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

dcbi — Data Cache Block Invalidate

Category: Memory · Form: X · Opcode: 0x7c0003ac

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
dcbi	dcbi	—	Data Cache Block Invalidate

Syntax

dcbi [RA0], [RB]

Encoding

dcbi — form X

• Opcode word: 0x7c0003ac
• Primary opcode (bits 0–5): 31
• Extended opcode: 470
• 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	dcbi: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
RB	dcbi: read	Source GPR.

Register Effects

dcbi

• 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

dcbi

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="dcbi"
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:19
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:811

Special Cases & Edge Conditions

• Privileged. Unlike dcbf and dcbst, dcbi is supervisor-only. Executing in problem state raises a privileged-instruction (program) interrupt. Game code never issues dcbi directly; only the kernel.
• Drops dirty data. The line is removed from cache without writing back, so any modifications that have not already been pushed to memory are lost. Used only when the underlying memory is being repurposed (e.g. DMA window flip, page demap) and stale dirty data would be incorrect.
• Cache line size. Xenon lines are 128 bytes. The low seven bits of EA are ignored — the operation targets the cache line that contains EA.
• RA0 semantics. When RA = 0, base is literal zero, so dcbi 0, RB invalidates the line containing address RB.
• Xenia treats it as a no-op. With no modelled cache, the emulator decodes and advances PC; memory is already authoritative.
• Sequencing. Not synchronising. Pair with sync when invalidation must precede a subsequent load on another thread.
• Architecturally subsumed by dcbf for problem state. Userspace that wants "this line is no longer valuable" must use dcbf, accepting the write-back cost.

Related Instructions

• dcbf — flush (write-back + invalidate); the unprivileged alternative.
• dcbst — write-back without invalidate.
• dcbz, dcbz128 — allocate-and-zero a line.
• dcbt, dcbtst — prefetch hints.
• icbi — instruction-cache analog (also problem-state, not privileged).
• sync, isync — pair with cache-control ops for ordering.

IBM Reference

• AIX 7.3 — dcbi (Data Cache Block Invalidate)
• PowerISA v2.07B Book II for the privilege model and cache-coherence rules.