Preface

The inclusion of all ratified extensions through March 2024.

Redefined misa.MXL to be read-only, making MXLEN a constant.

Added the constraint that SXLEN≥UXLEN.

Transliterated the document from LaTeX into AsciiDoc.

Defined the misa.V field to reflect that the V extension has been implemented.

Defined the RV32-only medelegh and hedelegh CSRs.

Defined the misaligned atomicity granule PMA, superseding the proposed Zam extension.

Allocated interrupt 13 for Sscofpmf LCOFI interrupt.

Defined hardware error and software check exception codes.

Specified synchronization requirements when changing the PBMTE fields in menvcfg and henvcfg.

Incorporated Svade and Svadu extension specifications.

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 VS-stage page-table accesses set G-stage A/D bits.

Clarified ordering rules when PBMT=IO is used on main-memory regions.

Clarified ordering rules for hardware A/D bit updates.

Clarified that, for a given exception cause, xtval might sometimes be set to a nonzero value but sometimes not.

Clarified exception behavior of unimplemented or inaccessible CSRs.

Clarified that Svpbmt allows implementations to override additional PMAs.

Exposed count-overflow interrups to VS-mode.

Changed MRET and SRET to clear mstatus.MPRV when leaving M-mode.

Reserved additional satp patterns for future use.

Stated that the scause Exception Code field must implement bits 4–0 at minimum.

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.

Allowed for speculative updates of page table entry A bits.

Clarify that if the address-translation algorithm non-speculatively reaches a PTE in which a bit reserved for future standard use is set, a page-fault exception must be raised.

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, henvcfg, and senvcfg CSRs (and RV32-only menvcfgh and henvcfgh CSRs), 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.

PMP reset values are now platform-defined.

An additional 48 optional PMP registers have been defined.

Slightly relaxed the atomicity requirement for A and D bit updates performed by the implementation.

Clarify the architectural behavior of address-translation caches

Added Sv57 and Sv57x4 address translation modes.

Software breakpoint exceptions are permitted to write either 0 or the pc to xtval.

Clarified that bare S-mode need not support the SFENCE.VMA instruction.

Specified relaxed constraints for implicit reads of non-idempotent regions.

Added the Svnapot Standard Extension, along with the N bit in Sv39, Sv48, and Sv57 PTEs.

Added the Svpbmt Standard Extension, along with the PBMT bits in Sv39, Sv48, and Sv57 PTEs.

Added the Svinval Standard Extension and associated instructions.

Moved Machine and Supervisor spec to Ratified status.

Improvements to the description and commentary.

Added a draft proposal for a hypervisor extension.

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.

The virtual-memory system no longer permits supervisor mode to execute instructions from user pages, regardless of the SUM setting.

Clarified that ASIDs are private to a hart, and added commentary about the possibility of a future global-ASID extension.

SFENCE.VMA semantics have been clarified.

Made the mstatus.MPP field WARL, rather than WLRL.

Made the unused xip fields WPRI, rather than WIRI.

Made the unused misa fields WARL, rather than WIRI.

Made the unused pmpaddr and pmpcfg fields WARL, rather than WIRI.

Required all harts in a system to employ the same PTE-update scheme as each other.

Rectified an editing error that misdescribed the mechanism by which mstatus.xIE is written upon an exception.

Described scheme for emulating misaligned AMOs.

Specified the behavior of the misa and xepc registers in systems with variable IALIGN.

Specified the behavior of writing self-contradictory values to the misa register.

Defined the mcountinhibit CSR, which stops performance counters from incrementing to reduce energy consumption.

Specified semantics for PMP regions coarser than four bytes.

Specified contents of CSRs across XLEN modification.

Moved PLIC chapter into its own document.

The previous version of this document was released under a Creative Commons Attribution 4.0 International License by the original authors, and this and future versions of this document will be released under the same license.

The explicit convention on shadow CSR addresses has been removed to reclaim CSR space. Shadow CSRs can still be added as needed.

The 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.

The interrupt-enable stack discipline has been simplified.

An optional mechanism to change the base ISA used by supervisor and user modes has been added to the mstatus CSR, and the field previously called Base in misa has been renamed to MXL for consistency.

Clarified expected use of XS to summarize additional extension state status fields in mstatus.

Optional vectored interrupt support has been added to the mtvec and stvec CSRs.

The SEIP and UEIP bits in the mip CSR have been redefined to support software injection of external interrupts.

The mbadaddr register has been subsumed by a more general mtval register that can now capture bad instruction bits on an illegal instruction fault to speed instruction emulation.

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 sptbr (now satp). Some of the motivation for the base and bound schemes are now covered by the PMP registers, but space remains available in mstatus to add these back at a later date if deemed useful.

In systems with only M-mode, or with both M-mode and U-mode but without U-mode trap support, the medeleg and mideleg registers now do not exist, whereas previously they returned zero.

Virtual-memory page faults now have mcause values distinct from physical-memory access faults. Page-fault exceptions can now be delegated to S-mode without delegating exceptions generated by PMA and PMP checks.

An optional physical-memory protection (PMP) scheme has been proposed.

The supervisor virtual memory configuration has been moved from the mstatus register to the sptbr register. Accordingly, the sptbr register has been renamed to satp (Supervisor Address Translation and Protection) to reflect its broadened role.

The SFENCE.VM instruction has been removed in favor of the improved SFENCE.VMA instruction.

The mstatus bit MXR has been exposed to S-mode via sstatus.

The polarity of the PUM bit in sstatus has been inverted to shorten code sequences involving MXR. The bit has been renamed to SUM.

Hardware management of page-table entry Accessed and Dirty bits has been made optional; simpler implementations may trap to software to set them.

The counter-enable scheme has changed, so that S-mode can control availability of counters to U-mode.

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.

A mechanism to improve virtualization performance by trapping S-mode virtual-memory management operations has been added.

The Supervisor Binary Interface (SBI) chapter has been removed, so that it can be maintained as a separate specification.

Numerous additions and improvements to the commentary sections.

Change configuration string proposal to be use a search process that supports various formats including Device Tree String and flattened Device Tree.

Made misa optionally writable to support modifying base and supported ISA extensions. CSR address of misa changed.

Added description of debug mode and debug CSRs.

Added a hardware performance monitoring scheme. Simplified the handling of existing hardware counters, removing privileged versions of the counters and the corresponding delta registers.

Fixed description of SPIE in presence of user-level interrupts.