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\chapter{Preface}
This is {\bf a draft of} version 1.10 of the RISC-V privileged
architecture proposal. Changes from version 1.9.1 include:
\begin{itemize}
\parskip 0pt
\itemsep 1pt
\item The previous version of this document was released under a
Creative Commons Attribution 4.0 International Licence by the
original authors, and this and future versions of this document will
be released under the same licence.
\item The explicit convention on shadow CSR addresses has been removed
to reclaim CSR space. Shadow CSRs can still be added as needed.
\item The {\tt mvendorid} register now contains the JEDEC code of the
core provider as opposed to a code supplied by the Foundation. This
avoids redundancy and offloads work from the Foundation.
\item The interrupt-enable stack discipline has been simplified.
\item An optional mechanism to change the base ISA used by supervisor
and user modes has been added to the {\tt mstatus} CSR, and the
field previously called Base in {\tt misa} has been renamed to {\tt
MXL} for consistency.
\item Clarified expected use of XS to summarize additional extension
state status fields in {\tt mstatus}.
\item Optional vectored interrupt support has been added to the
{\tt mtvec} and {\tt stvec} CSRs.
\item The SEIP and UEIP bits in the {\tt mip} CSR have been redefined
to support software injection of external interrupts.
\item The {\tt mbadaddr} register has been subsumed by a more
general {\tt mtval} register that can now capture bad
instruction bits on an illegal instruction fault to speed
instruction emulation.
\item The machine-mode base-and-bounds translation and protection
schemes have been removed from the specification as part of moving
the virtual memory configuration to {\tt sptbr} (now {\tt satp}). Some of the
motivation for the base and bound schemes are now covered by the PMP
registers, but space remains available in {\tt mstatus} to add these
back at a later date if deemed useful.
\item In systems with only M-mode, or with both M-mode and U-mode but
without U-mode trap support, the {\tt medeleg} and {\tt mideleg}
registers now do not exist, whereas previously they returned zero.
\item Virtual-memory page faults now have {\tt mcause} values distinct from
physical-memory access exceptions. Page-fault exceptions can now be
delegated to S-mode without delegating exceptions generated by PMA
and PMP checks.
\item An optional physical-memory protection (PMP) scheme has been proposed.
\item The supervisor virtual memory configuration has been moved from the
{\tt mstatus} register to the {\tt sptbr} register. Accordingly, the
{\tt sptbr} register has been renamed to {\tt satp} (Supervisor Address
Translation and Protection) to reflect is broadened role.
\item The SFENCE.VM instruction has been removed in favor of the improved
SFENCE.VMA instruction.
\item The {\tt mstatus} bit MXR has been exposed to S-mode via {\tt sstatus}.
\item The polarity of the PUM bit in {\tt sstatus} has been inverted to
shorten code sequences involving MXR. The bit has been renamed to SUM.
\item Hardware management of page-table entry Accessed and Dirty bits has
been made optional; simpler implementations may trap to software to
set them.
\item The counter-enable scheme has changed, so that S-mode can
control availability of counters to U-mode.
\item H-mode has been removed, as we are focusing on recursive
virtualization support in S-mode. The encoding space has been
reserved and may be repurposed at a later date.
\item A mechanism to improve virtualization performance by
trapping S-mode virtual-memory management operations has been added.
\end{itemize}
\newpage
\section*{Preface to Version 1.9.1}
This is version 1.9.1 of the RISC-V privileged architecture
proposal. Changes from version 1.9 include:
\begin{itemize}
\parskip 0pt
\itemsep 1pt
\item Numerous additions and improvements to the commentary sections.
\item Change configuration string proposal to be use a search process
that supports various formats including Device Tree String and
flattened Device Tree.
\item Made {\tt misa} optionally writable to support modifying base
and supported ISA extensions. CSR address of {\tt misa} changed.
\item Added description of debug mode and debug CSRs.
\item Added a hardware performance monitoring scheme. Simplified the
handling of existing hardware counters, removing privileged versions
of the counters and the corresponding delta registers.
\item Fixed description of SPIE in presence of user-level interrupts.
\end{itemize}
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