Sylpheed/tools/ppc-manual/memory/lfs.md

lfs — Load Floating-Point Single

Category: Memory · Form: D · Opcode: 0xc0000000

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
lfs	lfs	—	Load Floating-Point Single
lfsu	lfsu	—	Load Floating-Point Single with Update
lfsux	lfsux	—	Load Floating-Point Single with Update Indexed
lfsx	lfsx	—	Load Floating-Point Single Indexed

Syntax

lfs [FD], [d]([RA0])
lfsu [FD], [d]([RA])
lfsux [FD], [RA], [RB]
lfsx [FD], [RA0], [RB]

Encoding

lfs — form D

• Opcode word: 0xc0000000
• Primary opcode (bits 0–5): 48
• 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

lfsu — form D

• Opcode word: 0xc4000000
• Primary opcode (bits 0–5): 49
• 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

lfsux — form X

• Opcode word: 0x7c00046e
• Primary opcode (bits 0–5): 31
• Extended opcode: 567
• 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

lfsx — form X

• Opcode word: 0x7c00042e
• Primary opcode (bits 0–5): 31
• Extended opcode: 535
• 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
RA0	lfs: read; lfsx: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
d	lfs: read; lfsu: read	16-bit signed displacement (d) added to the base address register.
FD	lfs: write; lfsu: write; lfsux: write; lfsx: write	Destination floating-point register.
RA	lfsu: read; lfsu: write; lfsux: read; lfsux: write	Source GPR (r0–r31).
RB	lfsux: read; lfsx: read	Source GPR.

Register Effects

lfs

• Reads (always): RA0, d
• Reads (conditional): none
• Writes (always): FD
• Writes (conditional): none

lfsu

• Reads (always): RA, d
• Reads (conditional): none
• Writes (always): FD, RA
• Writes (conditional): none

lfsux

• Reads (always): RA, RB
• Reads (conditional): none
• Writes (always): FD, RA
• Writes (conditional): none

lfsx

• Reads (always): RA0, RB
• Reads (conditional): none
• Writes (always): FD
• Writes (conditional): none

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

EA <- (RA|0) + EXTS(d)
FRT <- DoubleFromSingle(MEM(EA, 4))

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

lfs

• Canary XML: tools/ppc-instructions.xml — search for mnem="lfs"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_memory.cc:960
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:117
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:486

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_lfs(PPCHIRBuilder& f, const InstrData& i) {
  // if RA = 0 then
  //   b <- 0
  // else
  //   b <- (RA)
  // EA <- b + EXTS(D)
  // FRT <- DOUBLE(MEM(EA, 4))
  Value* ea = CalculateEA_0_i(f, i.D.RA, XEEXTS16(i.D.DS));
  Value* rt = f.Convert(
      f.Cast(f.ByteSwap(f.Load(ea, INT32_TYPE)), FLOAT32_TYPE), FLOAT64_TYPE);
  f.StoreFPR(i.D.RT, rt);
  return 0;
}

lfsu

• Canary XML: tools/ppc-instructions.xml — search for mnem="lfsu"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_memory.cc:974
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:118
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:487

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_lfsu(PPCHIRBuilder& f, const InstrData& i) {
  // EA <- (RA) + EXTS(D)
  // FRT <- DOUBLE(MEM(EA, 4))
  // RA <- EA
  Value* ea = CalculateEA_i(f, i.D.RA, XEEXTS16(i.D.DS));
  Value* rt = f.Convert(
      f.Cast(f.ByteSwap(f.Load(ea, INT32_TYPE)), FLOAT32_TYPE), FLOAT64_TYPE);
  f.StoreFPR(i.D.RT, rt);
  StoreEA(f, i.D.RA, ea);
  return 0;
}

lfsux

• Canary XML: tools/ppc-instructions.xml — search for mnem="lfsux"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_memory.cc:986
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:119
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:938

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_lfsux(PPCHIRBuilder& f, const InstrData& i) {
  // EA <- (RA) + (RB)
  // FRT <- DOUBLE(MEM(EA, 4))
  // RA <- EA
  Value* ea = CalculateEA(f, i.X.RA, i.X.RB);
  Value* rt = f.Convert(
      f.Cast(f.ByteSwap(f.Load(ea, INT32_TYPE)), FLOAT32_TYPE), FLOAT64_TYPE);
  f.StoreFPR(i.X.RT, rt);
  StoreEA(f, i.X.RA, ea);
  return 0;
}

lfsx

• Canary XML: tools/ppc-instructions.xml — search for mnem="lfsx"
• Canary emitter: src/xenia/cpu/ppc/ppc_emit_memory.cc:998
• Sylpheed opcode: crates/sylpheed-ppc/src/opcode.rs:120
• Sylpheed decoder: crates/sylpheed-ppc/src/decoder.rs:934

Canary emitter (frozen snapshot @ f21ebd49e9)

int InstrEmit_lfsx(PPCHIRBuilder& f, const InstrData& i) {
  // if RA = 0 then
  //   b <- 0
  // else
  //   b <- (RA)
  // EA <- b + (RB)
  // FRT <- DOUBLE(MEM(EA, 4))
  Value* ea = CalculateEA_0(f, i.X.RA, i.X.RB);
  Value* rt = f.Convert(
      f.Cast(f.ByteSwap(f.Load(ea, INT32_TYPE)), FLOAT32_TYPE), FLOAT64_TYPE);
  f.StoreFPR(i.X.RT, rt);
  return 0;
}

Special Cases & Edge Conditions

• Single → double in-register. Reads 4 bytes as IEEE binary32, then exactly converts to binary64 (every binary32 has a representation in binary64). The result occupies all 64 bits of the FPR; subsequent FP arithmetic operates in double regardless of the value's origin.
• No FPSCR side effects. The single→double widening is exact, so lfs cannot raise inexact, overflow, underflow, or invalid. A signalling NaN passes through unchanged into the FPR — it will signal at the next FP arithmetic instruction.
• Subnormals. A binary32 subnormal expands to a binary64 normal — lfs quietly normalises. There is no "FPSCR[NI] non-IEEE mode" subnormal-to-zero behaviour applied at this stage on Xenon (NI affects arithmetic, not loads).
• RA0 semantics. In lfs / lfsx, RA = 0 selects literal zero. Update forms lfsu / lfsux are invalid with RA = 0.
• Alignment. Xenon tolerates unaligned 4-byte loads; PowerISA permits implementations to raise alignment exceptions for FP loads on cache-inhibited storage.
• Big-endian read. Bytes EA..EA+3 form the binary32 pattern, sign bit at EA[7]. Xenia's mem.read_f32 handles host byte-swap.
• MSR[FP] required. Disabled FP unit raises Floating-Point Unavailable.
• Pair with stfs. Store-single performs the inverse double→single rounding (which can raise FPSCR exceptions because that direction may be inexact).

Related Instructions

• lfd — double-precision load (no format conversion).
• stfs, stfsu, stfsx, stfsux — corresponding stores; these can round.
• stfiwx — store-FP-as-integer-word.
• lwz — integer word load (same width, GPR target).

IBM Reference

• AIX 7.3 — lfs (Load Floating-Point Single)
• AIX 7.3 — lfsu / lfsx / lfsux