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

subfcx — Subtract From Carrying

Category: Integer ALU · Form: XO · Opcode: 0x7c000010

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
subfc	subfcx	—	Subtract From Carrying
subfco	subfcx	OE=1	Subtract From Carrying
subfc.	subfcx	Rc=1	Subtract From Carrying
subfco.	subfcx	OE=1, Rc=1	Subtract From Carrying

Syntax

subfc[OE][Rc] [RD], [RA], [RB]

Encoding

subfcx — form XO

• Opcode word: 0x7c000010
• Primary opcode (bits 0–5): 31
• Extended opcode: 8
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (31)
6–10	RT	destination GPR
11–15	RA	source A
16–20	RB	source B
21	OE	overflow-enable flag
22–30	XO	extended opcode (9 bits)
31	Rc	record-form flag

Operands

Field	Role	Description
RA	subfcx: read	Source GPR (r0–r31).
RB	subfcx: read	Source GPR.
RD	subfcx: write	Destination GPR.
CR	subfcx: write (conditional)	Condition-register update. When Rc=1, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result.
OE	subfcx: write (conditional)	Overflow-enable bit. When 1, the instruction updates XER[OV] and stickies XER[SO] on signed overflow.

Register Effects

subfcx

• Reads (always): RA, RB
• Reads (conditional): none
• Writes (always): RD
• Writes (conditional): CR, OE

Status-Register Effects

• subfcx: CR0 ← signed-compare(result, 0) with SO ← XER[SO], when Rc=1.; XER[OV] ← signed-overflow(result); XER[SO] stickies, when OE=1.

Operation (pseudocode)

RT <- ~(RA) + (RB) + 1
CA <- carry_out

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

subfcx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="subfcx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_alu.cc:441
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:83
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:859
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:270-287

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::subfcx => {
            // PPCBUG-007: 32-bit ABI. The `rb >= ra` u64 unsigned compare is
            // exactly the shape that broke addis. Defensive 32-bit truncation
            // is required for correct CA even after upstream cleanup.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let rb32 = ctx.gpr[instr.rb()] as u32;
            let result32 = rb32.wrapping_sub(ra32);
            ctx.xer_ca = if rb32 >= ra32 { 1 } else { 0 };
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                let true_diff = (rb32 as i32 as i128) - (ra32 as i32 as i128);
                overflow::apply(ctx, true_diff != (result32 as i32) as i128);
            }
            if instr.rc_bit() {
                ctx.update_cr_signed(0, result32 as i32 as i64);
            }
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• RT ← RB − RA with XER[CA] set on no-borrow. Same operand-order convention as subfx: the first source is subtracted from the second.
• XER[CA] = 1 means no borrow occurred — i.e. RB >= RA as unsigned. PowerISA encodes this as the carry-out of ~RA + RB + 1, not as a borrow flag. Xenia's if rb >= ra test (interpreter.rs:157) is the correct boolean encoding.
• No trap on signed overflow. subfco / subfco. set XER[OV] and sticky XER[SO]; xenia-rs leaves the OE arm unimplemented.
• 64-bit CR update on Xenon, 32-bit in xenia-rs. interpreter.rs:160 truncates with as i32 as i64. Spec demands a full 64-bit signed compare for subfc..
• Seeds a multi-word subtract chain. Use as the low-word op; continue with subfex for middle words and subfmex/subfzex for the high word.
• Operand aliasing fine. subfc r3, r3, r3 always yields 0 with CA = 1.

Related Instructions

• subfx — same op without XER[CA].
• subfex — ~RA + RB + CA (chain continuation).
• subfmex, subfzex — chain terminators.
• subfic — D-form: RT ← SIMM − RA with XER[CA].
• addcx — dual: addition seed-of-chain.

IBM Reference

• AIX 7.3 — subfc (Subtract From Carrying)