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

subfex — Subtract From Extended

Category: Integer ALU · Form: XO · Opcode: 0x7c000110

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
subfe	subfex	—	Subtract From Extended
subfeo	subfex	OE=1	Subtract From Extended
subfe.	subfex	Rc=1	Subtract From Extended
subfeo.	subfex	OE=1, Rc=1	Subtract From Extended

Syntax

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

Encoding

subfex — form XO

• Opcode word: 0x7c000110
• Primary opcode (bits 0–5): 31
• Extended opcode: 136
• 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	subfex: read	Source GPR (r0–r31).
RB	subfex: read	Source GPR.
RD	subfex: write	Destination GPR.
CR	subfex: 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	subfex: write (conditional)	Overflow-enable bit. When 1, the instruction updates XER[OV] and stickies XER[SO] on signed overflow.

Register Effects

subfex

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

Status-Register Effects

• subfex: 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) + CA
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

subfex

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="subfex"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_alu.cc:468
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:83
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:867
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:288-306

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::subfex => {
            // PPCBUG-008: 32-bit ABI. Compute in u32 space — `!ra` on u64 always
            // pollutes the upper 32 bits, making this an active poisoner.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let rb32 = ctx.gpr[instr.rb()] as u32;
            let ca = ctx.xer_ca as u32;
            let result32 = (!ra32).wrapping_add(rb32).wrapping_add(ca);
            ctx.xer_ca = if rb32 > ra32 || (rb32 == ra32 && ca != 0) { 1 } else { 0 };
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                // RT <- !RA + RB + CA  ==  RB - RA - 1 + CA  (32-bit semantics).
                let true_sum = (rb32 as i32 as i128) - (ra32 as i32 as i128) - 1 + (ca as i128);
                overflow::apply(ctx, true_sum != (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 ← ~RA + RB + XER[CA]. The middle link of a multi-word subtract chain seeded by subfcx. XER[CA] propagates the borrow from the previous word.
• Carry-out predicate handles the boundary case. Xenia computes CA' = (rb > ra) || (rb == ra && CA != 0) (interpreter.rs:170). The second clause covers when RB == RA and the previous chain added a +1 from the input carry — without it, the carry-out would be wrong.
• OE=1 should set XER[OV] on signed overflow; xenia-rs ignores it.
• 64-bit CR update on Xenon, 32-bit in xenia-rs. interpreter.rs:173.
• XER[CA] must be initialised (typically by subfcx). Reading stale CA is a frequent multi-word-subtract bug.
• Symmetry with addex. subfe RT, RA, RB ≡ adde RT, ~RA, RB (with the implicit complement).

Related Instructions

• subfcx — seeds the chain (no CA read).
• subfmex, subfzex — terminate the chain (~RA + −1 + CA, ~RA + 0 + CA).
• subfx — plain subtract.
• addex — dual.

IBM Reference

• AIX 7.3 — subfe (Subtract From Extended)