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

dcbz — Data Cache Block Clear to Zero

Category: Memory · Form: DCBZ · Opcode: 0x7c0007ec

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
dcbz	dcbz	—	Data Cache Block Clear to Zero
dcbz128	dcbz128	—	Data Cache Block Clear to Zero 128

Syntax

dcbz [RA0], [RB]
dcbz128 [RA0], [RB]

Encoding

dcbz — form DCBZ

• Opcode word: 0x7c0007ec
• Primary opcode (bits 0–5): 31
• Extended opcode: 1014
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (31)
6–10	—	reserved
11–15	RA	base register (0 ⇒ literal 0)
16–20	RB	offset register
21–30	XO	extended opcode (1014 for dcbz / 1010 for dcbz128)
31	—	reserved

dcbz128 — form DCBZ

• Opcode word: 0x7c2007ec
• Primary opcode (bits 0–5): 31
• Extended opcode: 1014
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (31)
6–10	—	reserved
11–15	RA	base register (0 ⇒ literal 0)
16–20	RB	offset register
21–30	XO	extended opcode (1014 for dcbz / 1010 for dcbz128)
31	—	reserved

Operands

Field	Role	Description
RA0	dcbz: read; dcbz128: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
RB	dcbz: read; dcbz128: read	Source GPR.

Register Effects

dcbz

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

dcbz128

• 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

dcbz

• Canary XML: tools/ppc-instructions.xml — search for mnem="dcbz"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_memory.cc:1171
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:49
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:1001

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_dcbz(PPCHIRBuilder& f, const InstrData& i) {
  // EA <- (RA) + (RB)
  // memset(EA & ~31, 0, 32)
  // On Xbox360 there is no short cache line. Normal dcbz always clears 128
  // bytes.
  return InstrEmit_dcbz128(f, i);
}

// ── delegates to (src/xenia/cpu/ppc/ppc_emit_memory.cc:1159) ──
int InstrEmit_dcbz128(PPCHIRBuilder& f, const InstrData& i) {
  // EA <- (RA) + (RB)
  // memset(EA & ~31, 0, 32)
  Value* ea = CalculateEA_0(f, i.X.RA, i.X.RB);
  // dcbz128 - 128 byte set
  int block_size = 128;
  int address_mask = ~127;
  f.Memset(f.And(ea, f.LoadConstantInt64(address_mask)), f.LoadZeroInt8(),
           f.LoadConstantInt64(block_size));
  return 0;
}

dcbz128

• Canary XML: tools/ppc-instructions.xml — search for mnem="dcbz128"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_memory.cc:1159
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:50
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:1002

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_dcbz128(PPCHIRBuilder& f, const InstrData& i) {
  // EA <- (RA) + (RB)
  // memset(EA & ~31, 0, 32)
  Value* ea = CalculateEA_0(f, i.X.RA, i.X.RB);
  // dcbz128 - 128 byte set
  int block_size = 128;
  int address_mask = ~127;
  f.Memset(f.And(ea, f.LoadConstantInt64(address_mask)), f.LoadZeroInt8(),
           f.LoadConstantInt64(block_size));
  return 0;
}

Special Cases & Edge Conditions

• Cache-line size mismatch. Stock PowerPC dcbz zeroes one architectural cache line — 32 bytes on classic POWER, but the Xenon's L1 line is 128 bytes. Microsoft added dcbz128 (encoded with bit-9 set so RT field reads as 1) to clear a true Xenon line in one instruction. Most Xbox 360 code therefore emits dcbz128; a stray dcbz only zeroes 32 bytes and silently leaves the rest of the line uncleared.
• Alignment is forced via mask. The effective address is masked by ~31 (dcbz) or ~127 (dcbz128) before writing — the low bits are dropped, not validated. Calling dcbz r0, r3 with r3 = 0x10037 writes zeros to 0x10000..0x1007F, not 0x10037..0x100B6.
• No memory read; pure write. Real hardware allocates the line in cache and may skip a read-from-memory fill ("cache-line zero" optimisation). Canary simulates the architectural effect without modelling cache state, and always clears 128 bytes at EA & ~127 — its comment: "On Xbox360 there is no short cache line."
• RA0 semantics. RA = 0 selects literal zero as the base, so dcbz128 0, RB zeros the line containing address RB. The update form does not exist for cache-control instructions.
• Block-fill idiom. Compilers and hand-written copy loops pair dcbz128 with stvx / stw sequences to avoid the cache-line read-allocate that a cold store would trigger. Skipping the read is the entire point.
• Privilege. dcbz is unprivileged (problem-state); does not require supervisor mode. It can fault on protection or unmapped memory like an ordinary store.
• Sequencing. Not synchronising. Pair with sync / lwsync when the zeros must be visible before subsequent loads on another thread.

Related Instructions

• dcbf — flush a line back to memory.
• dcbst — store-through (write-back without invalidate).
• dcbi — invalidate (privileged on most cores).
• dcbt, dcbtst — touch / touch-for-store hints.
• icbi — instruction-cache invalidate (companion to data-cache control).
• stvx, stw — typical pair-mates in block-fill loops.

IBM Reference

• AIX 7.3 — dcbz (Data Cache Block Set to Zero)
• Microsoft Xbox 360 XDK / Xenon Programming Guide — for dcbz128 specifics; PowerISA v2.07B Book II § "Storage Control Instructions" for the architectural baseline.