Sylpheed/tools/ppc-manual/fpu/fmsubx.md

fmsubx — Floating Multiply-Subtract

Category: Floating-Point · Form: A · Opcode: 0xfc000038

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
fmsub	fmsubx	—	Floating Multiply-Subtract
fmsub.	fmsubx	Rc=1	Floating Multiply-Subtract

Syntax

fmsub[Rc] [FD], [FA], [FC], [FB]

Encoding

fmsubx — form A

• Opcode word: 0xfc000038
• Primary opcode (bits 0–5): 63
• Extended opcode: 28
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (59 or 63)
6–10	FRT	destination FPR
11–15	FRA	source A FPR
16–20	FRB	source B FPR
21–25	FRC	source C FPR (multiplier for madd-style ops)
26–30	XO	extended opcode (5 bits)
31	Rc	record-form flag (updates CR1)

Operands

Field	Role	Description
FA	fmsubx: read	Source A floating-point register (fr0–fr31).
FC	fmsubx: read	Source C floating-point register (for madd-style ops).
FB	fmsubx: read	Source B floating-point register.
FD	fmsubx: write	Destination floating-point register.
CR	fmsubx: write (conditional)	Condition-register update. When Rc=1, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result.
FPSCR	fmsubx: write	Floating-Point Status and Control Register.

Register Effects

fmsubx

• Reads (always): FA, FC, FB
• Reads (conditional): none
• Writes (always): FD, FPSCR
• Writes (conditional): CR

Status-Register Effects

• fmsubx: CR1 ← FPSCR[FX, FEX, VX, OX] when Rc=1.; FPSCR updated per IEEE-754 flags (FX, FEX, FPRF, FR, FI, exceptions).

Operation (pseudocode)

FRT <- (FRA × FRC) − FRB

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

fmsubx

• Canary XML: tools/ppc-instructions.xml — search for mnem="fmsubx"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_fpu.cc:205
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:80
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:1042

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_fmsubx(PPCHIRBuilder& f, const InstrData& i) {
  return InstrEmit_fmsub(f, i, false);
}

Special Cases & Edge Conditions

• Single rounding step. fmsub computes (FRA × FRC) − FRB with one rounding at the end. Canary emits vfmsub213sd on FMA3 hosts; without FMA3 it falls back to vmulsd + vsubsd, which rounds twice.
• Subtle: negate-then-FMA. Negating b before passing to FMA matters for sign of zero and overflow. (+0×+0) − (+0) = +0 in round-to-nearest, but (+0×+0) − (−0) = +0 (the negation flips it before the FMA). Standard IEEE rules apply.
• Operand order. Assembler: FD, FA, FC, FB.
• Invalid operations. 0×∞ − finite → VXIMZ; same-signed infinity collision (e.g. (+∞×+1) − (+∞)) → VXISI. Quiet NaN result with FPSCR[VX, FX].
• FPSCR side effects. Hardware updates FPRF, FR, FI, FX, OX, UX, XX, VXIMZ, VXISI, VXSNAN. Canary does not update FPSCR (UpdateFPSCR is a stub).
• Rc=1 (fmsub.) copies FPSCR[FX, FEX, VX, OX] into CR1.
• NaN propagation. Quiet-NaN result for any NaN operand; signalling NaNs are quietened.
• Use case. Newton-Raphson refinement of reciprocal estimates: x_new = x*(2 - d*x) = -((d*x) - 2) uses fnmsub, but fmsub shows up wherever (a*c) - b appears (residuals, error correction).
• Denormal flush. That Xenon boots with FPSCR[NI]=1 is unverified. Canary starts every guest thread in IEEE mode (MXCSR 0x1F80; its init comment flags the startup state as unchecked) and turns on the host's flush-to-zero (MXCSR.FZ) only when the guest sets NI through mtfsf/mtfsfi.

Related Instructions

• fmsubsx — single-precision sibling.
• fmaddx, fnmaddx, fnmsubx — other fused multiply-add variants.
• fmulx, fsubx — non-fused decomposition (two rounding steps).
• fresx, frsqrtex — reciprocal helpers refined by fused multiply-subtracts.
• fnegx — sign flip; Canary applies the same Neg to the fused result in fnmsub.

IBM Reference

• AIX 7.3 — fmsub (Floating Multiply-Subtract)
• PowerISA v2.07B, Book I, Chapter 4 — Floating-Point Processor.