Sylpheed/tools/ppc-manual/alu/subfmex.md

subfmex — Subtract From Minus One Extended

Category: Integer ALU · Form: XO · Opcode: 0x7c0001d0

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
subfme	subfmex	—	Subtract From Minus One Extended
subfmeo	subfmex	OE=1	Subtract From Minus One Extended
subfme.	subfmex	Rc=1	Subtract From Minus One Extended
subfmeo.	subfmex	OE=1, Rc=1	Subtract From Minus One Extended

Syntax

subfme[OE][Rc] [RD], [RA]

Encoding

subfmex — form XO

• Opcode word: 0x7c0001d0
• Primary opcode (bits 0–5): 31
• Extended opcode: 232
• 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	subfmex: read	Source GPR (r0–r31).
CA	subfmex: read; subfmex: write	XER[CA] carry bit. Read by add-with-carry/subtract-with-borrow instructions, written by carrying instructions.
RD	subfmex: write	Destination GPR.
CR	subfmex: 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	subfmex: write (conditional)	Overflow-enable bit. When 1, the instruction updates XER[OV] and stickies XER[SO] on signed overflow.

Register Effects

subfmex

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

Status-Register Effects

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

subfmex

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="subfmex"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_alu.cc:486
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:83
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:871
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:325-341

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::subfmex => {
            // PPCBUG-019: also fixes the always-true CA edge — `!ra` on u64
            // is non-zero when ra32==0xFFFFFFFF and ca==0, so CA was stuck at 1.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let ca = ctx.xer_ca as u32;
            let result32 = (!ra32).wrapping_add(ca).wrapping_sub(1);
            ctx.xer_ca = if (!ra32) != 0 || ca != 0 { 1 } else { 0 };
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                let true_sum = -(ra32 as i32 as i128) - 2 + (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 + (−1) + XER[CA] ≡ ~RA − 1 + CA. Terminator for a multi-word subtract chain when the high "minuend" word is implicitly all-ones (e.g. when computing ~x style negation across many words).
• RB field unused. XO-form but only RA is read.
• Carry-out predicate. CA' = (~RA != 0) || (CA != 0) (interpreter.rs:191). Only when RA == ~0 (all ones) AND CA == 0 does CA' become 0 — every other case produces no borrow on this final word.
• 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:194.
• XER[CA] must be initialised (typically by a subfcx or subfex earlier in the chain).
• Symmetric with addmex, the add-side terminator.

Related Instructions

• subfzex — terminate with ~RA + 0 + CA instead.
• subfex — middle-of-chain.
• subfcx — chain seed.
• addmex — dual.

IBM Reference

• AIX 7.3 — subfme (Subtract From Minus One Extended)