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

subfex — Subtract From Extended

Category: Integer ALU · Form: XO · Opcode: 0x7c000110

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
subfe	subfex	—	Subtract From Extended
subfeo	subfex	OE=1	Subtract From Extended
subfe.	subfex	Rc=1	Subtract From Extended
subfeo.	subfex	OE=1, Rc=1	Subtract From Extended

Syntax

subfe[OE][Rc] [RD], [RA], [RB]

Encoding

subfex — form XO

• Opcode word: 0x7c000110
• Primary opcode (bits 0–5): 31
• Extended opcode: 136
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (31)
6–10	RT	destination GPR
11–15	RA	source A
16–20	RB	source B
21	OE	overflow-enable flag
22–30	XO	extended opcode (9 bits)
31	Rc	record-form flag

Operands

Field	Role	Description
RA	subfex: read	Source GPR (r0–r31).
RB	subfex: read	Source GPR.
RD	subfex: write	Destination GPR.
CR	subfex: write (conditional)	Condition-register update. When Rc=1, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result.
OE	subfex: write (conditional)	Overflow-enable bit. When 1, the instruction updates XER[OV] and stickies XER[SO] on signed overflow.

Register Effects

subfex

• Reads (always): RA, RB
• Reads (conditional): none
• Writes (always): RD
• Writes (conditional): CR, OE

Status-Register Effects

• subfex: CR0 ← signed-compare(result, 0) with SO ← XER[SO], when Rc=1.; XER[OV] ← signed-overflow(result); XER[SO] stickies, when OE=1.

Operation (pseudocode)

RT <- ~(RA) + (RB) + CA
CA <- carry_out

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

subfex

• Canary XML: tools/ppc-instructions.xml — search for mnem="subfex"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_alu.cc:468
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:282
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:982

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_subfex(PPCHIRBuilder& f, const InstrData& i) {
  // RT <- ¬(RA) + (RB) + CA
  Value* not_ra = f.Not(f.LoadGPR(i.XO.RA));
  Value* rb = f.LoadGPR(i.XO.RB);
  Value* v = f.AddWithCarry(not_ra, rb, f.LoadCA());
  f.StoreGPR(i.XO.RT, v);
  if (i.XO.OE) {
    XEINSTRNOTIMPLEMENTED();
    // e.update_xer_with_overflow_and_carry(b.CreateExtractValue(v, 1));
  } else {
    f.StoreCA(AddWithCarryDidCarry(f, not_ra, rb, f.LoadCA()));
  }
  if (i.XO.Rc) {
    f.UpdateCR(0, v);
  }
  return 0;
}

Special Cases & Edge Conditions

• RT ← ~RA + RB + XER[CA]. The middle link of a multi-word subtract chain seeded by subfcx. XER[CA] propagates the borrow from the previous word.
• Carry-out predicate handles the boundary case. CA' = (RB > RA) || (RB == RA && CA != 0). The second clause covers RB == RA with an incoming carry supplying the +1 — without it, the carry-out would be wrong. Canary evaluates it on the low word via AddWithCarryDidCarry(~RA, RB, CA).
• OE=1 should set XER[OV] on signed overflow; Canary's OE branch is XEINSTRNOTIMPLEMENTED() and does not store CA.
• 64-bit CR update on Xenon, 32-bit in Canary (f.UpdateCR(0, v)).
• XER[CA] must be initialised (typically by subfcx). Reading stale CA is a frequent multi-word-subtract bug.
• Symmetry with addex. subfe RT, RA, RB ≡ adde RT, ~RA, RB (with the implicit complement).

Related Instructions

• subfcx — seeds the chain (no CA read).
• subfmex, subfzex — terminate the chain (~RA + −1 + CA, ~RA + 0 + CA).
• subfx — plain subtract.
• addex — dual.

IBM Reference

• AIX 7.3 — subfe (Subtract From Extended)