4.1.1 Supervisor Status Register (sstatus)

The sstatus register is an SXLEN-bit read/write register formatted as shown in Figure 1.1 when SXLEN=32 and Figure 1.2 when SXLEN=64. The sstatus register keeps track of the processor’s current operating state.

The SPP bit indicates the privilege level at which a hart was executing before entering supervisor mode. When a trap is taken, SPP is set to 0 if the trap originated from user mode, or 1 otherwise. When an SRET instruction (see Section [otherpriv]) is executed to return from the trap handler, the privilege level is set to user mode if the SPP bit is 0, or supervisor mode if the SPP bit is 1; SPP is then set to 0.

The SIE bit enables or disables all interrupts in supervisor mode. When SIE is clear, interrupts are not taken while in supervisor mode. When the hart is running in user-mode, the value in SIE is ignored, and supervisor-level interrupts are enabled. The supervisor can disable individual interrupt sources using the sie CSR.

The SPIE bit indicates whether supervisor interrupts were enabled prior to trapping into supervisor mode. When a trap is taken into supervisor mode, SPIE is set to SIE, and SIE is set to 0. When an SRET instruction is executed, SIE is set to SPIE, then SPIE is set to 1.

The sstatus register is a subset of the mstatus register.

4.1.2 Supervisor Trap Vector Base Address Register (stvec)

The stvec register is an SXLEN-bit read/write register that holds trap vector configuration, consisting of a vector base address (BASE) and a vector mode (MODE).

The BASE field in stvec is a WARL field that can hold any valid virtual or physical address, subject to the following alignment constraints: the address must be 4-byte aligned, and MODE settings other than Direct might impose additional alignment constraints on the value in the BASE field.

The encoding of the MODE field is shown in Table [stvec-mode]. When MODE=Direct, all traps into supervisor mode cause the pc to be set to the address in the BASE field. When MODE=Vectored, all synchronous exceptions into supervisor mode cause the pc to be set to the address in the BASE field, whereas interrupts cause the pc to be set to the address in the BASE field plus four times the interrupt cause number. For example, a supervisor-mode timer interrupt (see Table [scauses]) causes the pc to be set to BASE+0x14. Setting MODE=Vectored may impose a stricter alignment constraint on BASE.

4.1.3 Supervisor Interrupt Registers (sip and sie)

The sip register is an SXLEN-bit read/write register containing information on pending interrupts, while sie is the corresponding SXLEN-bit read/write register containing interrupt enable bits. Interrupt cause number i (as reported in CSR scause, Section 1.1.8) corresponds with bit i in both sip and sie. Bits 15:0 are allocated to standard interrupt causes only, while bits 16 and above are designated for platform or custom use.

An interrupt i will trap to S-mode if both of the following are true: (a) either the current privilege mode is S and the SIE bit in the sstatus register is set, or the current privilege mode has less privilege than S-mode; and (b) bit i is set in both sip and sie.

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 sip, and must also be evaluated immediately following the execution of an SRET instruction or an explicit write to a CSR on which these interrupt trap conditions expressly depend (including sip, sie and sstatus).

Interrupts to S-mode take priority over any interrupts to lower privilege modes.

Each individual bit in register sip may be writable or may be read-only. When bit i in sip is writable, a pending interrupt i can be cleared by writing 0 to this bit. If interrupt i can become pending but bit i in sip is read-only, the implementation must provide some other mechanism for clearing the pending interrupt (which may involve a call to the execution environment).

A bit in sie must be writable if the corresponding interrupt can ever become pending. Bits of sie that are not writable are read-only zero.

The standard portions (bits 15:0) of registers sip and sie are formatted as shown in Figures 1.6 and 1.7 respectively.

Bits sip.SEIP and sie.SEIE are the interrupt-pending and interrupt-enable bits for supervisor-level external interrupts. If implemented, SEIP is read-only in sip, and is set and cleared by the execution environment, typically through a platform-specific interrupt controller.

Bits sip.STIP and sie.STIE are the interrupt-pending and interrupt-enable bits for supervisor-level timer interrupts. If implemented, STIP is read-only in sip, and is set and cleared by the execution environment.

Bits sip.SSIP and sie.SSIE are the interrupt-pending and interrupt-enable bits for supervisor-level software interrupts. If implemented, SSIP is writable in sip and may also be set to 1 by a platform-specific interrupt controller.

Each standard interrupt type (SEI, STI, or SSI) may not be implemented, in which case the corresponding interrupt-pending and interrupt-enable bits are read-only zeros. All bits in sip and sie are WARL fields. The implemented interrupts may be found by writing one to every bit location in sie, then reading back to see which bit positions hold a one.

Multiple simultaneous interrupts destined for supervisor mode are handled in the following decreasing priority order: SEI, SSI, STI.

4.1.4 Supervisor Timers and Performance Counters

Supervisor software uses the same hardware performance monitoring facility as user-mode software, including the time, cycle, and instret CSRs. The implementation should provide a mechanism to modify the counter values.

The implementation must provide a facility for scheduling timer interrupts in terms of the real-time counter, time.

4.1.5 Counter-Enable Register (scounteren)

The counter-enable register scounteren is a 32-bit register that controls the availability of the hardware performance monitoring counters to U-mode.

When the CY, TM, IR, or HPMn bit in the scounteren register is clear, attempts to read the cycle, time, instret, or hpmcountern register while executing in U-mode will cause an illegal instruction exception. When one of these bits is set, access to the corresponding register is permitted.

scounteren must be implemented. However, any of the bits may be read-only zero, indicating reads to the corresponding counter will cause an exception when executing in U-mode. Hence, they are effectively WARL fields.

4.1.6 Supervisor Scratch Register (sscratch)

The sscratch register is an SXLEN-bit read/write register, dedicated for use by the supervisor. Typically, sscratch is used to hold a pointer to the hart-local supervisor context while the hart is executing user code. At the beginning of a trap handler, sscratch is swapped with a user register to provide an initial working register.

4.1.7 Supervisor Exception Program Counter (sepc)

sepc is an SXLEN-bit read/write register formatted as shown in Figure 1.10. The low bit of sepc (sepc[0]) is always zero. On implementations that support only IALIGN=32, the two low bits (sepc[1:0]) are always zero.

If an implementation allows IALIGN to be either 16 or 32 (by changing CSR misa, for example), then, whenever IALIGN=32, bit sepc[1] is masked on reads so that it appears to be 0. This masking occurs also for the implicit read by the SRET instruction. Though masked, sepc[1] remains writable when IALIGN=32.

sepc 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 sepc, implementations may convert an invalid address into some other invalid address that sepc is capable of holding.

When a trap is taken into S-mode, sepc is written with the virtual address of the instruction that was interrupted or that encountered the exception. Otherwise, sepc is never written by the implementation, though it may be explicitly written by software.

4.1.8 Supervisor Cause Register (scause)

The scause register is an SXLEN-bit read-write register formatted as shown in Figure 1.11. When a trap is taken into S-mode, scause is written with a code indicating the event that caused the trap. Otherwise, scause is never written by the implementation, though it may be explicitly written by software.

The Interrupt bit in the scause 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 [scauses] lists the possible exception codes for the current supervisor ISAs. The Exception Code is a WLRL field. It is required to hold the values 0–31 (i.e., bits 4–0 must be implemented), but otherwise it is only guaranteed to hold supported exception codes.

4.1.9 Supervisor Trap Value (stval) Register

The stval register is an SXLEN-bit read-write register formatted as shown in Figure 1.12. When a trap is taken into S-mode, stval is written with exception-specific information to assist software in handling the trap. Otherwise, stval is never written by the implementation, though it may be explicitly written by software. The hardware platform will specify which exceptions must set stval informatively and which may unconditionally set it to zero.

If stval is written with a nonzero value when a breakpoint, address-misaligned, access-fault, or page-fault exception occurs on an instruction fetch, load, or store, then stval will contain the faulting virtual address.

If stval is written with a nonzero value when a misaligned load or store causes an access-fault or page-fault exception, then stval will contain the virtual address of the portion of the access that caused the fault.

If stval is written with a nonzero value when an instruction access-fault or page-fault exception occurs on a system with variable-length instructions, then stval will contain the virtual address of the portion of the instruction that caused the fault, while sepc will point to the beginning of the instruction.

The stval register can optionally also be used to return the faulting instruction bits on an illegal instruction exception (sepc points to the faulting instruction in memory). If stval is written with a nonzero value when an illegal-instruction exception occurs, then stval will contain the shortest of:

the actual faulting instruction

the first ILEN bits of the faulting instruction

the first SXLEN bits of the faulting instruction

The value loaded into stval on an illegal-instruction exception is right-justified and all unused upper bits are cleared to zero.

For other traps, stval is set to zero, but a future standard may redefine stval’s setting for other traps.

stval is a WARL register that must be able to hold all valid virtual addresses and the value 0. It need not be capable of holding all possible invalid addresses. Prior to writing stval, implementations may convert an invalid address into some other invalid address that stval is capable of holding. If the feature to return the faulting instruction bits is implemented, stval must also be able to hold all values less than 2^N, where N is the smaller of SXLEN and ILEN.

4.1.10 Supervisor Environment Configuration Register (senvcfg)

The senvcfg CSR is an SXLEN-bit read/write register, formatted as shown in Figure 1.13, that controls certain characteristics of the U-mode execution environment.

If bit FIOM (Fence of I/O implies Memory) is set to one in senvcfg, FENCE instructions executed in U-mode are modified so the requirement to order accesses to device I/O implies also the requirement to order main memory accesses. Table [tab:senvcfg-FIOM] details the modified interpretation of FENCE instruction bits PI, PO, SI, and SO in U-mode when FIOM=1.

Similarly, for U-mode when FIOM=1, if an atomic instruction that accesses a region ordered as device I/O has its aq and/or rl bit set, then that instruction is ordered as though it accesses both device I/O and memory.

If satp.MODE is read-only zero (always Bare), the implementation may make FIOM read-only zero.

The definition of the CBZE field will be furnished by the forthcoming Zicboz extension. Its allocation within senvcfg 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 senvcfg may change prior to the ratification of that extension.

4.1.11 Supervisor Address Translation and Protection (satp) Register

The satp register is an SXLEN-bit read/write register, formatted as shown in Figure 1.14 for SXLEN=32 and Figure 1.15 for SXLEN=64, which controls supervisor-mode address translation and protection. This register holds the physical page number (PPN) of the root page table, i.e., its supervisor physical address divided by 4 KiB; an address space identifier (ASID), which facilitates address-translation fences on a per-address-space basis; and the MODE field, which selects the current address-translation scheme. Further details on the access to this register are described in Section [virt-control].

Table 1.16 shows the encodings of the MODE field when SXLEN=32 and SXLEN=64. When MODE=Bare, supervisor virtual addresses are equal to supervisor physical addresses, and there is no additional memory protection beyond the physical memory protection scheme described in Section [sec:pmp]. To select MODE=Bare, software must write zero to the remaining fields of satp (bits 30–0 when SXLEN=32, or bits 59–0 when SXLEN=64). Attempting to select MODE=Bare with a nonzero pattern in the remaining fields has an  effect on the value that the remaining fields assume and an  effect on address translation and protection behavior.

When SXLEN=32, the satp encodings corresponding to MODE=Bare and ASID[8:7]=3 are designated for custom use, whereas the encodings corresponding to MODE=Bare and ASID[8:7]≠3 are reserved for future standard use. When SXLEN=64, all satp encodings corresponding to MODE=Bare are reserved for future standard use.

When SXLEN=32, the only other valid setting for MODE is Sv32, a paged virtual-memory scheme described in Section 1.3.

When SXLEN=64, three paged virtual-memory schemes are defined: Sv39, Sv48, and Sv57, described in Sections 1.4, 1.5, and 1.6, respectively. One additional scheme, Sv64, will be defined in a later version of this specification. The remaining MODE settings are reserved for future use and may define different interpretations of the other fields in satp.

Implementations are not required to support all MODE settings, and if satp is written with an unsupported MODE, the entire write has no effect; no fields in satp are modified.

The number of ASID bits is  and may be zero. The number of implemented ASID bits, termed ASIDLEN , may be determined by writing one to every bit position in the ASID field, then reading back the value in satp to see which bit positions in the ASID field hold a one. The least-significant bits of ASID are implemented first: that is, if ASIDLEN > 0, ASID[ASIDLEN-1:0] is writable. The maximal value of ASIDLEN, termed ASIDMAX, is 9 for Sv32 or 16 for Sv39, Sv48, and Sv57.

The satp register is considered active when the effective privilege mode is S-mode or U-mode. Executions of the address-translation algorithm may only begin using a given value of satp when satp is active.

Note that writing satp does not imply any ordering constraints between page-table updates and subsequent address translations, nor does it imply any invalidation of address-translation caches. If the new address space’s page tables have been modified, or if an ASID is reused, it may be necessary to execute an SFENCE.VMA instruction (see Section 1.2.1) after, or in some cases before, writing satp.

4.2.1 Supervisor Memory-Management Fence Instruction

The supervisor memory-management fence instruction SFENCE.VMA is used to synchronize updates to in-memory memory-management data structures with current execution. Instruction execution causes implicit reads and writes to these data structures; however, these implicit references are ordinarily not ordered with respect to explicit loads and stores. Executing an SFENCE.VMA instruction guarantees that any previous stores already visible to the current RISC-V hart are ordered before certain implicit references by subsequent instructions in that hart to the memory-management data structures. The specific set of operations ordered by SFENCE.VMA is determined by rs1 and rs2, as described below. SFENCE.VMA is also used to invalidate entries in the address-translation cache associated with a hart (see Section 1.3.2). Further details on the behavior of this instruction are described in Section [virt-control] and Section [pmp-vmem].

SFENCE.VMA orders only the local hart’s implicit references to the memory-management data structures.

For the common case that the translation data structures have only been modified for a single address mapping (i.e., one page or superpage), rs1 can specify a virtual address within that mapping to effect a translation fence for that mapping only. Furthermore, for the common case that the translation data structures have only been modified for a single address-space identifier, rs2 can specify the address space. The behavior of SFENCE.VMA depends on rs1 and rs2 as follows:

• If rs1=x0 and rs2=x0, the fence orders all reads and writes made to any level of the page tables, for all address spaces. The fence also invalidates all address-translation cache entries, for all address spaces.

• If rs1=x0 and rs2≠x0, the fence orders all reads and writes made to any level of the page tables, but only for the address space identified by integer register rs2. Accesses to global mappings (see Section 1.3.1) are not ordered. The fence also invalidates all address-translation cache entries matching the address space identified by integer register rs2, except for entries containing global mappings.

• If rs1≠x0 and rs2=x0, the fence orders only reads and writes made to leaf page table entries corresponding to the virtual address in rs1, for all address spaces. The fence also invalidates all address-translation cache entries that contain leaf page table entries corresponding to the virtual address in rs1, for all address spaces.

• If rs1≠x0 and rs2≠x0, the fence orders only reads and writes made to leaf page table entries corresponding to the virtual address in rs1, for the address space identified by integer register rs2. Accesses to global mappings are not ordered. The fence also invalidates all address-translation cache entries that contain leaf page table entries corresponding to the virtual address in rs1 and that match the address space identified by integer register rs2, except for entries containing global mappings.

If the value held in rs1 is not a valid virtual address, then the SFENCE.VMA instruction has no effect. No exception is raised in this case.

When rs2≠x0, bits SXLEN-1:ASIDMAX of the value held in rs2 are reserved for future standard use. Until their use is defined by a standard extension, they should be zeroed by software and ignored by current implementations. Furthermore, if ASIDLEN < ASIDMAX, the implementation shall ignore bits ASIDMAX-1:ASIDLEN of the value held in rs2.

An implicit read of the memory-management data structures may return any translation for an address that was valid at any time since the most recent SFENCE.VMA that subsumes that address. The ordering implied by SFENCE.VMA does not place implicit reads and writes to the memory-management data structures into the global memory order in a way that interacts cleanly with the standard RVWMO ordering rules. In particular, even though an SFENCE.VMA orders prior explicit accesses before subsequent implicit accesses, and those implicit accesses are ordered before their associated explicit accesses, SFENCE.VMA does not necessarily place prior explicit accesses before subsequent explicit accesses in the global memory order. These implicit loads also need not otherwise obey normal program order semantics with respect to prior loads or stores to the same address.

Implementations must only perform implicit reads of the translation data structures pointed to by the current contents of the satp register or a subsequent valid (V=1) translation data structure entry, and must only raise exceptions for implicit accesses that are generated as a result of instruction execution, not those that are performed speculatively.

Changes to the sstatus fields SUM and MXR take effect immediately, without the need to execute an SFENCE.VMA instruction. Changing satp.MODE from Bare to other modes and vice versa also takes effect immediately, without the need to execute an SFENCE.VMA instruction. Likewise, changes to satp.ASID take effect immediately.

If a hart employs an address-translation cache, that cache must appear to be private to that hart. In particular, the meaning of an ASID is local to a hart; software may choose to use the same ASID to refer to different address spaces on different harts.

For implementations that make satp.MODE read-only zero (always Bare), attempts to execute an SFENCE.VMA instruction might raise an illegal instruction exception.

4.3.1 Addressing and Memory Protection

Sv32 implementations support a 32-bit virtual address space, divided into 4 KiB pages. An Sv32 virtual address is partitioned into a virtual page number (VPN) and page offset, as shown in Figure 1.17. When Sv32 virtual memory mode is selected in the MODE field of the satp register, supervisor virtual addresses are translated into supervisor physical addresses via a two-level page table. The 20-bit VPN is translated into a 22-bit physical page number (PPN), while the 12-bit page offset is untranslated. The resulting supervisor-level physical addresses are then checked using any physical memory protection structures (Sections [sec:pmp]), before being directly converted to machine-level physical addresses. If necessary, supervisor-level physical addresses are zero-extended to the number of physical address bits found in the implementation.

Sv32 page tables consist of 2¹⁰ page-table entries (PTEs), each of four bytes. A page table is exactly the size of a page and must always be aligned to a page boundary. The physical page number of the root page table is stored in the satp register.

The PTE format for Sv32 is shown in Figures 1.19. The V bit indicates whether the PTE is valid; if it is 0, all other bits in the PTE are don’t-cares and may be used freely by software. The permission bits, R, W, and X, indicate whether the page is readable, writable, and executable, respectively. When all three are zero, the PTE is a pointer to the next level of the page table; otherwise, it is a leaf PTE. Writable pages must also be marked readable; the contrary combinations are reserved for future use. Table [pteperm] summarizes the encoding of the permission bits.

Attempting to fetch an instruction from a page that does not have execute permissions raises a fetch page-fault exception. Attempting to execute a load or load-reserved instruction whose effective address lies within a page without read permissions raises a load page-fault exception. Attempting to execute a store, store-conditional, or AMO instruction whose effective address lies within a page without write permissions raises a store page-fault exception.

The U bit indicates whether the page is accessible to user mode. U-mode software may only access the page when U=1. If the SUM bit in the sstatus register is set, supervisor mode software may also access pages with U=1. However, supervisor code normally operates with the SUM bit clear, in which case, supervisor code will fault on accesses to user-mode pages. Irrespective of SUM, the supervisor may not execute code on pages with U=1.

The G bit designates a global mapping. Global mappings are those that exist in all address spaces. For non-leaf PTEs, the global setting implies that all mappings in the subsequent levels of the page table are global. Note that failing to mark a global mapping as global merely reduces performance, whereas marking a non-global mapping as global is a software bug that, after switching to an address space with a different non-global mapping for that address range, can unpredictably result in either mapping being used.

The RSW field is reserved for use by supervisor software; the implementation shall ignore this field.

Each leaf PTE contains an accessed (A) and dirty (D) bit. The A bit indicates the virtual page has been read, written, or fetched from since the last time the A bit was cleared. The D bit indicates the virtual page has been written since the last time the D bit was cleared.

Two schemes to manage the A and D bits are permitted:

• When a virtual page is accessed and the A bit is clear, or is written and the D bit is clear, a page-fault exception is raised.

• When a virtual page is accessed and the A bit is clear, or is written and the D bit is clear, the implementation sets the corresponding bit(s) in the PTE. The PTE update must be atomic with respect to other accesses to the PTE, and must atomically check that the PTE is valid and grants sufficient permissions. Updates of the A bit may be performed as a result of speculation, but updates to the D bit must be exact (i.e., not speculative), and observed in program order by the local hart. Furthermore, the PTE update must appear in the global memory order no later than the explicit memory access, or any subsequent explicit memory access to that virtual page by the local hart. The ordering on loads and stores provided by FENCE instructions and the acquire/release bits on atomic instructions also orders the PTE updates associated with those loads and stores as observed by remote harts.

  The PTE update is not required to be atomic with respect to the explicit memory access that caused the update, and the sequence is interruptible. However, the hart must not perform the explicit memory access before the PTE update is globally visible.

All harts in a system must employ the same PTE-update scheme as each other.

Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv32 supports 4 MiB megapages. A megapage must be virtually and physically aligned to a 4 MiB boundary; a page-fault exception is raised if the physical address is insufficiently aligned.

For non-leaf PTEs, the D, A, and U bits are reserved for future standard use. Until their use is defined by a standard extension, they must be cleared by software for forward compatibility.

For implementations with both page-based virtual memory and the “A” standard extension, the LR/SC reservation set must lie completely within a single base physical page (i.e., a naturally aligned 4 KiB physical-memory region).

4.3.2 Virtual Address Translation Process

A virtual address va is translated into a physical address pa as follows:

1. Let a be ${\tt satp}.ppn \times \textrm{PAGESIZE}$, and let i = LEVELS − 1. (For Sv32, PAGESIZE=2¹² and LEVELS=2.) The satp register must be active, i.e., the effective privilege mode must be S-mode or U-mode.

2. Let pte be the value of the PTE at address a + va.vpn[i] × PTESIZE. (For Sv32, PTESIZE=4.) If accessing pte violates a PMA or PMP check, raise an access-fault exception corresponding to the original access type.

3. If pte.v = 0, or if pte.r = 0 and pte.w = 1, or if any bits or encodings that are reserved for future standard use are set within pte, stop and raise a page-fault exception corresponding to the original access type.

4. Otherwise, the PTE is valid. If pte.r = 1 or pte.x = 1, go to step 5. Otherwise, this PTE is a pointer to the next level of the page table. Let i = i − 1. If i < 0, stop and raise a page-fault exception corresponding to the original access type. Otherwise, let a = pte.ppn × PAGESIZE and go to step 2.

5. A leaf PTE has been found. Determine if the requested memory access is allowed by the pte.r, pte.w, pte.x, and pte.u bits, given the current privilege mode and the value of the SUM and MXR fields of the mstatus register. If not, stop and raise a page-fault exception corresponding to the original access type.

6. If i > 0 and pte.ppn[i − 1 : 0] ≠ 0, this is a misaligned superpage; stop and raise a page-fault exception corresponding to the original access type.

7. If pte.a = 0, or if the original memory access is a store and pte.d = 0, either raise a page-fault exception corresponding to the original access type, or:

   • If a store to pte would violate a PMA or PMP check, raise an access-fault exception corresponding to the original access type.

   • Perform the following steps atomically:

     • Compare pte to the value of the PTE at address a + va.vpn[i] × PTESIZE.

     • If the values match, set pte.a to 1 and, if the original memory access is a store, also set pte.d to 1.

     • If the comparison fails, return to step 2

8. The translation is successful. The translated physical address is given as follows:

   • pa.pgoff = va.pgoff.

   • If i > 0, then this is a superpage translation and pa.ppn[i − 1 : 0] = va.vpn[i − 1 : 0].

   • pa.ppn[LEVELS − 1 : i] = pte.ppn[LEVELS − 1 : i].

All implicit accesses to the address-translation data structures in this algorithm are performed using width PTESIZE.

The results of implicit address-translation reads in step 2 may be held in a read-only, incoherent address-translation cache but not shared with other harts. The address-translation cache may hold an arbitrary number of entries, including an arbitrary number of entries for the same address and ASID. Entries in the address-translation cache may then satisfy subsequent step 2 reads if the ASID associated with the entry matches the ASID loaded in step 0 or if the entry is associated with a global mapping. To ensure that implicit reads observe writes to the same memory locations, an SFENCE.VMA instruction must be executed after the writes to flush the relevant cached translations.

The address-translation cache cannot be used in step 7; accessed and dirty bits may only be updated in memory directly.

Implementations may also execute the address-translation algorithm speculatively at any time, for any virtual address, as long as satp is active (as defined in Section 1.1.11). Such speculative executions have the effect of pre-populating the address-translation cache.

Speculative executions of the address-translation algorithm behave as non-speculative executions of the algorithm do, except that they must not set the dirty bit for a PTE, they must not trigger an exception, and they must not create address-translation cache entries if those entries would have been invalidated by any SFENCE.VMA instruction executed by the hart since the speculative execution of the algorithm began.

4.4.1 Addressing and Memory Protection

Sv39 implementations support a 39-bit virtual address space, divided into 4 KiB pages. An Sv39 address is partitioned as shown in Figure 1.20. Instruction fetch addresses and load and store effective addresses, which are 64 bits, must have bits 63–39 all equal to bit 38, or else a page-fault exception will occur. The 27-bit VPN is translated into a 44-bit PPN via a three-level page table, while the 12-bit page offset is untranslated.

Sv39 page tables contain 2⁹ page table entries (PTEs), eight bytes each. A page table is exactly the size of a page and must always be aligned to a page boundary. The physical page number of the root page table is stored in the satp register’s PPN field.

The PTE format for Sv39 is shown in Figure 1.22. Bits 9–0 have the same meaning as for Sv32. Bit 63 is reserved for use by the Svnapot extension in Chapter 2. If Svnapot is not implemented, bit 63 remains reserved and must be zeroed by software for forward compatibility, or else a page-fault exception is raised. Bits 62–61 are reserved for use by the Svpbmt extension in Chapter 3. If Svpbmt is not implemented, bits 62–61 remain reserved and must be zeroed by software for forward compatibility, or else a page-fault exception is raised. Bits 60–54 are reserved for future standard use and, until their use is defined by some standard extension, must be zeroed by software for forward compatibility. If any of these bits are set, a page-fault exception is raised.

Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv39 supports 2 MiB megapages and 1 GiB gigapages, each of which must be virtually and physically aligned to a boundary equal to its size. A page-fault exception is raised if the physical address is insufficiently aligned.

The algorithm for virtual-to-physical address translation is the same as in Section 1.3.2, except LEVELS equals 3 and PTESIZE equals 8.

4.5.1 Addressing and Memory Protection

Sv48 implementations support a 48-bit virtual address space, divided into 4 KiB pages. An Sv48 address is partitioned as shown in Figure 1.23. Instruction fetch addresses and load and store effective addresses, which are 64 bits, must have bits 63–48 all equal to bit 47, or else a page-fault exception will occur. The 36-bit VPN is translated into a 44-bit PPN via a four-level page table, while the 12-bit page offset is untranslated.

The PTE format for Sv48 is shown in Figure 1.25. Bits 63–54 and 9–0 have the same meaning as for Sv39. Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv48 supports 2 MiB megapages, 1 GiB gigapages, and 512 GiB terapages, each of which must be virtually and physically aligned to a boundary equal to its size. A page-fault exception is raised if the physical address is insufficiently aligned.

The algorithm for virtual-to-physical address translation is the same as in Section 1.3.2, except LEVELS equals 4 and PTESIZE equals 8.

4.6.1 Addressing and Memory Protection

Sv57 implementations support a 57-bit virtual address space, divided into 4 KiB pages. An Sv57 address is partitioned as shown in Figure 1.26. Instruction fetch addresses and load and store effective addresses, which are 64 bits, must have bits 63–57 all equal to bit 56, or else a page-fault exception will occur. The 45-bit VPN is translated into a 44-bit PPN via a five-level page table, while the 12-bit page offset is untranslated.

The PTE format for Sv57 is shown in Figure 1.28. Bits 63–54 and 9–0 have the same meaning as for Sv39. Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv57 supports 2 MiB megapages, 1 GiB gigapages, 512 GiB terapages, and 256 TiB petapages, each of which must be virtually and physically aligned to a boundary equal to its size. A page-fault exception is raised if the physical address is insufficiently aligned.

The algorithm for virtual-to-physical address translation is the same as in Section 1.3.2, except LEVELS equals 5 and PTESIZE equals 8.