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

fabsx — Floating Absolute Value

Category: Floating-Point · Form: X · Opcode: 0xfc000210

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
fabs	fabsx	—	Floating Absolute Value
fabs.	fabsx	Rc=1	Floating Absolute Value

Syntax

fabs[Rc] [FD], [FB]

Encoding

fabsx — form X

• Opcode word: 0xfc000210
• Primary opcode (bits 0–5): 63
• Extended opcode: 264
• 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
FB	fabsx: read	Source B floating-point register.
FD	fabsx: write	Destination floating-point register.
CR	fabsx: write (conditional)	Condition-register update. When Rc=1, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result.

Register Effects

fabsx

• Reads (always): FB
• Reads (conditional): none
• Writes (always): FD
• Writes (conditional): CR

Status-Register Effects

• fabsx: CR0 ← signed-compare(result, 0) with SO ← XER[SO], when Rc=1.

Operation (pseudocode)

FRT <- clear_sign(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

fabsx

• Canary XML: tools/ppc-instructions.xml — search for mnem="fabsx"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_fpu.cc:478
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:64
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:1024

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_fabsx(PPCHIRBuilder& f, const InstrData& i) {
  // frD <- abs(frB)
  Value* v = f.Abs(f.LoadFPR(i.X.RB));
  f.StoreFPR(i.X.RT, v);
  /*
  The contents of frB with bit 0 cleared are placed into frD.
Note that the fabs instruction treats NaNs just like any other kind of value.
That is, the sign bit of a NaN may be altered by fabs. This instruction does not
alter the FPSCR. Other registers altered: • Condition Register (CR1 field):
Affected: FX, FEX, VX, OX (if Rc = 1)
  */
  // f.UpdateFPSCR(v, i.X.Rc);
  if (i.X.Rc) {
    // todo
  }
  return 0;
}

Special Cases & Edge Conditions

• Bit-pattern operation, no rounding. fabs clears the sign bit (bit 0) of the source FPR's binary64 representation and writes the 64-bit value to the destination unchanged otherwise. No precision loss, no FPSCR exception bits. The mnemonic does not have an s variant — there is one form regardless of whether the operand is interpreted as binary32 or binary64.
• NaN handling. fabs(NaN) returns the same NaN with the sign bit cleared. The signalling/quiet bit is not modified, and FPSCR[VXSNAN] is not raised. Canary emits f.Abs, a pure sign-bit clear. ⚠️ Its Rc=1 branch is empty, so fabs. does not update CR1 in Canary.
• Special values. fabs(±0) = +0; fabs(±∞) = +∞; fabs(±NaN) = +NaN (sign cleared, payload preserved).
• FPSCR is largely untouched. Hardware specifies FPRF is not updated by fabs, and no exception bits are raised. Notation in the page header about FPSCR write is generic — the only meaningful write is via Rc=1.
• Rc=1 (fabs.) copies FPSCR[FX, FEX, VX, OX] into CR1 (these bits are typically stale or zero).
• No FRA operand. X-form, primary 63, XO 264. Reads FRB only; bits 11–15 are don't-care.
• Common idiom. fabs followed by fcmpu against a small constant for ULP-sized "near zero" tests; or paired with fneg/fnabs for sign-set-to-known operations.

Related Instructions

• fnegx — flip sign bit.
• fnabsx — absolute value with sign set (always negative result).
• fmrx — copy FPR (no sign manipulation).
• fselx — branch-free select; combined with fabs for min/max/clamp patterns.
• fcmpux, fcmpox — compares often paired with fabs for magnitude tests.

IBM Reference

• AIX 7.3 — fabs (Floating Absolute Value)
• PowerISA v2.07B, Book I, Chapter 4 — Floating-Point Processor (sign-bit manipulation defined as bit-pattern, not arithmetic).