Sm Extension
Versions
- 1.11.0
-
- State
-
ratified
- Ratification date
-
2019-12
- Changes
-
-
Moved Machine spec to Ratified status.
-
Improvements to the description and commentary.
-
Specified which interrupt sources are reserved for standard use.
-
Allocated some synchronous exception causes for custom use.
-
Specified the priority ordering of synchronous exceptions.
-
Added specification that xRET instructions may, but are not required to, clear LR reservations if A extension present.
-
Made the mstatus.MPP field WARL, rather than WLRL.
-
Made the unused
xipfields WPRI, rather than WIRI. -
Made the unused misa fields WARL, rather than WIRI.
-
Rectified an editing error that misdescribed the mechanism by which mstatus is written upon an exception.
-
Described scheme for emulating misaligned AMOs.
-
Specified the behavior of the misa and
xepcregisters in systems with variable IALIGN. -
Specified the behavior of writing self-contradictory values to the misa register.
-
Specified contents of CSRs across XLEN modification.
-
Moved PLIC chapter into its own document.
-
- 1.12.0
-
- State
-
ratified
- Ratification date
-
2021-12
- Changes
-
-
Changed MRET to clear mstatus.MPRV when leaving M-mode.
-
Relaxed I/O regions have been specified to follow RVWMO. The previous specification implied that PPO rules other than fences and acquire/release annotations did not apply.
-
Constrained the LR/SC reservation set size and shape when using page-based virtual memory.
-
PMP changes require an SFENCE.VMA on any hart that implements page-based virtual memory, even if VM is not currently enabled.
-
Removed the N extension.
-
Defined the mandatory RV32-only CSR mstatush, which contains most of the same fields as the upper 32 bits of RV64’s mstatus.
-
Defined the mandatory CSR mconfigptr, which if nonzero contains the address of a configuration data structure.
-
Defined optional mseccfg and mseccfgh CSRs, which control the machine’s security configuration.
-
Defined menvcfg CSR (and RV32-only menvcfgh), which control various characteristics of the execution environment.
-
Designated part of SYSTEM major opcode for custom use.
-
Permitted the unconditional delegation of less-privileged interrupts.
-
Added optional big-endian and bi-endian support.
-
Made priority of load/store/AMO address-misaligned exceptions implementation-defined relative to load/store/AMO page-fault and access-fault exceptions.
-
Software breakpoint exceptions are permitted to write either 0 or the
pctoxtval. -
Specified relaxed constraints for implicit reads of non-idempotent regions.
-
- 1.13.0
-
- State
-
frozen
- Changes
-
-
Redefined misa.MXL to be read-only, making MXLEN a constant.
-
Defined the misa.B field to reflect that the B extension has been implemented.
-
Defined the misa.V field to reflect that the V extension has been implemented.
-
Defined the RV32-only medelegh CSR.
-
Defined the misaligned atomicity granule PMA, superseding the proposed Zam extension.
-
Defined hardware error and software check exception codes.
-
Specified synchronization requirements when changing the PBMTE fields in menvcfg and henvcfg.
-
Exposed count-overflow interrupts to VS-mode via the Shlcofideleg extension.
-
Relaxed behavior of some HINTs when MXLEN > XLEN.
-
Transliterated the document from LaTeX into AsciiDoc.
-
Included all ratified extensions through March 2024.
-
Clarified that "platform- or custom-use" interrupts are actually "platform-use interrupts", where the platform can choose to make some custom.
-
Clarified semantics of explicit accesses to CSRs wider than XLEN bits.
-
Clarified that MXLEN≥SXLEN.
-
Clarified that WFI is not a HINT instruction.
-
Clarified that, for a given exception cause,
xtvalmight sometimes be set to a nonzero value but sometimes not. -
Clarified exception behavior of unimplemented or inaccessible CSRs.
-
Replaced the concept of vacant memory regions with inaccessible memory or I/O regions.
-
Clarified that timer and count-overflow interrupts' arrival in interrupt-pending registers is not immediate.
-
Clarified that MXR affects only explicit memory accesses.
-
Synopsis
This chapter describes the machine-level operations available in machine-mode (M-mode), which is the highest privilege mode in a RISC-V hart. 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. This chapter describes the RISC-V machine-level architecture, which contains a common core that is used with various supervisor-level address translation and protection schemes.
Parameters
This extension has the following implementation options:
- ARCH_ID
-
Type
integer
Valid Values
64-bit integer
Description
Vendor-specific architecture ID in marchid
- CONFIG_PTR_ADDRESS
-
Type
integer
Valid Values
integer
Description
Physical address of the unified discovery configuration data structure. This address is reported in the mconfigptr CSR. TODO: GitHub issue 53
- IMP_ID
-
Type
integer
Valid Values
64-bit integer
Description
Vendor-specific implementation ID in mimpid
- MISALIGNED_LDST
-
Type
boolean
Valid Values
boolean
Description
Does the implementation perform non-atomic misaligned loads and stores to main memory (does not affect misaligned support to device memory)? If not, the implementation always throws a misaligned exception.
- MISALIGNED_LDST_EXCEPTION_PRIORITY
-
Type
string
Valid Values
[low, high]
Description
The relative priority of a load/store/AMO exception vs. load/store/AMO page-fault or access-fault exceptions.
May be one of:
low
Misaligned load/store/AMO exceptions are always lower priority than load/store/AMO page-fault and access-fault exceptions.
high
Misaligned load/store/AMO exceptions are always higher priority than load/store/AMO page-fault and access-fault exceptions.
MISALIGNED_LDST_EXCEPTION_PRIORITY cannot be "high" when MISALIGNED_MAX_ATOMICITY_GRANULE_SIZE is non-zero, since the atomicity of an access cannot be determined in that case until after address translation.
- MISALIGNED_MAX_ATOMICITY_GRANULE_SIZE
-
Type
integer
Valid Values
[0, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096]
Description
The maximum granule size, in bytes, that the hart can atomically perform a misaligned load/store/AMO without raising a Misaligned exception. When MISALIGNED_MAX_ATOMICITY_GRANULE_SIZE is 0, the hart cannot atomically perform a misaligned load/store/AMO. When a power of two, the hart can atomically load/store/AMO a misaligned access that is fully contained in a MISALIGNED_MAX_ATOMICITY_GRANULE_SIZE-aligned region.
Even if the hart is capable of performing a misaligned load/store/AMO atomically, a misaligned exception may still occur if the access does not have the appropriate Misaligned Atomicity Granule PMA set. - MISALIGNED_SPLIT_STRATEGY
-
Type
string
Valid Values
[by_byte, custom]
Description
When misaligned accesses are supported, this determines the order in the implementation appears to process the load/store, which determines how/which exceptions will be reported
Options:
-
by_byte: The load/store appears to be broken into byte-sized accesses that processed sequentially from smallest address to largest address
-
custom: Something else. Will result in a call to unpredictable() in the execution
-
- MISA_CSR_IMPLEMENTED
- MTVAL_WIDTH
-
Type
integer
Valid Values
≤ 64
Description
The number of implemented bits in the mtval CSR. This is the CSR that may be written when a trap is taken into M-mode with exception-specific information to assist software in handling the trap (e.g., address associated with exception).
Must be greater than or equal to max(
PHYS_ADDR_WIDTH,VA_SIZE) - MTVEC_BASE_ALIGNMENT_DIRECT
-
Type
integer
Valid Values
≥ 4
Description
Byte alignment for mtvec.BASE when mtvec.MODE is Direct.
Cannot be less than 4-byte alignment.
- MTVEC_BASE_ALIGNMENT_VECTORED
-
Type
integer
Valid Values
≥ 4
Description
Byte alignment for mtvec.BASE when mtvec.MODE is Vectored.
Cannot be less than 4-byte alignment.
- MTVEC_MODES
-
Type
array
Valid Values
1-element to 2-element array of [0, 1]
Description
Modes supported by mtvec.MODE. If only one, it is assumed to be read-only with that value.
- M_MODE_ENDIANESS
-
Type
string
Valid Values
[little, big, dynamic]
Description
Endianess of data in M-mode. Can be one of:
little
M-mode data is always little endian
big
M-mode data is always big endian
dynamic
M-mode data can be either little or big endian, depending on the CSR field mstatus.MBE
- PHYS_ADDR_WIDTH
-
Type
integer
Valid Values
≤ 64
Description
Number of bits in the physical address space.
- PMA_GRANULARITY
-
Type
integer
Valid Values
2 to 66
Description
log2 of the smallest supported PMA region.
Generally, for systems with an MMU, should not be smaller than 12, as that would preclude caching PMP results in the TLB along with virtual memory translations
- PRECISE_SYNCHRONOUS_EXCEPTIONS
-
Type
boolean
Valid Values
boolean
Description
Whether or not all synchronous exceptions are precise.
If false, any exception not otherwise mandated to precise (e.g., PMP violation) will cause execution to enter an unpredictable state.
- REPORT_ENCODING_IN_MTVAL_ON_ILLEGAL_INSTRUCTION
- REPORT_VA_IN_MTVAL_ON_BREAKPOINT
-
Type
boolean
Valid Values
boolean
Description
- REPORT_VA_IN_MTVAL_ON_INSTRUCTION_ACCESS_FAULT
- REPORT_VA_IN_MTVAL_ON_INSTRUCTION_MISALIGNED
-
Type
boolean
Valid Values
boolean
Description
When true, mtval is written with the virtual PC when an instruction fetch is misaligned.
When false, mtval is written with 0 when an instruction fetch is misaligned.
Note that when IALIGN=16 (i.e., when the C or one of the
Zc*extensions are implemented), it is impossible to generate a misaligned fetch, and so this parameter has no effect. - REPORT_VA_IN_MTVAL_ON_INSTRUCTION_PAGE_FAULT
- REPORT_VA_IN_MTVAL_ON_LOAD_ACCESS_FAULT
- REPORT_VA_IN_MTVAL_ON_LOAD_MISALIGNED
- REPORT_VA_IN_MTVAL_ON_LOAD_PAGE_FAULT
- REPORT_VA_IN_MTVAL_ON_STORE_AMO_ACCESS_FAULT
- REPORT_VA_IN_MTVAL_ON_STORE_AMO_MISALIGNED
- REPORT_VA_IN_MTVAL_ON_STORE_AMO_PAGE_FAULT
- TRAP_ON_EBREAK
-
Type
boolean
Valid Values
boolean
Description
Whether or not an EBREAK causes a synchronous exception.
The spec states that implementations may handle EBREAKs transparently without raising a trap, in which case the EEI must provide a builtin.
- TRAP_ON_ECALL_FROM_M
-
Type
boolean
Valid Values
boolean
Description
Whether or not an ECALL-from-M-mode causes a synchronous exception.
The spec states that implementations may handle ECALLs transparently without raising a trap, in which case the EEI must provide a builtin.
- TRAP_ON_ILLEGAL_WLRL
-
Type
boolean
Valid Values
boolean
Description
When true, writing an illegal value to a WLRL CSR field raises an
IllegalInstructionexception.When false, writing an illegal value to a WLRL CSR field is
unpredictable. - TRAP_ON_RESERVED_INSTRUCTION
-
Type
boolean
Valid Values
boolean
Description
When true, fetching an unimplemented and/or undefined instruction from the standard/reserved encoding space will cause an
IllegalInstructionexception.When false, fetching such an instruction is
UNPREDICTABLE. - TRAP_ON_UNIMPLEMENTED_CSR
- TRAP_ON_UNIMPLEMENTED_INSTRUCTION
-
Type
boolean
Valid Values
boolean
Description
When true, fetching an unimplemented instruction from the custom encoding space will cause an
IllegalInstructionexception.When false, fetching an unimplemented instruction is
UNPREDICTABLE. - VENDOR_ID_BANK
-
Type
integer
Valid Values
25-bit integer
Description
JEDEC Vendor ID bank, for mvendorid
- VENDOR_ID_OFFSET
-
Type
integer
Valid Values
7-bit integer
Description
Vendor JEDEC code offset, for mvendorid
- XLEN
-
Type
integer
Valid Values
[32, 64]
Description
XLEN in M-mode (AKA MXLEN, tracked by issue #52)