xenia-rs/migration/project-root/ppc-manual/alu/subficx.md

subficx — Subtract From Immediate Carrying

Category: Integer ALU · Form: D · Opcode: 0x20000000

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
subfic	subficx	—	Subtract From Immediate Carrying

Syntax

subfic [RD], [RA], [SIMM]

Encoding

subficx — form D

• Opcode word: 0x20000000
• Primary opcode (bits 0–5): 8
• 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

Operands

Field	Role	Description
RA	subficx: read	Source GPR (r0–r31).
SIMM	subficx: read	16-bit signed immediate. Sign-extended to 64 bits before use.
RD	subficx: write	Destination GPR.
CA	subficx: write	XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions.

Register Effects

subficx

• Reads (always): RA, SIMM
• Reads (conditional): none
• Writes (always): RD, CA
• Writes (conditional): none

Status-Register Effects

• subficx: XER[CA] ← carry-out of the add / borrow-in of the subtract (always).

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

subficx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="subficx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_alu.cc:459
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:83
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:333
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:155-164

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::subficx => {
            // PPCBUG-005: 32-bit ABI. Sign-extended imm has bits 32-63 set for
            // negative SIMM, poisoning the writeback. Canary uses 32-bit form.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let imm32 = instr.simm16() as i32 as u32;
            let result32 = imm32.wrapping_sub(ra32);
            ctx.xer_ca = if imm32 >= ra32 { 1 } else { 0 };
            ctx.gpr[instr.rd()] = result32 as u64;
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• RT ← SIMM − RA with XER[CA] always set. Note the operand order: the immediate is the minuend, not the subtrahend. subfic rD, rA, 1 computes 1 - rA, useful for negation-plus-one or one's-complement-style operations.
• Immediate is sign-extended to 64 bits before the subtract. So subfic rD, rA, -1 computes -1 - rA, equivalent to ~rA.
• XER[CA] = 1 when SIMM >= RA (no borrow). Computed in xenia as if imm >= ra (interpreter.rs:73) — comparing the sign-extended unsigned representations.
• No Rc, no OE. This D-form has no flag bits beyond the implicit CA write.
• No record-form variant. There is no subfic. in the ISA; if you need CR0 to also reflect the result, follow with a cmpwi.
• Synthesised "subtract immediate". Assemblers sometimes accept subi rD, rA, value as a shorthand for addi rD, rA, -value, but for the carry-producing variant you must use subfic explicitly.
• Common idiom: subfic rD, rA, 0 computes -rA and sets CA according to whether rA == 0 (no borrow) or rA != 0 (borrow). Equivalent to negx when CA doesn't matter.

Related Instructions

• subfcx — XO-form register version.
• subfx — register subtract without XER[CA].
• addic, addicx — dual: add immediate carrying.
• negx — equivalent to subfic rD, rA, 0 when CA is unwanted.

IBM Reference

• AIX 7.3 — subfic (Subtract From Immediate Carrying)