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

subfmex — Subtract From Minus One Extended

Category: Integer ALU · Form: XO · Opcode: 0x7c0001d0

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
subfme	subfmex	—	Subtract From Minus One Extended
subfmeo	subfmex	OE=1	Subtract From Minus One Extended
subfme.	subfmex	Rc=1	Subtract From Minus One Extended
subfmeo.	subfmex	OE=1, Rc=1	Subtract From Minus One Extended

Syntax

subfme[OE][Rc] [RD], [RA]

Encoding

subfmex — form XO

• Opcode word: 0x7c0001d0
• Primary opcode (bits 0–5): 31
• Extended opcode: 232
• 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	subfmex: read	Source GPR (r0–r31).
CA	subfmex: read; subfmex: write	XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions.
RD	subfmex: write	Destination GPR.
CR	subfmex: 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	subfmex: write (conditional)	Overflow-enable bit. When 1, the instruction updates XER[OV] and stickies XER[SO] on signed overflow.

Register Effects

subfmex

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

Status-Register Effects

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

subfmex

• Canary XML: tools/ppc-instructions.xml — search for mnem="subfmex"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_alu.cc:486
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:284
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:986

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_subfmex(PPCHIRBuilder& f, const InstrData& i) {
  // RT <- ¬(RA) + CA - 1
  Value* not_ra = f.Not(f.LoadGPR(i.XO.RA));
  Value* v = f.AddWithCarry(not_ra, f.LoadConstantInt64(-1), 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, f.LoadConstantInt64(-1), f.LoadCA()));
  }
  if (i.XO.Rc) {
    f.UpdateCR(0, v);
  }
  return 0;
}

Special Cases & Edge Conditions

• RT ← ~RA + (−1) + XER[CA] ≡ ~RA − 1 + CA. Terminator for a multi-word subtract chain when the high "minuend" word is implicitly all-ones (e.g. when computing ~x style negation across many words).
• RB field unused. XO-form but only RA is read.
• Carry-out predicate. CA' = (~RA != 0) || (CA != 0). Only when RA == ~0 (all ones) AND CA == 0 does CA' become 0 — every other case produces no borrow on this final word. Canary evaluates it on the low word (AddWithCarryDidCarry).
• 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 a subfcx or subfex earlier in the chain).
• Symmetric with addmex, the add-side terminator.

Related Instructions

• subfzex — terminate with ~RA + 0 + CA instead.
• subfex — middle-of-chain.
• subfcx — chain seed.
• addmex — dual.

IBM Reference

• AIX 7.3 — subfme (Subtract From Minus One Extended)