Sylpheed/tools/ppc-manual/alu/subficx.md

subficx — Subtract From Immediate Carrying

Category: Integer ALU · Form: D · Opcode: 0x20000000

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
subfic	subficx	—	Subtract From Immediate Carrying

Syntax

subfic [RD], [RA], [SIMM]

Encoding

subficx — form D

• Opcode word: 0x20000000
• Primary opcode (bits 0–5): 8
• Extended opcode: —
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode
6–10	RT	destination GPR (or RS when storing)
11–15	RA	source GPR (0 ⇒ literal 0 for RA0 forms)
16–31	D/SI/UI	16-bit signed or unsigned immediate

Operands

Field	Role	Description
RA	subficx: read	Source GPR (r0–r31).
SIMM	subficx: read	16-bit signed immediate. Sign-extended to 64 bits before use.
RD	subficx: write	Destination GPR.
CA	subficx: write	XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions.

Register Effects

subficx

• Reads (always): RA, SIMM
• Reads (conditional): none
• Writes (always): RD, CA
• Writes (conditional): none

Status-Register Effects

• subficx: XER[CA] ← carry-out of the add / borrow-in of the subtract (always).

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

subficx

• Canary XML: tools/ppc-instructions.xml — search for mnem="subficx"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_alu.cc:459
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:283
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:448

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_subficx(PPCHIRBuilder& f, const InstrData& i) {
  // RT <- ¬(RA) + EXTS(SI) + 1
  Value* ra = f.LoadGPR(i.D.RA);
  Value* v = f.Sub(f.LoadConstantInt64(XEEXTS16(i.D.DS)), ra);
  f.StoreGPR(i.D.RT, v);
  f.StoreCA(SubDidCarry(f, f.LoadConstantInt64(XEEXTS16(i.D.DS)), ra));
  return 0;
}

Special Cases & Edge Conditions

• RT ← SIMM − RA with XER[CA] always set. Note the operand order: the immediate is the minuend, not the subtrahend. subfic rD, rA, 1 computes 1 - rA, useful for negation-plus-one or one's-complement-style operations.
• Immediate is sign-extended to 64 bits before the subtract. So subfic rD, rA, -1 computes -1 - rA, equivalent to ~rA.
• XER[CA] = 1 when SIMM >= RA (no borrow). Computed in xenia as if imm >= ra (interpreter.rs:73) — comparing the sign-extended unsigned representations.
• No Rc, no OE. This D-form has no flag bits beyond the implicit CA write.
• No record-form variant. There is no subfic. in the ISA; if you need CR0 to also reflect the result, follow with a cmpwi.
• Synthesised "subtract immediate". Assemblers sometimes accept subi rD, rA, value as a shorthand for addi rD, rA, -value, but for the carry-producing variant you must use subfic explicitly.
• Common idiom: subfic rD, rA, 0 computes -rA and sets CA according to whether rA == 0 (no borrow) or rA != 0 (borrow). Equivalent to negx when CA doesn't matter.

Related Instructions

• subfcx — XO-form register version.
• subfx — register subtract without XER[CA].
• addic, addicx — dual: add immediate carrying.
• negx — equivalent to subfic rD, rA, 0 when CA is unwanted.

IBM Reference

• AIX 7.3 — subfic (Subtract From Immediate Carrying)