xenia-rs/migration/project-root/ppc-manual/memory/stfd.md

stfd — Store Floating-Point Double

Category: Memory · Form: D · Opcode: 0xd8000000

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
stfd	stfd	—	Store Floating-Point Double
stfdu	stfdu	—	Store Floating-Point Double with Update
stfdux	stfdux	—	Store Floating-Point Double with Update Indexed
stfdx	stfdx	—	Store Floating-Point Double Indexed

Syntax

stfd [FS], [d]([RA0])
stfdu [FS], [d]([RA])
stfdux [FS], [RA], [RB]
stfdx [FS], [RA0], [RB]

Encoding

stfd — form D

• Opcode word: 0xd8000000
• Primary opcode (bits 0–5): 54
• 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

stfdu — form D

• Opcode word: 0xdc000000
• Primary opcode (bits 0–5): 55
• 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

stfdux — form X

• Opcode word: 0x7c0005ee
• Primary opcode (bits 0–5): 31
• Extended opcode: 759
• 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

stfdx — form X

• Opcode word: 0x7c0005ae
• Primary opcode (bits 0–5): 31
• Extended opcode: 727
• 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
FS	stfd: read; stfdu: read; stfdux: read; stfdx: read	Source floating-point register.
RA0	stfd: read; stfdx: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
d	stfd: read; stfdu: read	16-bit signed displacement (d) added to the base address register.
RA	stfdu: read; stfdu: write; stfdux: read; stfdux: write	Source GPR (r0–r31).
RB	stfdux: read; stfdx: read	Source GPR.

Register Effects

stfd

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

stfdu

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

stfdux

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

stfdx

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

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

EA <- (RA|0) + EXTS(d)
MEM(EA, 8) <- (FRS)

C Translation Example

/* C translation: the xenia-rs interpreter arm below in           */
/* Implementation References is the authoritative semantic        */
/* snapshot. Translate it line-by-line:                            */
/*   - ctx.gpr[N]  -> r[N]       (or f[]/v[] for FPRs/VRs)        */
/*   - mem.read_u*/write_u* -> mem_read_u*_be / mem_write_u*_be   */
/*   - ctx.update_cr_signed(fld, v) -> update_cr_signed(fld, v)   */
/*   - ctx.xer_ca / xer_ov / xer_so -> xer.CA / xer.OV / xer.SO   */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */

Implementation References

stfd

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="stfd"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:1014
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:71
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:377
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1473-1481

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::stfd => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(instr.d() as i64 as u64) as u32;
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() { t.invalidate_for_write(ea); }
            }
            mem.write_f64(ea, ctx.fpr[instr.rs()]);
            ctx.pc += 4;
        }

stfdu

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="stfdu"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:1026
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:71
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:378
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1482-1490

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::stfdu => {
            let ea = ctx.gpr[instr.ra()].wrapping_add(instr.d() as i64 as u64) as u32;
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() { t.invalidate_for_write(ea); }
            }
            mem.write_f64(ea, ctx.fpr[instr.rs()]);
            ctx.gpr[instr.ra()] = ea as u64;
            ctx.pc += 4;
        }

stfdux

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="stfdux"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:1036
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:71
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:837
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1500-1508

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::stfdux => {
            let ea = ctx.gpr[instr.ra()].wrapping_add(ctx.gpr[instr.rb()]) as u32;
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() { t.invalidate_for_write(ea); }
            }
            mem.write_f64(ea, ctx.fpr[instr.rs()]);
            ctx.gpr[instr.ra()] = ea as u64;
            ctx.pc += 4;
        }

stfdx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="stfdx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:1046
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:71
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:836
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1491-1499

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::stfdx => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(ctx.gpr[instr.rb()]) as u32;
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() { t.invalidate_for_write(ea); }
            }
            mem.write_f64(ea, ctx.fpr[instr.rs()]);
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Bit-exact double store. Writes the 64-bit IEEE binary64 contents of FRS directly to memory; no rounding, no format conversion. The xenia snapshot calls mem.write_f64(ea, ctx.fpr[instr.rs()]), which preserves the exact bit pattern (including signalling NaNs).
• No FPSCR side effects. Like lfd, stfd cannot raise IEEE exceptions: there is no rounding step. Contrast stfs, where double→single rounding can raise inexact / overflow / underflow.
• RA0 (non-update forms). RA = 0 in stfd and stfdx selects literal zero. Update forms stfdu / stfdux invoke RA = 0 as an invalid form.
• Update-form post-write. stfdu / stfdux write the computed EA back to RA after the store. No FRS / RA collision possible — RS is an FPR, RA is a GPR.
• Big-endian write. Byte at EA is the FPR's most-significant byte (sign + part of exponent), byte at EA+7 is the least-significant mantissa byte. Xenia's mem.write_f64 performs host-side byte-swap.
• Alignment. Xenon tolerates unaligned 8-byte FP stores. PowerISA permits implementations to raise alignment exceptions on cache-inhibited storage.
• MSR[FP] required. Disabled FP unit raises Floating-Point Unavailable.

Related Instructions

• lfd, lfdu, lfdx, lfdux — corresponding loads.
• stfs — single-precision store with format conversion (can raise FPSCR).
• stfiwx — store low 32 bits of FPR as integer word.
• std — integer doubleword store (same width, GPR source).

IBM Reference

• AIX 7.3 — stfd (Store Floating-Point Double)
• AIX 7.3 — stfdu / stfdx / stfdux