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CSR Bit Fields

RISC-V pmpaddr21 CSR Register

Address 0x3C5Privilege MachineAccess RW / MXLEN: RV32 encodes PA[33:2]; RV64 [53:0] encodes PA[55:2] and [63:54] is zeroMachine physical memory protection CSRs

pmpaddr21 (0x3C5) is the WARL address register for machine PMP entry 21 when 64 PMP entries are implemented; it encodes physical address bits 33:2 on RV32 and 55:2 on RV64.

Related Search Terms
pmpaddr21 CSRpmpaddr21 0x3C5pmpaddr21 TORpmpaddr21 NAPOTpmpaddr21 grainpmp21cfg A
Bit Overview
bit 3 = only bit 3; bits 12..11 = bits 12 down to 11
MSBLSB
063:54WARL/reads zeroaddress[55:2]53:0WARL/may be RO0
Field Map

Understand pmpaddr21 By Bit Fields

2 key fields
63:54

0

WARL/reads zero

RV64: the upper ten bits are zero in the standard pmpaddr format; this segment is part of the WARL format.

0 (bits 63:54) — RV64: the upper ten bits are zero in the standard pmpaddr format; this segment is part of the WARL format.

What This Field Controls

  • - RV64: the upper ten bits are zero in the standard pmpaddr format; this segment is part of the WARL format.

Common Values

This field is better understood together with surrounding context than as a fixed memorized enumeration.

↗Open Official Manual
53:0

address[55:2]

WARL/may be RO0

RV64: encodes bits 55:2 of a 56-bit physical address. Unimplemented physical-address bits make the field WARL; every PMP CSR field may be implemented as read-only zero.

address[55:2] (bits 53:0) — RV64: encodes bits 55:2 of a 56-bit physical address. Unimplemented physical-address bits make the field WARL; every PMP CSR field may be implemented as read-only zero.

What This Field Controls

  • - RV64: encodes bits 55:2 of a 56-bit physical address. Unimplemented physical-address bits make the field WARL; every PMP CSR field may be implemented as read-only zero.

Common Values

This field is better understood together with surrounding context than as a fixed memorized enumeration.

↗Open Official Manual
Official Basis & Search Notes

pmpaddr21 is the MXLEN-bit WARL address encoding for entry 21 when 64 PMP entries are implemented. pmp21cfg.A determines its meaning; TOR uses pmpaddr20 as the lower bound.

RV32 pmpaddr encodes PA[33:2]; RV64 [53:0] encodes PA[55:2], with [63:54] zero.
TOR entry 21 uses pmpaddr20 as its lower bound and pmpaddr21 as its upper bound; NAPOT uses low pmpaddr21 bits to encode a naturally aligned power-of-two region of at least 8 bytes.
Platform grain changes low-bit readback; pmp21cfg.L or a locked TOR entry 22 can cause writes to pmpaddr21 to be ignored. Smepmp MML=1 changes L's permission meaning, but the relevant L=1 rules remain locked unless RLB=1 temporarily bypasses locking.
RISC-V ISA Manual 2026-07-09 release - Machine ISA PMPRISC-V Privileged Architecture - Physical Memory ProtectionRISC-V Privileged Architecture - Smepmp Physical Memory Protection Rules

What To Check First When Reading This CSR

  • - PMP is optional; implementations may provide 0, 16, or 64 entries, with the lowest-numbered entries implemented first. Therefore, pmpaddr21 exists only when 64 entries are implemented; every PMP CSR field is WARL and may be read-only zero.
  • - pmpaddr21 is accessible only to M-mode. Its readback depends on pmp21cfg.A and the platform-wide PMP grain G.
  • - With G>=2 and A=NAPOT, pmpaddr21[G-2:0] reads as ones; with G>=1 and A=OFF/TOR, pmpaddr21[G-1:0] reads as zeros, but those bits do not affect TOR matching. Changing pmp21cfg.A[1] changes readback, not the underlying stored value.
  • - Reset specifies defaults only for writable PMP A/L fields (subject to platform overrides); pmpaddr21's reset value is unspecified, but no WARL field contains an illegal value.

Risk Checks Before Writing

  • - Writes to pmpaddr21 are ignored when pmp21cfg.L=1; pmpaddr21 is also unwritable as entry 22's lower bound when that entry is locked with pmp22cfg.A=TOR. Without Smepmp, locking persists until hart reset. With Smepmp MML=1, L changes permission meaning, but the L=1 M-mode-only or locked Shared-Region rules it defines remain locked unless mseccfg.RLB=1 temporarily bypasses locking.
  • - pmpaddr21 is WARL; read it back after writing and interpret the masked value using the current pmp21cfg.A and grain.
  • - With pmp21cfg.A=TOR, entry 21 matches pmpaddr20 <= y < pmpaddr21; if the lower bound is not less than the upper bound, it matches no addresses. NA4 is not selectable when G>=1.

Put It Back Into A Real Flow

1

Read pmp21cfg.A to determine whether entry 21 uses OFF, TOR, NA4, or NAPOT.

2

Write pmpaddr21 for the selected mode; TOR also requires lower bound pmpaddr20, while NAPOT encodes range size in low bits.

3

Read back pmpaddr21 and the corresponding pmpcfg CSR to confirm the effective WARL/grain encoding; set pmp21cfg.L only after final verification.

FAQ

Where do pmpaddr21's TOR bounds come from?

With pmp21cfg.A=TOR, pmpaddr20 supplies the lower bound and pmpaddr21 the upper bound, so entry 21 matches the half-open range pmpaddr20 <= y < pmpaddr21; it matches no addresses when the lower bound is not less than the upper bound.

Why can pmpaddr21 read back differently from the written value?

pmpaddr21 is WARL, and platform grain plus pmp21cfg.A[1] masks low-bit readback: NAPOT can read ones while OFF/TOR reads zeros. Changing A[1] does not alter the underlying stored value.