Sylpheed/tools/ppc-manual/memory/icbi.md

icbi — Instruction Cache Block Invalidate

Category: Memory · Form: X · Opcode: 0x7c0007ac

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
icbi	icbi	—	Instruction Cache Block Invalidate

Syntax

icbi [RA], [RB]

Encoding

icbi — form X

• Opcode word: 0x7c0007ac
• Primary opcode (bits 0–5): 31
• Extended opcode: 982
• 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	icbi: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
RB	icbi: read	Source GPR.

Register Effects

icbi

• 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)

; No hand-written pseudocode for this instruction yet.
; The authoritative semantics are the Canary emitter snapshot under
; Implementation References; about half of Canary's emitters open
; with the PPC-style definition as a comment (`RD <- (RA) + (RB)`).
; Every side effect is also enumerated in the Register Effects and
; Status-Register Effects tables above.

C Translation Example

/* No hand-written C yet. Translate the Canary emitter snapshot   */
/* under Implementation References; its HIR maps directly:        */
/*   f.LoadGPR(n) / f.StoreGPR(n, v)  -> r[n] / r[n] = v          */
/*   f.LoadFPR / StoreFPR, f.LoadVR / StoreVR -> f[n], v[n]        */
/*   f.Load(ea, T), f.Store(ea, v) -> raw read / write; emitters   */
/*     wrap them in f.ByteSwap for the big-endian guest value      */
/*   f.UpdateCR(n, v)  -> CR field n from v's LOW 32 BITS vs 0     */
/*   f.LoadCA / f.StoreCA -> xer.CA;  f.StoreSAT -> vscr.SAT       */
/*   i.XO.RA, i.D.DS, ... -> the bit-fields listed under Operands  */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */

Implementation References

icbi

• Canary XML: tools/ppc-instructions.xml — search for mnem="icbi"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_memory.cc:1179
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:98
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:965

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_icbi(PPCHIRBuilder& f, const InstrData& i) {
  // XEINSTRNOTIMPLEMENTED();
  f.Nop();
  return 0;
}

Special Cases & Edge Conditions

• Self-modifying code primitive. Removes the line containing EA from the instruction cache so a subsequent fetch reads from memory. Required after writing new instructions because the I-cache is not coherent with the D-cache or with main memory.
• Standard recipe. The full sequence is: stw (write new code) → dcbst (push dirty data through D-cache to memory) → sync (wait for memory) → icbi (drop stale I-cache line) → isync (drain prefetch / refetch). Skipping any of these can leave the CPU executing stale instructions.
• Cache line size. Xenon's I-cache line is 128 bytes; the low seven bits of EA are ignored.
• RA0 semantics. When RA = 0, base is the literal zero. icbi 0, RB invalidates the line containing address RB.
• Unprivileged. icbi is problem-state, unlike its data-side cousin dcbi.
• No exception on bad address. Treated as a hint at the hardware level — invalidating an absent line is harmless.
• Per-thread effect. On the multithreaded Xenon core, icbi propagates across hardware threads sharing the same L1 I-cache; cross-core invalidation requires bus broadcast handled implicitly by the cache coherence protocol.
• Xenia models as no-op. No I-cache is simulated; rebuilds of generated code (when applicable) are triggered by the JIT cache-watcher, not by icbi itself.

Related Instructions

• dcbst — D-cache write-back (paired step before icbi).
• dcbf, dcbi — D-cache push / invalidate.
• isync — instruction-stream barrier (paired step after icbi).
• sync — full memory barrier between dcbst and icbi.

IBM Reference

• AIX 7.3 — icbi (Instruction Cache Block Invalidate)
• PowerISA v2.07B Book II § "Instruction Storage" for the canonical self-modifying-code sequence.