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A guide to reading the specification
------------------------------------
The model is written in the Sail language. Although specifications in
Sail are quite readable as pseudocode, it would help to have the [Sail
manual](https://github.com/rems-project/sail/blob/sail2/manual.pdf) handy.
The model contains the following Sail modules in the `model` directory:
- `prelude.sail` contains useful Sail library functions. This file
should be referred to as needed. The lowest level memory access
primitives are defined in this file, and are typically implemented
by the various Sail backends.
- `riscv_types.sail` contains some basic RISC-V definitions. This
file should be read first, since it provides basic definitions that
are pervasively used throughout the specification, such as privilege
levels, registers and register access, interrupt and exception
definitions and numbering, and types used to define memory accesses.
- `riscv_sys.sail` describes M-mode and S-mode CSRs, interrupt and
exception delegation and dispatch, and handling privilege
transitions. In addition, this file contains functions to handle
the reading and writing of CSRs with WPRI, WLRL and WARL fields;
more details are provided in comments in the file.
Since these functions are intended to capture platform-specific
functionality, future versions of the model might separate these
functions out into a separate platform-defined file. The current
implementation of these functions usually implement the same
behavior as the Spike emulator.
- `riscv_platform.sail` contains platform-specific functionality for
the model. It contains the physical memory map, the local interrupt
controller, and the MMIO interfaces to the clock, timer and terminal
devices. Sail `extern` definitions are used to connect externally
provided (i.e. external to the Sail model) platform functionality,
such as those provided by the platform support in the C and OCaml
emulators. This file also contains the externally selectable
options for platform behavior, such as the handling of misaligned
memory accesses, the handling of PTE dirty-bit updates during
address translation, etc. These platform options can be specified
via command line switches in the C and OCaml emulators.
- `riscv_mem.sail` contains the functions that convert accesses to
physical addresses into accesses to physical memory, or MMIO
accesses to the devices provided by the platform, or into the
appropriate access fault. This file also contains definitions that
are used in the weak memory concurrency model.
- `riscv_vmem.sail` describes the S-mode address translation. It
contains the definitions and processing of the page-table entries
and their various permission and status bits, the specification of
page-table walks, and the selection of the address translation mode.
- `riscv.sail` captures the instruction definitions and their assembly
language formats. Each instruction is represented as a variant
clause of the `ast` type, and its execution semantics are
represented as a clause of the `execute` function. `mapping` clauses
specify the encoding and decoding of each instruction to and from
assembly language formats.
- `riscv_step.sail` implements the top-level fetch and execute loop.
The `fetch` is done in 16-bit granules to handle RVC instructions.
The `step` function performs the instruction fetch, handles any
fetch errors, dispatches the execution of each instruction, and
checks for any pending interrupts that may need to be handled. A
`loop` function implements the execute loop, and uses the same HTIF
(host-target interface) mechanism as the Spike emulator to detect
termination of execution.
- `riscv_analysis.sail` is used in the formal operational RVWMO memory
model.
Note that the files above are listed in dependency order, i.e. files
earlier in the order do not depend on later files.
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