xenia-rs/migration/project-root/ppc-manual/control/mtfsfix.md

mtfsfix — Move to FPSCR Field Immediate

Category: Control / CR / SPR · Form: X · Opcode: 0xfc00010c

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
mtfsfi	mtfsfix	—	Move to FPSCR Field Immediate
mtfsfi.	mtfsfix	Rc=1	Move to FPSCR Field Immediate

Syntax

mtfsfi[Rc] [CRFD], [IMM]

Encoding

mtfsfix — form X

• Opcode word: 0xfc00010c
• Primary opcode (bits 0–5): 63
• Extended opcode: 134
• 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
IMM	mtfsfix: read	Generic immediate field.
CRFD	mtfsfix: write	CR destination field (crf, 0–7).
CR	mtfsfix: 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

mtfsfix

• Reads (always): IMM
• Reads (conditional): none
• Writes (always): CRFD
• Writes (conditional): CR

Status-Register Effects

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

Operation (pseudocode)

; Pseudocode derives directly from the xenia-rs interpreter
; arm (see Implementation References). Operation semantics:
;   - Read source operands from the fields listed under Operands.
;   - Apply the arithmetic / logical / memory action described
;     in the Description field above.
;   - Write results to the destination register(s); update any
;     status bits enumerated under Status-Register Effects.
; Consult the IBM AIX reference link under IBM Reference for
; canonical PPC-style pseudocode where xenia's expression is
; terse.

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

mtfsfix

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="mtfsfix"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_fpu.cc:452
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:55
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:907
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:3069-3077

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::mtfsfix => {
            // Move to FPSCR field immediate: crfD = IMM (4 bits)
            let crfd = instr.crfd();
            let imm = (instr.raw >> 12) & 0xF;
            let shift = 28 - crfd as u32 * 4;
            ctx.fpscr = (ctx.fpscr & !(0xF << shift)) | (imm << shift);
            if instr.rc_bit() { update_cr1_from_fpscr(ctx); }
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Operation. Loads a 4-bit immediate (IMM, encoded in instruction bits 16..19) into a single FPSCR field selected by CRFD (3-bit, 0..7). All other FPSCR fields are preserved.
• Most common use: rounding-mode set. mtfsfi 7, 0 selects round-to-nearest, mtfsfi 7, 1 round-toward-zero, mtfsfi 7, 2 round-toward-+∞, mtfsfi 7, 3 round-toward-−∞. The four immediate values map to RN per IEEE-754. Compilers emit this when transitioning into and out of strict-IEEE regions.
• No FPR source. Unlike mtfsfx, mtfsfi doesn't need an FPR — it carries its 4-bit value in the instruction word, making it cheaper for constant updates.
• Rc=1. mtfsfi. copies FPSCR's top 4 bits (FX, FEX, VX, OX) into CR1 after the write.
• Restrictions in newer PowerISA. v2.05+ disallows writing FEX/VX (summary bits) via mtfsfi. xenia-rs does not enforce this — the immediate goes straight into the chosen field.
• xenia simplification. xenia stores FPSCR as a u32 and applies the field-shift correctly. Same caveat as mtfsf: most xenia FP paths don't honour FPSCR, so the rounding-mode change is architecturally visible (via mffsx) but typically does not change subsequent FP results.
• Not synchronising. PowerISA recommends isync after rounding-mode changes if subsequent FP correctness depends on the new mode.

Related Instructions

• mtfsfx — write FPSCR fields from an FPR (uses 8-bit field-mask).
• mtfsb0x, mtfsb1x — clear / set a single FPSCR bit.
• mffsx — read FPSCR.
• mcrfs — FPSCR field → CR field (also clears sticky bits).

mtfsfi is the simplified form (Rc=0); mtfsfi. is the recording variant.

IBM Reference

• AIX 7.3 — mtfsfi (Move to FPSCR Field Immediate)
• PowerISA v2.07B, Book I §4.6.6 — FPSCR-field write semantics and the FEX/VX restriction.