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

dcbst — Data Cache Block Store

Category: Memory · Form: X · Opcode: 0x7c00006c

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
dcbst	dcbst	—	Data Cache Block Store

Syntax

dcbst [RA0], [RB]

Encoding

dcbst — form X

• Opcode word: 0x7c00006c
• Primary opcode (bits 0–5): 31
• Extended opcode: 54
• 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	dcbst: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
RB	dcbst: read	Source GPR.

Register Effects

dcbst

• 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

dcbst

• Canary XML: tools/ppc-instructions.xml — search for mnem="dcbst"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_memory.cc:1134
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:46
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:880

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_dcbst(PPCHIRBuilder& f, const InstrData& i) {
  if (!cvars::disable_prefetch_and_cachecontrol) {
    Value* ea = CalculateEA_0(f, i.X.RA, i.X.RB);
    f.CacheControl(ea, 128, CacheControlType::CACHE_CONTROL_TYPE_DATA_STORE);
  }
  return 0;
}

Special Cases & Edge Conditions

• Write-through, no invalidate. If the addressed line is dirty, it is written back to memory; the line itself remains in the cache (clean afterwards). Lighter than dcbf — the cache stays warm.
• Cache line size. Xenon's line is 128 bytes; the low seven bits of EA are ignored. There is no dcbst128; the operation is sized to the architectural line.
• RA0 semantics. RA = 0 selects literal zero as base. dcbst 0, RB pushes the line containing address RB to memory.
• Self-modifying code stage 1. The canonical "patch then run" sequence is stw (modify) → dcbst (push dirty data to memory) → sync → icbi (invalidate I-cache for the same address) → isync. dcbst is preferred over dcbf here because it leaves the data in D-cache for any subsequent normal reads.
• DMA hand-off. Used before initiating a GPU or DMA read of a buffer the CPU has just written, to ensure memory holds the latest data.
• Unprivileged. Available from problem state.
• Xenia models as no-op. No cache state is simulated; PC advances and memory is already authoritative.

Related Instructions

• dcbf — flush + invalidate (heavier alternative).
• dcbi — invalidate without write-back (privileged).
• dcbz, dcbz128 — allocate-and-zero.
• dcbt, dcbtst — prefetch hints.
• icbi — instruction-cache invalidate, sequenced after dcbst in self-modifying-code recipes.
• sync, isync — ordering primitives that bracket cache control.

IBM Reference

• AIX 7.3 — dcbst (Data Cache Block Store)
• PowerISA v2.07B Book II § "Storage Control Instructions".