xenia-rs/migration/project-root/ppc-manual/memory/dcbf.md

dcbf — Data Cache Block Flush

Category: Memory · Form: X · Opcode: 0x7c0000ac

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
dcbf	dcbf	—	Data Cache Block Flush

Syntax

dcbf [RA0], [RB]

Encoding

dcbf — form X

• Opcode word: 0x7c0000ac
• Primary opcode (bits 0–5): 31
• Extended opcode: 86
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode
6–10	RT/FRT/VRT	destination
11–15	RA/FRA/VRA	source A
16–20	RB/FRB/VRB	source B
21–30	XO	extended opcode (10 bits)
31	Rc	record-form flag

Operands

Field	Role	Description
RA0	dcbf: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
RB	dcbf: read	Source GPR.

Register Effects

dcbf

• Reads (always): RA0, RB
• Reads (conditional): none
• Writes (always): none
• Writes (conditional): none

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

; Pseudocode derives directly from the xenia-rs interpreter
; arm (see Implementation References). Operation semantics:
;   - Read source operands from the fields listed under Operands.
;   - Apply the arithmetic / logical / memory action described
;     in the Description field above.
;   - Write results to the destination register(s); update any
;     status bits enumerated under Status-Register Effects.
; Consult the IBM AIX reference link under IBM Reference for
; canonical PPC-style pseudocode where xenia's expression is
; terse.

C Translation Example

/* C translation: the xenia-rs interpreter arm below in           */
/* Implementation References is the authoritative semantic        */
/* snapshot. Translate it line-by-line:                            */
/*   - ctx.gpr[N]  -> r[N]       (or f[]/v[] for FPRs/VRs)        */
/*   - mem.read_u*/write_u* -> mem_read_u*_be / mem_write_u*_be   */
/*   - ctx.update_cr_signed(fld, v) -> update_cr_signed(fld, v)   */
/*   - ctx.xer_ca / xer_ov / xer_so -> xer.CA / xer.OV / xer.SO   */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */

Implementation References

dcbf

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="dcbf"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:1125
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:19
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:773

Special Cases & Edge Conditions

• Flush = write-back + invalidate. If the addressed line is dirty in the data cache, it is written to memory; whether dirty or clean, the line is then removed from the cache. Subsequent loads must refill from memory.
• Cache line size. Xenon's L1/L2 lines are 128 bytes. The hardware ignores the low seven bits of EA, so dcbf RA, RB flushes the line containing EA regardless of where in that line EA lies. There is no dcbf128 variant — the hint is sized to the architectural line.
• RA0 semantics. When RA = 0, the base is the literal zero — dcbf 0, RB flushes the line containing address RB. The instruction has no destination register.
• Xenia models a no-op. Xenia-rs's emulator does not maintain a coherent cache model; the decode entry exists but the interpreter typically advances PC without further effect, since target memory is always coherent on the host. This is correct behaviour for an emulator.
• Unprivileged. dcbf is a problem-state instruction — usable from user code. Storage protection still applies; flushing an unmapped page raises a DSI exception.
• Pair with sync. Hardware dcbf does not by itself impose ordering; software that needs the flushed data visible to other masters (DMA, GPU) issues a sync afterwards.
• Self-modifying code companion. When patching code, the recipe is dcbst (push dirty data through to memory) → sync → icbi (invalidate I-cache) → isync. dcbf is the heavier alternative when the writer also wants the line out of D-cache.

Related Instructions

• dcbst — write-back without invalidate (lighter than dcbf).
• dcbi — invalidate without write-back (privileged; loses dirty data).
• dcbt, dcbtst — touch hints to bring lines in.
• dcbz, dcbz128 — allocate-and-zero a line.
• icbi — instruction-cache invalidate, used together for self-modifying code.
• sync — full memory barrier, typically follows dcbf.

IBM Reference

• AIX 7.3 — dcbf (Data Cache Block Flush)
• PowerISA v2.07B Book II § "Storage Control Instructions" for cache-coherence semantics.