tt pc} or effective address. \end{commentary} When a trap is taken into M-mode, {\tt mepc} is written with the virtual address of the instruction that was interrupted or that encountered the exception. Otherwise, {\tt mepc} is never written by the implementation, though it may be explicitly written by software. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{\tt mepc} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine exception program counter register.} \label{mepcreg} \end{figure} \subsection{Machine Cause Register ({\tt mcause})} \label{sec:mcause} The {\tt mcause} register is an MXLEN-bit read-write register formatted as shown in Figure~\ref{mcausereg}. When a trap is taken into M-mode, {\tt mcause} is written with a code indicating the event that caused the trap. Otherwise, {\tt mcause} is never written by the implementation, though it may be explicitly written by software. The Interrupt bit in the {\tt mcause} register is set if the trap was caused by an interrupt. The Exception Code field contains a code identifying the last exception or interrupt. Table~\ref{mcauses} lists the possible machine-level exception codes. The Exception Code is a \wlrl\ field, so is only guaranteed to hold supported exception codes. \begin{figure*}[h!] {\footnotesize \begin{center} \begin{tabular}{c@{}U} \instbit{MXLEN-1} & \instbitrange{MXLEN-2}{0} \\ \hline \multicolumn{1}{|c|}{Interrupt} & \multicolumn{1}{c|}{Exception Code (\wlrl)} \\ \hline 1 & MXLEN-1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Cause register {\tt mcause}.} \label{mcausereg} \end{figure*} \begin{table*}[htbp] \begin{center} \begin{tabular}{|r|r|l|l|} \hline Interrupt & Exception Code & Description \\ \hline 1 & 0 & {\em Reserved} \\ 1 & 1 & Supervisor software interrupt \\ 1 & 2 & {\em Reserved} \\ 1 & 3 & Machine software interrupt \\ \hline 1 & 4 & {\em Reserved} \\ 1 & 5 & Supervisor timer interrupt \\ 1 & 6 & {\em Reserved} \\ 1 & 7 & Machine timer interrupt \\ \hline 1 & 8 & {\em Reserved} \\ 1 & 9 & Supervisor external interrupt \\ 1 & 10 & {\em Reserved} \\ 1 & 11 & Machine external interrupt \\ \hline 1 & 12--15 & {\em Reserved} \\ 1 & $\ge$16 & {\em Designated for platform use} \\ \hline 0 & 0 & Instruction address misaligned \\ 0 & 1 & Instruction access fault \\ 0 & 2 & Illegal instruction \\ 0 & 3 & Breakpoint \\ 0 & 4 & Load address misaligned \\ 0 & 5 & Load access fault \\ 0 & 6 & Store/AMO address misaligned \\ 0 & 7 & Store/AMO access fault \\ 0 & 8 & Environment call from U-mode\\ 0 & 9 & Environment call from S-mode \\ 0 & 10 & {\em Reserved} \\ 0 & 11 & Environment call from M-mode \\ 0 & 12 & Instruction page fault \\ 0 & 13 & Load page fault \\ 0 & 14 & {\em Reserved} \\ 0 & 15 & Store/AMO page fault \\ 0 & 16--23 & {\em Reserved} \\ 0 & 24--31 & {\em Designated for custom use} \\ 0 & 32--47 & {\em Reserved} \\ 0 & 48--63 & {\em Designated for custom use} \\ 0 & $\ge$64 & {\em Reserved} \\ \hline \end{tabular} \end{center} \caption{Machine cause register ({\tt mcause}) values after trap.} \label{mcauses} \end{table*} Note that load and load-reserved instructions generate load exceptions, whereas store, store-conditional, and AMO instructions generate store/AMO exceptions. \begin{commentary} Interrupts can be separated from other traps with a single branch on the sign of the {\tt mcause} register value. A shift left can remove the interrupt bit and scale the exception codes to index into a trap vector table. \end{commentary} \begin{commentary} We do not distinguish privileged instruction exceptions from illegal opcode exceptions. This simplifies the architecture and also hides details of which higher-privilege instructions are supported by an implementation. The privilege level servicing the trap can implement a policy on whether these need to be distinguished, and if so, whether a given opcode should be treated as illegal or privileged. \end{commentary} If an instruction may raise multiple synchronous exceptions, the decreasing priority order of Table~\ref{exception-priority} indicates which exception is taken and reported in {\tt mcause}. The priority of any custom synchronous exceptions is implementation-defined. \begin{table*}[htbp] \begin{center} \begin{tabular}{|l|r|l|} \hline Priority & Exc.\@ Code & Description \\ \hline {\em Highest} & 3 & Instruction address breakpoint \\ \hline & & During instruction address translation: \\ & 12, 1 & \quad First encountered page fault or access fault \\ \hline & & With physical address for instruction: \\ & 1 & \quad Instruction access fault \\ \hline & 2 & Illegal instruction \\ & 0 & Instruction address misaligned \\ & 8, 9, 11 & Environment call \\ & 3 & Environment break \\ & 3 & Load/store/AMO address breakpoint \\ \hline & & Optionally: \\ & 4, 6 & \quad Load/store/AMO address misaligned \\ \hline & & During address translation for an explicit memory access: \\ & 13, 15, 5, 7 & \quad First encountered page fault or access fault \\ \hline & & With physical address for an explicit memory access: \\ & 5, 7 & \quad Load/store/AMO access fault \\ \hline & & If not higher priority: \\ {\em Lowest} & 4, 6 & \quad Load/store/AMO address misaligned \\ \hline \end{tabular} \end{center} \caption{Synchronous exception priority in decreasing priority order.} \label{exception-priority} \end{table*} When a virtual address is translated into a physical address, the address translation algorithm determines what specific exception may be raised. Load/store/AMO address-misaligned exceptions may have either higher or lower priority than load/store/AMO page-fault and access-fault exceptions. \begin{commentary} The relative priority of load/store/AMO address-misaligned and page-fault exceptions is implementation-defined to flexibly cater to two design points. Implementations that never support misaligned accesses can unconditionally raise the misaligned-address exception without performing address translation or protection checks. Implementations that support misaligned accesses only to some physical addresses must translate and check the address before determining whether the misaligned access may proceed, in which case raising the page-fault exception or access is more appropriate. \end{commentary} \begin{commentary} Instruction address breakpoints have the same cause value as, but different priority than, data address breakpoints (a.k.a. watchpoints) and environment break exceptions (which are raised by the EBREAK instruction). \end{commentary} \begin{commentary} Instruction address misaligned exceptions are raised by control-flow instructions with misaligned targets, rather than by the act of fetching an instruction. Therefore, these exceptions have lower priority than other instruction address exceptions. \end{commentary} \FloatBarrier \subsection{Machine Trap Value Register ({\tt mtval})} The {\tt mtval} register is an MXLEN-bit read-write register formatted as shown in Figure~\ref{mtvalreg}. When a trap is taken into M-mode, {\tt mtval} is either set to zero or written with exception-specific information to assist software in handling the trap. Otherwise, {\tt mtval} is never written by the implementation, though it may be explicitly written by software. The hardware platform will specify which exceptions must set {\tt mtval} informatively and which may unconditionally set it to zero. If the hardware platform specifies that no exceptions set {\tt mtval} to a nonzero value, then {\tt mtval} is hardwired to zero. If {\tt mtval} is not hardwired to zero, then when a breakpoint, address-misaligned, access-fault, or page-fault exception occurs on an instruction fetch, load, or store, {\tt mtval} is written with the faulting virtual address. On an illegal instruction trap, {\tt mtval} may be written with the first XLEN or ILEN bits of the faulting instruction as described below. For other traps, {\tt mtval} is set to zero, but a future standard may redefine {\tt mtval}'s setting for other traps. \begin{commentary} The {\tt mtval} register replaces the {\tt mbadaddr} register in the previous specification. In addition to providing bad addresses, the register can now provide the bad instruction that triggered an illegal instruction trap (and may in future be used to return other information). Returning the instruction bits accelerates instruction emulation and also removes some races that might be present when trying to emulate illegal instructions. \end{commentary} \begin{commentary} When page-based virtual memory is enabled, {\tt mtval} is written with the faulting virtual address, even for physical-memory access-fault exceptions. This design reduces datapath cost for most implementations, particularly those with hardware page-table walkers. \end{commentary} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{\tt mtval} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Trap Value register.} \label{mtvalreg} \end{figure} For misaligned loads and stores that cause access-fault or page-fault exceptions, {\tt mtval} will contain the virtual address of the portion of the access that caused the fault (unless {\tt mtval} is hardwired to zero). For instruction access-fault or page-fault exceptions on systems with variable-length instructions, {\tt mtval} will contain the virtual address of the portion of the instruction that caused the fault (unless {\tt mtval} is hardwired to zero). {\tt mepc} will point to the beginning of the instruction. The {\tt mtval} register can optionally also be used to return the faulting instruction bits on an illegal instruction exception ({\tt mepc} points to the faulting instruction in memory). If this feature is not provided, then {\tt mtval} is set to zero on an illegal instruction fault. If this feature is provided, after an illegal instruction trap, {\tt mtval} will contain the shortest of: \begin{compactitem} \item the actual faulting instruction \item the first ILEN bits of the faulting instruction \item the first XLEN bits of the faulting instruction \end{compactitem} The value loaded into {\tt mtval} is right-justified and all unused upper bits are cleared to zero. \begin{commentary} Capturing the faulting instruction in {\tt mtval} reduces the overhead of instruction emulation, potentially avoiding several partial instruction loads if the instruction is misaligned, and likely data cache misses or slow uncached accesses when loads are used to fetch the instruction into a data register. There is also a problem of atomicity if another agent is manipulating the instruction memory, as might occur in a dynamic translation system. A requirement is that the entire instruction (or at least the first XLEN bits) are fetched into {\tt mtval} before taking the trap. This should not constrain implementations, which would typically fetch the entire instruction before attempting to decode the instruction, and avoids complicating software handlers. A value of zero in {\tt mtval} signifies either that the feature is not supported, or an illegal zero instruction was fetched. A load from the instruction memory pointed to by {\tt mepc} can be used to distinguish these two cases (or alternatively, the system configuration information can be interrogated to install the appropriate trap handling before runtime). \end{commentary} If {\tt mtval} is not hardwired to zero, it is a \warl\ register that must be able to hold all valid virtual addresses and the value zero. It need not be capable of holding all possible invalid addresses. Prior to writing {\tt mtval}, implementations may convert an invalid address into some other invalid address that {\tt mtval} is capable of holding. If the feature to return the faulting instruction bits is implemented, {\tt mtval} must also be able to hold all values less than $2^N$, where $N$ is the smaller of XLEN and ILEN. \subsection{Machine Configuration Pointer Register ({\tt mconfigptr})} {\tt mconfigptr} is an MXLEN-bit read-only CSR, formatted as shown in Figure~\ref{mconfigptrreg}, that holds the physical address of a configuration data structure. Software can traverse this data structure to discover information about the harts, the platform, and their configuration. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{MXLEN-1}{0} \\ \hline \multicolumn{1}{|c|}{\tt mconfigptr} \\ \hline MXLEN \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine Configuration Pointer register.} \label{mconfigptrreg} \end{figure} The pointer alignment in bits must be no smaller than the greatest supported MXLEN: i.e., if the greatest supported MXLEN is $8\times n$, then {\tt mconfigptr}[$\log_2n$-1:0] must be hardwired to zero. {\tt mconfigptr} must be implemented, but it may be hardwired to zero to indicate the configuration data structure does not exist or that an alternative mechanism must be used to locate it. \begin{commentary} The format and schema of the configuration data structure have yet to be standardized. \end{commentary} \begin{commentary} While {\tt mconfigptr} will simply be hardwired in some implementations, other implementations may provide a means to configure the value returned on CSR reads. For example, {\tt mconfigptr} might present the value of a memory-mapped register that is programmed by the platform or by M-mode software towards the beginning of the boot process. \end{commentary} \subsection{% Machine Environment Configuration Registers ({\tt menvcfg} and {\tt menvcfgh})% } The {\tt menvcfg} CSR is an MXLEN-bit read/write register, formatted for MXLEN=64 as shown in Figure~\ref{fig:menvcfg}, that controls certain characteristics of the execution environment for modes less privileged than M. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{c@{}Kcc@{}W@{}Wc} \instbit{63} & \instbitrange{62}{8} & \instbit{7} & \instbit{6} & \instbitrange{5}{4} & \instbitrange{3}{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{STCE} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{CBZE} & \multicolumn{1}{c|}{CBCFE} & \multicolumn{1}{c|}{CBIE} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{FIOM} \\ \hline 1 & 55 & 1 & 1 & 2 & 3 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine environment configuration register ({\tt menvcfg}) for MXLEN=64.} \label{fig:menvcfg} \end{figure} If bit FIOM (Fence of I/O implies Memory) is set to one in {\tt menvcfg}, FENCE instructions executed in modes less privileged than M are modified so the requirement to order accesses to device I/O implies also the requirement to order main memory accesses. Table~\ref{tab:menvcfg-FIOM} details the modified interpretation of FENCE instruction bits PI, PO, SI, and SO for modes less privileged than M when FIOM=1. Similarly, for modes less privileged than M when FIOM=1, if an atomic instruction that accesses a region ordered as device I/O has its {\em aq} and/or {\em rl} bit set, then that instruction is ordered as though it accesses both device I/O and memory. If S-mode is not supported, or if {\tt satp}.MODE is hardwired to Bare, the implementation may hardwire FIOM to zero. \begin{table}[h!] \begin{center} \begin{tabular}{|c|l|} \hline Instruction bit & Meaning when set \\ \hline PI & Predecessor device input and memory reads (PR implied) \\ PO & Predecessor device output and memory writes (PW implied) \\ \hline SI & Successor device input and memory reads (SR implied) \\ SO & Successor device output and memory writes (SW implied) \\ \hline \end{tabular} \end{center} \vspace{-0.1in} \caption{% Modified interpretation of FENCE predecessor and successor sets for modes less privileged than M when FIOM=1.% } \label{tab:menvcfg-FIOM} \end{table} \begin{commentary} Bit FIOM is needed in {\tt menvcfg} so M-mode can emulate the hypervisor extension of Chapter~\ref{hypervisor}, which has an equivalent FIOM bit in the hypervisor CSR {\tt henvcfg}. \end{commentary} The definition of the STCE field will be furnished by the forthcoming Sstc extension. Its allocation within {\tt menvcfg} may change prior to the ratification of that extension. The definition of the CBZE field will be furnished by the forthcoming Zicboz extension. Its allocation within {\tt menvcfg} may change prior to the ratification of that extension. The definitions of the CBCFE and CBIE fields will be furnished by the forthcoming Zicbom extension. Their allocations within {\tt menvcfg} may change prior to the ratification of that extension. When MXLEN=32, {\tt menvcfg} contains the same fields as bits 31:0 of {\tt menvcfg} when MXLEN=64. Additionally, when MXLEN=32, {\tt menvcfgh} is a 32-bit read/write register that contains the same fields as bits 63:32 of {\tt menvcfg} when MXLEN=64. Register {\tt menvcfgh} does not exist when MXLEN=64. If U-mode is not supported, then registers {\tt menvcfg} and {\tt menvcfgh} do not exist. \subsection{Machine Security Configuration Register ({\tt mseccfg})} \label{sec:mseccfg} {\tt mseccfg} is an optional MXLEN-bit read/write register, formatted as shown in Figure~\ref{fig:mseccfg}, that controls security features. When MXLEN=32 only, {\tt mseccfgh} is a 32-bit read/write register that contains the same fields as {\tt mseccfg} bits 63:32 when MXLEN=64. \begin{figure*}[h!] {\footnotesize \begin{center} \setlength{\tabcolsep}{4pt} \begin{tabular}{MccFccc} \instbitrange{XLEN-1}{10} & \instbit{9} & \instbit{8} & \instbitrange{7}{3} & \instbit{2} & \instbit{1} & \instbit{0} \\ \hline \multicolumn{1}{|c|}{\wpri} & \multicolumn{1}{c|}{SSEED} & \multicolumn{1}{c|}{USEED} & \multicolumn{1}{c|}{\wpri} & \multicolumn{1}{c|}{RLB} & \multicolumn{1}{c|}{MMWP} & \multicolumn{1}{c|}{MML} \\ \hline XLEN-10 & 1 & 1 & 5 & 1 & 1 & 1 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine security configuration register ({\tt mseccfg}).} \label{fig:mseccfg} \end{figure*} The definitions of the SSEED and USEED fields will be furnished by the forthcoming entropy-source extension, Zkr. Their allocations within {\tt mseccfg} may change prior to the ratification of that extension. The definitions of the RLB, MMWP, and MML fields will be furnished by the forthcoming PMP-enhancement extension, Smepmp. Their allocations within {\tt mseccfg} may change prior to the ratification of that extension. \section{Machine-Level Memory-Mapped Registers} \subsection{Machine Timer Registers ({\tt mtime} and {\tt mtimecmp})} Platforms provide a real-time counter, exposed as a memory-mapped machine-mode read-write register, {\tt mtime}. {\tt mtime} must increment at constant frequency, and the platform must provide a mechanism for determining the timebase of {\tt mtime}. The {\tt mtime} register will wrap around if the count overflows. The {\tt mtime} register has a 64-bit precision on all RV32 and RV64 systems. Platforms provide a 64-bit memory-mapped machine-mode timer compare register ({\tt mtimecmp}). A machine timer interrupt becomes pending whenever {\tt mtime} contains a value greater than or equal to {\tt mtimecmp}, treating the values as unsigned integers. The interrupt remains posted until {\tt mtimecmp} becomes greater than {\tt mtime} (typically as a result of writing {\tt mtimecmp}). The interrupt will only be taken if interrupts are enabled and the MTIE bit is set in the {\tt mie} register. \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{63}{0} \\ \hline \multicolumn{1}{|c|}{\tt mtime} \\ \hline 64 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine time register (memory-mapped control register).} \end{figure} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{63}{0} \\ \hline \multicolumn{1}{|c|}{\tt mtimecmp} \\ \hline 64 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{Machine time compare register (memory-mapped control register).} \end{figure} \begin{commentary} The timer facility is defined to use wall-clock time rather than a cycle counter to support modern processors that run with a highly variable clock frequency to save energy through dynamic voltage and frequency scaling. Accurate real-time clocks (RTCs) are relatively expensive to provide (requiring a crystal or MEMS oscillator) and have to run even when the rest of system is powered down, and so there is usually only one in a system located in a different frequency/voltage domain from the processors. Hence, the RTC must be shared by all the harts in a system and accesses to the RTC will potentially incur the penalty of a voltage-level-shifter and clock-domain crossing. It is thus more natural to expose {\tt mtime} as a memory-mapped register than as a CSR. Lower privilege levels do not have their own {\tt timecmp} registers. Instead, machine-mode software can implement any number of virtual timers on a hart by multiplexing the next timer interrupt into the {\tt mtimecmp} register. Simple fixed-frequency systems can use a single clock for both cycle counting and wall-clock time. \end{commentary} Writes to {\tt mtime} and {\tt mtimecmp} are guaranteed to be reflected in MTIP eventually, but not necessarily immediately. \begin{commentary} A spurious timer interrupt might occur if an interrupt handler increments {\tt mtimecmp} then immediately returns, because MTIP might not yet have fallen in the interim. All software should be written to assume this event is possible, but most software should assume this event is extremely unlikely. It is almost always more performant to incur an occasional spurious timer interrupt than to poll MTIP until it falls. \end{commentary} In RV32, memory-mapped writes to {\tt mtimecmp} modify only one 32-bit part of the register. The following code sequence sets a 64-bit {\tt mtimecmp} value without spuriously generating a timer interrupt due to the intermediate value of the comparand: \begin{figure}[h!] \begin{center} \begin{verbatim} # New comparand is in a1:a0. li t0, -1 la t1, mtimecmp sw t0, 0(t1) # No smaller than old value. sw a1, 4(t1) # No smaller than new value. sw a0, 0(t1) # New value. \end{verbatim} \end{center} \caption{Sample code for setting the 64-bit time comparand in RV32, assuming a little-endian memory system and that the registers live in a strongly ordered I/O region. Storing -1 to the low-order bits of {\tt mtimecmp} prevents {\tt mtimecmp} from temporarily becoming smaller than the lesser of the old and new values.} \label{mtimecmph} \end{figure} For RV64, naturally aligned 64-bit memory accesses to the {\tt mtime} and {\tt mtimecmp} registers are additionally supported and are atomic. \section{Machine-Mode Privileged Instructions} \subsection{Environment Call and Breakpoint} \vspace{-0.2in} \begin{center} \begin{tabular}{M@{}R@{}F@{}R@{}S} \\ \instbitrange{31}{20} & \instbitrange{19}{15} & \instbitrange{14}{12} & \instbitrange{11}{7} & \instbitrange{6}{0} \\ \hline \multicolumn{1}{|c|}{funct12} & \multicolumn{1}{c|}{rs1} & \multicolumn{1}{c|}{funct3} & \multicolumn{1}{c|}{rd} & \multicolumn{1}{c|}{opcode} \\ \hline 12 & 5 & 3 & 5 & 7 \\ ECALL & 0 & PRIV & 0 & SYSTEM \\ EBREAK & 0 & PRIV & 0 & SYSTEM \\ \end{tabular} \end{center} The ECALL instruction is used to make a request to the supporting execution environment. When executed in U-mode, S-mode, or M-mode, it generates an environment-call-from-U-mode exception, environment-call-from-S-mode exception, or environment-call-from-M-mode exception, respectively, and performs no other operation. \begin{commentary} ECALL generates a different exception for each originating privilege mode so that environment call exceptions can be selectively delegated. A typical use case for Unix-like operating systems is to delegate to S-mode the environment-call-from-U-mode exception but not the others. \end{commentary} The EBREAK instruction is used by debuggers to cause control to be transferred back to a debugging environment. It generates a breakpoint exception and performs no other operation. \begin{commentary} As described in the ``C'' Standard Extension for Compressed Instructions in Volume I of this manual, the C.EBREAK instruction performs the same operation as the EBREAK instruction. \end{commentary} ECALL and EBREAK cause the receiving privilege mode's {\tt epc} register to be set to the address of the ECALL or EBREAK instruction itself, {\em not} the address of the following instruction. As ECALL and EBREAK cause synchronous exceptions, they are not considered to retire, and should not increment the {\tt minstret} CSR. \subsection{Trap-Return Instructions} \label{otherpriv} Instructions to return from trap are encoded under the PRIV minor opcode. \vspace{-0.2in} \begin{center} \begin{tabular}{M@{}R@{}F@{}R@{}S} \\ \instbitrange{31}{20} & \instbitrange{19}{15} & \instbitrange{14}{12} & \instbitrange{11}{7} & \instbitrange{6}{0} \\ \hline \multicolumn{1}{|c|}{funct12} & \multicolumn{1}{c|}{rs1} & \multicolumn{1}{c|}{funct3} & \multicolumn{1}{c|}{rd} & \multicolumn{1}{c|}{opcode} \\ \hline 12 & 5 & 3 & 5 & 7 \\ MRET/SRET & 0 & PRIV & 0 & SYSTEM \\ \end{tabular} \end{center} To return after handling a trap, there are separate trap return instructions per privilege level, MRET and SRET. MRET is always provided. SRET must be provided if supervisor mode is supported, and should raise an illegal instruction exception otherwise. SRET should also raise an illegal instruction exception when TSR=1 in {\tt mstatus}, as described in Section~\ref{virt-control}. An {\em x}\/RET instruction can be executed in privilege mode {\em x} or higher, where executing a lower-privilege {\em x}\/RET instruction will pop the relevant lower-privilege interrupt enable and privilege mode stack. In addition to manipulating the privilege stack as described in Section~\ref{privstack}, {\em x}\/RET sets the {\tt pc} to the value stored in the {\em x}\/{\tt epc} register. If the A extension is supported, the {\em x}\/RET instruction is allowed to clear any outstanding LR address reservation but is not required to. Trap handlers should explicitly clear the reservation if required (e.g., by using a dummy SC) before executing the {\em x}\/RET. \begin{commentary} If {\em x}\/RET instructions always cleared LR reservations, it would be impossible to single-step through LR/SC sequences using a debugger. \end{commentary} \subsection{Wait for Interrupt} \label{wfi} The Wait for Interrupt instruction (WFI) provides a hint to the implementation that the current hart can be stalled until an interrupt might need servicing. Execution of the WFI instruction can also be used to inform the hardware platform that suitable interrupts should preferentially be routed to this hart. WFI is available in all privileged modes, and optionally available to U-mode. This instruction may raise an illegal instruction exception when TW=1 in {\tt mstatus}, as described in Section~\ref{virt-control}. \vspace{-0.2in} \begin{center} \begin{tabular}{M@{}R@{}F@{}R@{}S} \\ \instbitrange{31}{20} & \instbitrange{19}{15} & \instbitrange{14}{12} & \instbitrange{11}{7} & \instbitrange{6}{0} \\ \hline \multicolumn{1}{|c|}{funct12} & \multicolumn{1}{c|}{rs1} & \multicolumn{1}{c|}{funct3} & \multicolumn{1}{c|}{rd} & \multicolumn{1}{c|}{opcode} \\ \hline 12 & 5 & 3 & 5 & 7 \\ WFI & 0 & PRIV & 0 & SYSTEM \\ \end{tabular} \end{center} If an enabled interrupt is present or later becomes present while the hart is stalled, the interrupt exception will be taken on the following instruction, i.e., execution resumes in the trap handler and {\tt mepc} = {\tt pc} + 4. \begin{commentary} The following instruction takes the interrupt exception and trap, so that a simple return from the trap handler will execute code after the WFI instruction. \end{commentary} The purpose of the WFI instruction is to provide a hint to the implementation, and so a legal implementation is to simply implement WFI as a NOP. \begin{commentary} If the implementation does not stall the hart on execution of the instruction, then the interrupt will be taken on some instruction in the idle loop containing the WFI, and on a simple return from the handler, the idle loop will resume execution. \end{commentary} The WFI instruction can also be executed when interrupts are disabled. The operation of WFI must be unaffected by the global interrupt bits in {\tt mstatus} (MIE and SIE) and the delegation register {\tt mideleg} (i.e., the hart must resume if a locally enabled interrupt becomes pending, even if it has been delegated to a less-privileged mode), but should honor the individual interrupt enables (e.g, MTIE) (i.e., implementations should avoid resuming the hart if the interrupt is pending but not individually enabled). WFI is also required to resume execution for locally enabled interrupts pending at any privilege level, regardless of the global interrupt enable at each privilege level. If the event that causes the hart to resume execution does not cause an interrupt to be taken, execution will resume at {\tt pc} + 4, and software must determine what action to take, including looping back to repeat the WFI if there was no actionable event. \begin{commentary} By allowing wakeup when interrupts are disabled, an alternate entry point to an interrupt handler can be called that does not require saving the current context, as the current context can be saved or discarded before the WFI is executed. As implementations are free to implement WFI as a NOP, software must explicitly check for any relevant pending but disabled interrupts in the code following an WFI, and should loop back to the WFI if no suitable interrupt was detected. The {\tt mip} or {\tt sip} registers can be interrogated to determine the presence of any interrupt in machine or supervisor mode respectively. The operation of WFI is unaffected by the delegation register settings. WFI is defined so that an implementation can trap into a higher privilege mode, either immediately on encountering the WFI or after some interval to initiate a machine-mode transition to a lower power state, for example. \end{commentary} \begin{commentary} The same ``wait-for-event'' template might be used for possible future extensions that wait on memory locations changing, or message arrival. \end{commentary} \subsection{Custom SYSTEM Instructions} \label{sec:customsys} The subspace of the SYSTEM major opcode shown in Figure~\ref{fig:customsys} is designated for custom use. It is recommended that these instructions use bits 29:28 to designate the minimum required privilege mode, as do other SYSTEM instructions. \begin{figure}[h!] \begin{center} \begin{tabular}{Y@{}S@{}F@{}Y@{}Rc} \\ \instbitrange{31}{26} & \instbitrange{25}{15} & \instbitrange{14}{12} & \instbitrange{11}{7} & \instbitrange{6}{0} \\ \cline{1-5} \multicolumn{1}{|c|}{funct6} & \multicolumn{1}{c|}{\em custom} & \multicolumn{1}{c|}{funct3} & \multicolumn{1}{c|}{\em custom} & \multicolumn{1}{c|}{opcode} & Recommended Purpose \\ \cline{1-5} 6 & 11 & 3 & 5 & 7 \\ 100011 & {\em custom} & 0 & {\em custom} & SYSTEM & Unprivileged or User-Level \\ 110011 & {\em custom} & 0 & {\em custom} & SYSTEM & Unprivileged or User-Level \\ 100111 & {\em custom} & 0 & {\em custom} & SYSTEM & Supervisor-Level \\ 110111 & {\em custom} & 0 & {\em custom} & SYSTEM & Supervisor-Level \\ 101011 & {\em custom} & 0 & {\em custom} & SYSTEM & Hypervisor-Level \\ 111011 & {\em custom} & 0 & {\em custom} & SYSTEM & Hypervisor-Level \\ 101111 & {\em custom} & 0 & {\em custom} & SYSTEM & Machine-Level \\ 111111 & {\em custom} & 0 & {\em custom} & SYSTEM & Machine-Level \\ \end{tabular} \end{center} \caption{SYSTEM instruction encodings designated for custom use.} \label{fig:customsys} \end{figure} \section{Reset} \label{sec:reset} Upon reset, a hart's privilege mode is set to M. The {\tt mstatus} fields MIE and MPRV are reset to 0. If little-endian memory accesses are supported, the {\tt mstatus}/{\tt mstatush} field MBE is reset to 0. The {\tt misa} register is reset to enable the maximal set of supported extensions and widest MXLEN, as described in Section~\ref{sec:misa}. The {\tt pc} is set to an implementation-defined reset vector. The {\tt mcause} register is set to a value indicating the cause of the reset. Writable PMP registers' A and L fields are set to 0, unless the platform mandates a different reset value for some PMP registers' A and L fields. No \warl\ field contains an illegal value. All other hart state is \unspecified. The {\tt mcause} values after reset have implementation-specific interpretation, but the value 0 should be returned on implementations that do not distinguish different reset conditions. Implementations that distinguish different reset conditions should only use 0 to indicate the most complete reset. \begin{commentary} Some designs may have multiple causes of reset (e.g., power-on reset, external hard reset, brownout detected, watchdog timer elapse, sleep-mode wakeup), which machine-mode software and debuggers may wish to distinguish. {\tt mcause} reset values may alias {\tt mcause} values following synchronous exceptions. There should be no ambiguity in this overlap, since on reset the {\tt pc} is typically set to a different value than on other traps. \end{commentary} \section{Non-Maskable Interrupts} \label{sec:nmi} Non-maskable interrupts (NMIs) are only used for hardware error conditions, and cause an immediate jump to an implementation-defined NMI vector running in M-mode regardless of the state of a hart's interrupt enable bits. The {\tt mepc} register is written with the virtual address of the instruction that was interrupted, and {\tt mcause} is set to a value indicating the source of the NMI. The NMI can thus overwrite state in an active machine-mode interrupt handler. The values written to {\tt mcause} on an NMI are implementation-defined. The high Interrupt bit of {\tt mcause} should be set to indicate that this was an interrupt. An Exception Code of 0 is reserved to mean ``unknown cause'' and implementations that do not distinguish sources of NMIs via the {\tt mcause} register should return 0 in the Exception Code. Unlike resets, NMIs do not reset processor state, enabling diagnosis, reporting, and possible containment of the hardware error. \section{Physical Memory Attributes} \label{sec:pma} The physical memory map for a complete system includes various address ranges, some corresponding to memory regions, some to memory-mapped control registers, and some to vacant holes in the address space. Some memory regions might not support reads, writes, or execution; some might not support subword or subblock accesses; some might not support atomic operations; and some might not support cache coherence or might have different memory models. Similarly, memory-mapped control registers vary in their supported access widths, support for atomic operations, and whether read and write accesses have associated side effects. In RISC-V systems, these properties and capabilities of each region of the machine's physical address space are termed {\em physical memory attributes} (PMAs). This section describes RISC-V PMA terminology and how RISC-V systems implement and check PMAs. PMAs are inherent properties of the underlying hardware and rarely change during system operation. Unlike physical memory protection values described in Section~\ref{sec:pmp}, PMAs do not vary by execution context. The PMAs of some memory regions are fixed at chip design time---for example, for an on-chip ROM. Others are fixed at board design time, depending, for example, on which other chips are connected to off-chip buses. Off-chip buses might also support devices that could be changed on every power cycle (cold pluggable) or dynamically while the system is running (hot pluggable). Some devices might be configurable at run time to support different uses that imply different PMAs---for example, an on-chip scratchpad RAM might be cached privately by one core in one end-application, or accessed as a shared non-cached memory in another end-application. Most systems will require that at least some PMAs are dynamically checked in hardware later in the execution pipeline after the physical address is known, as some operations will not be supported at all physical memory addresses, and some operations require knowing the current setting of a configurable PMA attribute. While many other architectures specify some PMAs in the virtual memory page tables and use the TLB to inform the pipeline of these properties, this approach injects platform-specific information into a virtualized layer and can cause system errors unless attributes are correctly initialized in each page-table entry for each physical memory region. In addition, the available page sizes might not be optimal for specifying attributes in the physical memory space, leading to address-space fragmentation and inefficient use of expensive TLB entries. For RISC-V, we separate out specification and checking of PMAs into a separate hardware structure, the {\em PMA checker}. In many cases, the attributes are known at system design time for each physical address region, and can be hardwired into the PMA checker. Where the attributes are run-time configurable, platform-specific memory-mapped control registers can be provided to specify these attributes at a granularity appropriate to each region on the platform (e.g., for an on-chip SRAM that can be flexibly divided between cacheable and uncacheable uses). PMAs are checked for any access to physical memory, including accesses that have undergone virtual to physical memory translation. To aid in system debugging, we strongly recommend that, where possible, RISC-V processors precisely trap physical memory accesses that fail PMA checks. Precisely trapped PMA violations manifest as instruction, load, or store access-fault exceptions, distinct from virtual-memory page-fault exceptions. Precise PMA traps might not always be possible, for example, when probing a legacy bus architecture that uses access failures as part of the discovery mechanism. In this case, error responses from slave devices will be reported as imprecise bus-error interrupts. PMAs must also be readable by software to correctly access certain devices or to correctly configure other hardware components that access memory, such as DMA engines. As PMAs are tightly tied to a given physical platform's organization, many details are inherently platform-specific, as is the means by which software can learn the PMA values for a platform. Some devices, particularly legacy buses, do not support discovery of PMAs and so will give error responses or time out if an unsupported access is attempted. Typically, platform-specific machine-mode code will extract PMAs and ultimately present this information to higher-level less-privileged software using some standard representation. Where platforms support dynamic reconfiguration of PMAs, an interface will be provided to set the attributes by passing requests to a machine-mode driver that can correctly reconfigure the platform. For example, switching cacheability attributes on some memory regions might involve platform-specific operations, such as cache flushes, that are available only to machine-mode. \subsection{Main Memory versus I/O versus Vacant Regions} The most important characterization of a given memory address range is whether it holds regular main memory, or I/O devices, or is vacant. Regular main memory is required to have a number of properties, specified below, whereas I/O devices can have a much broader range of attributes. Memory regions that do not fit into regular main memory, for example, device scratchpad RAMs, are categorized as I/O regions. Vacant regions are also classified as I/O regions but with attributes specifying that no accesses are supported. \subsection{Supported Access Type PMAs} Access types specify which access widths, from 8-bit byte to long multi-word burst, are supported, and also whether misaligned accesses are supported for each access width. \begin{commentary} Although software running on a RISC-V hart cannot directly generate bursts to memory, software might have to program DMA engines to access I/O devices and might therefore need to know which access sizes are supported. \end{commentary} Main memory regions always support read and write of all access widths required by the attached devices, and can specify whether instruction fetch is supported. \begin{commentary} Some platforms might mandate that all of main memory support instruction fetch. Other platforms might prohibit instruction fetch from some main memory regions. \end{commentary} \begin{commentary} In some cases, the design of a processor or device accessing main memory might support other widths, but must be able to function with the types supported by the main memory. \end{commentary} I/O regions can specify which combinations of read, write, or execute accesses to which data widths are supported. For systems with page-based virtual memory, I/O and memory regions can specify which combinations of hardware page-table reads and hardware page-table writes are supported. \begin{commentary} Unix-like operating systems generally require that all of cacheable main memory supports page-table walks. \end{commentary} \subsection{Atomicity PMAs} Atomicity PMAs describes which atomic instructions are supported in this address region. Support for atomic instructions is divided into two categories: {\em LR/SC} and {\em AMOs}. \begin{commentary} Some platforms might mandate that all of cacheable main memory support all atomic operations required by the attached processors. \end{commentary} \subsubsection{AMO PMA} Within AMOs, there are four levels of support: {\em AMONone}, {\em AMOSwap}, {\em AMOLogical}, and {\em AMOArithmetic}. AMONone indicates that no AMO operations are supported. AMOSwap indicates that only {\tt amoswap} instructions are supported in this address range. AMOLogical indicates that swap instructions plus all the logical AMOs ({\tt amoand}, {\tt amoor}, {\tt amoxor}) are supported. AMOArithmetic indicates that all RISC-V AMOs are supported. For each level of support, naturally aligned AMOs of a given width are supported if the underlying memory region supports reads and writes of that width. Main memory and I/O regions may only support a subset or none of the processor-supported atomic operations. \begin{table*}[h!] \begin{center} \begin{tabular}{|l|l|} \hline AMO Class & Supported Operations \\ \hline AMONone & {\em None} \\ AMOSwap & {\tt amoswap} \\ AMOLogical & above + {\tt amoand}, {\tt amoor}, {\tt amoxor} \\ AMOArithmetic & above + {\tt amoadd}, {\tt amomin}, {\tt amomax}, {\tt amominu}, {\tt amomaxu} \\ \hline \end{tabular} \end{center} \caption{Classes of AMOs supported by I/O regions.} \label{amoclasses} \end{table*} \begin{commentary} We recommend providing at least AMOLogical support for I/O regions where possible. \end{commentary} \subsubsection{Reservability PMA} For {\em LR/SC}, there are three levels of support indicating combinations of the reservability and eventuality properties: {\em RsrvNone}, {\em RsrvNonEventual}, and {\em RsrvEventual}. RsrvNone indicates that no LR/SC operations are supported (the location is non-reservable). RsrvNonEventual indicates that the operations are supported (the location is reservable), but without the eventual success guarantee described in the unprivileged ISA specification. RsrvEventual indicates that the operations are supported and provide the eventual success guarantee. \begin{commentary} We recommend providing RsrvEventual support for main memory regions where possible. Most I/O regions will not support LR/SC accesses, as these are most conveniently built on top of a cache-coherence scheme, but some may support RsrvNonEventual or RsrvEventual. \end{commentary} \begin{commentary} When LR/SC is used for memory locations marked RsrvNonEventual, software should provide alternative fall-back mechanisms used when lack of progress is detected. \end{commentary} \subsubsection{Alignment} Memory regions that support aligned LR/SC or aligned AMOs might also support misaligned LR/SC or misaligned AMOs for some addresses and access widths. If, for a given address and access width, a misaligned LR/SC or AMO generates an address-misaligned exception, then {\em all} loads, stores, LRs/SCs, and AMOs using that address and access width must generate address-misaligned exceptions. \begin{commentary} The standard ``A'' extension does not support misaligned AMOs or LR/SC pairs. Support for misaligned AMOs is provided by the standard ``Zam'' extension. Support for misaligned LR/SC sequences is not currently standardized, so LR and SC to misaligned addresses must raise an exception. Mandating that misaligned loads and stores raise address-misaligned exceptions wherever misaligned AMOs raise address-misaligned exceptions permits the emulation of misaligned AMOs in an M-mode trap handler. The handler guarantees atomicity by acquiring a global mutex and emulating the access within the critical section. Provided that the handler for misaligned loads and stores uses the same mutex, all accesses to a given address that use the same word size will be mutually atomic. \end{commentary} Implementations may raise access-fault exceptions instead of address-misaligned exceptions for some misaligned accesses, indicating the instruction should not be emulated by a trap handler. If, for a given address and access width, all misaligned LRs/SCs and AMOs generate access-fault exceptions, then regular misaligned loads and stores using the same address and access width are not required to execute atomically. \subsection{Memory-Ordering PMAs} Regions of the address space are classified as either {\em main memory} or {\em I/O} for the purposes of ordering by the FENCE instruction and atomic-instruction ordering bits. Accesses by one hart to main memory regions are observable not only by other harts but also by other devices with the capability to initiate requests in the main memory system (e.g., DMA engines). Coherent main memory regions always have either the RVWMO or RVTSO memory model. Incoherent main memory regions have an implementation-defined memory model. Accesses by one hart to an I/O region are observable not only by other harts and bus mastering devices but also by targeted slave I/O devices, and I/O regions may be accessed with either {\em relaxed} or {\em strong} ordering. Accesses to an I/O region with relaxed ordering are generally observed by other harts and bus mastering devices in a manner similar to the ordering of accesses to an RVWMO memory region, as discussed in Section A.4.2 in Volume I of this specification. By contrast, accesses to an I/O region with strong ordering are generally observed by other harts and bus mastering devices in program order. Each strongly ordered I/O region specifies a numbered ordering channel, which is a mechanism by which ordering guarantees can be provided between different I/O regions. Channel 0 is used to indicate point-to-point strong ordering only, where only accesses by the hart to the single associated I/O region are strongly ordered. Channel 1 is used to provide global strong ordering across all I/O regions. Any accesses by a hart to any I/O region associated with channel 1 can only be observed to have occurred in program order by all other harts and I/O devices, including relative to accesses made by that hart to relaxed I/O regions or strongly ordered I/O regions with different channel numbers. In other words, any access to a region in channel 1 is equivalent to executing a {\tt fence io,io} instruction before and after the instruction. Other larger channel numbers provide program ordering to accesses by that hart across any regions with the same channel number. Systems might support dynamic configuration of ordering properties on each memory region. \begin{commentary} Strong ordering can be used to improve compatibility with legacy device driver code, or to enable increased performance compared to insertion of explicit ordering instructions when the implementation is known to not reorder accesses. Local strong ordering (channel 0) is the default form of strong ordering as it is often straightforward to provide if there is only a single in-order communication path between the hart and the I/O device. Generally, different strongly ordered I/O regions can share the same ordering channel without additional ordering hardware if they share the same interconnect path and the path does not reorder requests. \end{commentary} \subsection{Coherence and Cacheability PMAs} Coherence is a property defined for a single physical address, and indicates that writes to that address by one agent will eventually be made visible to other agents in the system. Coherence is not to be confused with the memory consistency model of a system, which defines what values a memory read can return given the previous history of reads and writes to the entire memory system. In RISC-V platforms, the use of hardware-incoherent regions is discouraged due to software complexity, performance, and energy impacts. The cacheability of a memory region should not affect the software view of the region except for differences reflected in other PMAs, such as main memory versus I/O classification, memory ordering, supported accesses and atomic operations, and coherence. For this reason, we treat cacheability as a platform-level setting managed by machine-mode software only. Where a platform supports configurable cacheability settings for a memory region, a platform-specific machine-mode routine will change the settings and flush caches if necessary, so the system is only incoherent during the transition between cacheability settings. This transitory state should not be visible to lower privilege levels. \begin{commentary} We categorize RISC-V caches into three types: {\em master-private}, {\em shared}, and {\em slave-private}. Master-private caches are attached to a single master agent, i.e., one that issues read/write requests to the memory system. Shared caches are located between masters and slaves and may be hierarchically organized. Slave-private caches do not impact coherence, as they are local to a single slave and do not affect other PMAs at a master, so are not considered further here. We use {\em private cache} to mean a master-private cache in the following section, unless explicitly stated otherwise. Coherence is straightforward to provide for a shared memory region that is not cached by any agent. The PMA for such a region would simply indicate it should not be cached in a private or shared cache. Coherence is also straightforward for read-only regions, which can be safely cached by multiple agents without requiring a cache-coherence scheme. The PMA for this region would indicate that it can be cached, but that writes are not supported. Some read-write regions might only be accessed by a single agent, in which case they can be cached privately by that agent without requiring a coherence scheme. The PMA for such regions would indicate they can be cached. The data can also be cached in a shared cache, as other agents should not access the region. If an agent can cache a read-write region that is accessible by other agents, whether caching or non-caching, a cache-coherence scheme is required to avoid use of stale values. In regions lacking hardware cache coherence (hardware-incoherent regions), cache coherence can be implemented entirely in software, but software coherence schemes are notoriously difficult to implement correctly and often have severe performance impacts due to the need for conservative software-directed cache-flushing. Hardware cache-coherence schemes require more complex hardware and can impact performance due to the cache-coherence probes, but are otherwise invisible to software. For each hardware cache-coherent region, the PMA would indicate that the region is coherent and which hardware coherence controller to use if the system has multiple coherence controllers. For some systems, the coherence controller might be an outer-level shared cache, which might itself access further outer-level cache-coherence controllers hierarchically. Most memory regions within a platform will be coherent to software, because they will be fixed as either uncached, read-only, hardware cache-coherent, or only accessed by one agent. \end{commentary} If a PMA indicates non-cacheability, then accesses to that region must be satisfied by the memory itself, not by any caches. \begin{commentary} For implementations with a cacheability-control mechanism, the situation may arise that a program uncacheably accesses a memory location that is currently cache-resident. In this situation, the cached copy must be ignored. This constraint is necessary to prevent more-privileged modes' speculative cache refills from affecting the behavior of less-privileged modes' uncacheable accesses. \end{commentary} \subsection{Idempotency PMAs} Idempotency PMAs describe whether reads and writes to an address region are idempotent. Main memory regions are assumed to be idempotent. For I/O regions, idempotency on reads and writes can be specified separately (e.g., reads are idempotent but writes are not). If accesses are non-idempotent, i.e., there is potentially a side effect on any read or write access, then speculative or redundant accesses must be avoided. For the purposes of defining the idempotency PMAs, changes in observed memory ordering created by redundant accesses are not considered a side effect. \begin{commentary} While hardware should always be designed to avoid speculative or redundant accesses to memory regions marked as non-idempotent, it is also necessary to ensure software or compiler optimizations do not generate spurious accesses to non-idempotent memory regions. \end{commentary} \begin{commentary} Non-idempotent regions might not support misaligned accesses. Misaligned accesses to such regions should raise access-fault exceptions rather than address-misaligned exceptions, indicating that software should not emulate the misaligned access using multiple smaller accesses, which could cause unexpected side effects. \end{commentary} For non-idempotent regions, implicit reads and writes must not be performed early or speculatively, with the following exceptions. When a non-speculative implicit read is performed, an implementation is permitted to additionally read any of the bytes within a naturally aligned power-of-2 region containing the address of the non-speculative implicit read. Furthermore, when a non-speculative instruction fetch is performed, an implementation is permitted to additionally read any of the bytes within the {\em next} naturally aligned power-of-2 region of the same size (with the address of the region taken modulo $2^{\text{XLEN}}$). The results of these additional reads may be used to satisfy subsequent early or speculative implicit reads. The size of these naturally aligned power-of-2 regions is implementation-defined, but, for systems with page-based virtual memory, must not exceed the smallest supported page size. \section{Physical Memory Protection} \label{sec:pmp} To support secure processing and contain faults, it is desirable to limit the physical addresses accessible by software running on a hart. An optional physical memory protection (PMP) unit provides per-hart machine-mode control registers to allow physical memory access privileges (read, write, execute) to be specified for each physical memory region. The PMP values are checked in parallel with the PMA checks described in Section~\ref{sec:pma}. The granularity of PMP access control settings are platform-specific, but the standard PMP encoding supports regions as small as four bytes. Certain regions' privileges can be hardwired---for example, some regions might only ever be visible in machine mode but in no lower-privilege layers. \begin{commentary} Platforms vary widely in demands for physical memory protection, and some platforms may provide other PMP structures in addition to or instead of the scheme described in this section. \end{commentary} PMP checks are applied to all accesses whose effective privilege mode is S or U, including instruction fetches in S and U mode, data accesses in S and U mode when the MPRV bit in the {\tt mstatus} register is clear, and data accesses in any mode when the MPRV bit in {\tt mstatus} is set and the MPP field in {\tt mstatus} contains S or U. PMP checks are also applied to page-table accesses for virtual-address translation, for which the effective privilege mode is S. Optionally, PMP checks may additionally apply to M-mode accesses, in which case the PMP registers themselves are locked, so that even M-mode software cannot change them until the hart is reset. In effect, PMP can {\em grant} permissions to S and U modes, which by default have none, and can {\em revoke} permissions from M-mode, which by default has full permissions. PMP violations are always trapped precisely at the processor. \subsection{Physical Memory Protection CSRs} PMP entries are described by an 8-bit configuration register and one MXLEN-bit address register. Some PMP settings additionally use the address register associated with the preceding PMP entry. Up to 64 PMP entries are supported. Implementations may implement zero, 16, or 64 PMP CSRs. All PMP CSR fields are \warl\ and may be hardwired to zero. PMP CSRs are only accessible to M-mode. The PMP configuration registers are densely packed into CSRs to minimize context-switch time. For RV32, sixteen CSRs, {\tt pmpcfg0}--{\tt pmpcfg15}, hold the configurations {\tt pmp0cfg}--{\tt pmp63cfg} for the 64 PMP entries, as shown in Figure~\ref{pmpcfg-rv32}. For RV64, eight even-numbered CSRs, {\tt pmpcfg0}, {\tt pmpcfg2}, \ldots, {\tt pmpcfg14}, hold the configurations for the 64 PMP entries, as shown in Figure~\ref{pmpcfg-rv64}. For RV64, the odd-numbered configuration registers, {\tt pmpcfg1}, {\tt pmpcfg3}, \ldots, {\tt pmpcfg15}, are illegal. \begin{commentary} RV64 systems use {\tt pmpcfg2}, rather than {\tt pmpcfg1}, to hold configurations for PMP entries 8--15. This design reduces the cost of supporting multiple MXLEN values, since the configurations for PMP entries 8--11 appear in {\tt pmpcfg2}[31:0] for both RV32 and RV64. \end{commentary} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}Y@{}Y@{}Y@{}Yl} \instbitrange{31}{24} & \instbitrange{23}{16} & \instbitrange{15}{8} & \instbitrange{7}{0} & \\ \cline{1-4} \multicolumn{1}{|c|}{pmp3cfg} & \multicolumn{1}{c|}{pmp2cfg} & \multicolumn{1}{c|}{pmp1cfg} & \multicolumn{1}{c|}{pmp0cfg} & \tt pmpcfg0 \\ \cline{1-4} 8 & 8 & 8 & 8 & \\ \instbitrange{31}{24} & \instbitrange{23}{16} & \instbitrange{15}{8} & \instbitrange{7}{0} & \\ \cline{1-4} \multicolumn{1}{|c|}{pmp7cfg} & \multicolumn{1}{c|}{pmp6cfg} & \multicolumn{1}{c|}{pmp5cfg} & \multicolumn{1}{c|}{pmp4cfg} & \tt pmpcfg1 \\ \cline{1-4} 8 & 8 & 8 & 8 & \\ ~ \\ \multicolumn{4}{c}{\Huge\vdots} & \ \\ ~ \\ \instbitrange{31}{24} & \instbitrange{23}{16} & \instbitrange{15}{8} & \instbitrange{7}{0} & \\ \cline{1-4} \multicolumn{1}{|c|}{pmp63cfg} & \multicolumn{1}{c|}{pmp62cfg} & \multicolumn{1}{c|}{pmp61cfg} & \multicolumn{1}{c|}{pmp60cfg} & \tt pmpcfg15 \\ \cline{1-4} 8 & 8 & 8 & 8 & \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{RV32 PMP configuration CSR layout.} \label{pmpcfg-rv32} \end{figure} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}Y@{}Y@{}Y@{}Y@{}Y@{}Y@{}Y@{}Yl} \instbitrange{63}{56} & \instbitrange{55}{48} & \instbitrange{47}{40} & \instbitrange{39}{32} & \instbitrange{31}{24} & \instbitrange{23}{16} & \instbitrange{15}{8} & \instbitrange{7}{0} & \\ \cline{1-8} \multicolumn{1}{|c|}{pmp7cfg} & \multicolumn{1}{c|}{pmp6cfg} & \multicolumn{1}{c|}{pmp5cfg} & \multicolumn{1}{c|}{pmp4cfg} & \multicolumn{1}{c|}{pmp3cfg} & \multicolumn{1}{c|}{pmp2cfg} & \multicolumn{1}{c|}{pmp1cfg} & \multicolumn{1}{c|}{pmp0cfg} & \tt pmpcfg0 \\ \cline{1-8} 8 & 8 & 8 & 8 & 8 & 8 & 8 & 8 & \\ \instbitrange{63}{56} & \instbitrange{55}{48} & \instbitrange{47}{40} & \instbitrange{39}{32} & \instbitrange{31}{24} & \instbitrange{23}{16} & \instbitrange{15}{8} & \instbitrange{7}{0} & \\ \cline{1-8} \multicolumn{1}{|c|}{pmp15cfg} & \multicolumn{1}{c|}{pmp14cfg} & \multicolumn{1}{c|}{pmp13cfg} & \multicolumn{1}{c|}{pmp12cfg} & \multicolumn{1}{c|}{pmp11cfg} & \multicolumn{1}{c|}{pmp10cfg} & \multicolumn{1}{c|}{pmp9cfg} & \multicolumn{1}{c|}{pmp8cfg} & \tt pmpcfg2 \\ \cline{1-8} 8 & 8 & 8 & 8 & 8 & 8 & 8 & 8 & \\ ~ \\ \multicolumn{8}{c}{\Huge\vdots} & \ \\ ~ \\ \instbitrange{63}{56} & \instbitrange{55}{48} & \instbitrange{47}{40} & \instbitrange{39}{32} & \instbitrange{31}{24} & \instbitrange{23}{16} & \instbitrange{15}{8} & \instbitrange{7}{0} & \\ \cline{1-8} \multicolumn{1}{|c|}{pmp63cfg} & \multicolumn{1}{c|}{pmp62cfg} & \multicolumn{1}{c|}{pmp61cfg} & \multicolumn{1}{c|}{pmp60cfg} & \multicolumn{1}{c|}{pmp59cfg} & \multicolumn{1}{c|}{pmp58cfg} & \multicolumn{1}{c|}{pmp57cfg} & \multicolumn{1}{c|}{pmp56cfg} & \tt pmpcfg14 \\ \cline{1-8} 8 & 8 & 8 & 8 & 8 & 8 & 8 & 8 & \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{RV64 PMP configuration CSR layout.} \label{pmpcfg-rv64} \end{figure} The PMP address registers are CSRs named {\tt pmpaddr0}--{\tt pmpaddr63}. Each PMP address register encodes bits 33--2 of a 34-bit physical address for RV32, as shown in Figure~\ref{pmpaddr-rv32}. For RV64, each PMP address register encodes bits 55--2 of a 56-bit physical address, as shown in Figure~\ref{pmpaddr-rv64}. Not all physical address bits may be implemented, and so the {\tt pmpaddr} registers are \warl. \begin{commentary} The Sv32 page-based virtual-memory scheme described in Section~\ref{sec:sv32} supports 34-bit physical addresses for RV32, so the PMP scheme must support addresses wider than XLEN for RV32. The Sv39 and Sv48 page-based virtual-memory schemes described in Sections~\ref{sec:sv39} and~\ref{sec:sv48} support a 56-bit physical address space, so the RV64 PMP address registers impose the same limit. \end{commentary} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}J} \instbitrange{31}{0} \\ \hline \multicolumn{1}{|c|}{address[33:2] (\warl)} \\ \hline 32 \\ \end{tabular} \end{center} } \vspace{-0.1in} \caption{PMP address register format, RV32.} \label{pmpaddr-rv32} \end{figure} \begin{figure}[h!] {\footnotesize \begin{center} \begin{tabular}{@{}F@{}J} \instbitrange{63}{54} & \instbitrange{53}{0} \\ \hline \multicolumn{1}{|c|}{0 (\warl)} & \multicolumn{1}{c|}{address[55:2] (\warl)} \\ \hline 10 & 54 \\ \end{tabular} \end{center} } \vspace{-0.1in}


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