xenia-rs/migration/project-root/ppc-manual/vmx/vsubcuw.md

vsubcuw — Vector Subtract Carryout Unsigned Word

Category: VMX (Altivec) · Form: VX · Opcode: 0x10000580

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
vsubcuw	vsubcuw	—	Vector Subtract Carryout Unsigned Word

Syntax

vsubcuw [VD], [VA], [VB]

Encoding

vsubcuw — form VX

• Opcode word: 0x10000580
• Primary opcode (bits 0–5): 4
• Extended opcode: 1408
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (4)
6–10	VRT/VD	destination vector register
11–15	VRA/VA	source A vector register
16–20	VRB/VB	source B vector register
21–31	XO	extended opcode (11 bits)

Operands

Field	Role	Description
VA	vsubcuw: read	Source A vector register.
VB	vsubcuw: read	Source B vector register.
VD	vsubcuw: write	Destination vector register.

Register Effects

vsubcuw

• Reads (always): VA, VB
• Reads (conditional): none
• Writes (always): VD
• Writes (conditional): none

Status-Register Effects

No condition-register or status-register effects.

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

vsubcuw

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="vsubcuw"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:1671
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:125
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:536
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:3391-3399

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::vsubcuw => {
            // "Subtract Carryout": r = 1 if a >= b (no borrow), 0 otherwise.
            let a = ctx.vr[instr.ra()].as_u32x4();
            let b = ctx.vr[instr.rb()].as_u32x4();
            let mut r = [0u32; 4];
            for i in 0..4 { r[i] = if a[i] >= b[i] { 1 } else { 0 }; }
            ctx.vr[instr.rd()] = xenia_types::Vec128::from_u32x4_array(r);
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• "Borrow out" producer for unsigned word subtract. Each of the 4 lanes produces 1 if VA.w[i] >= VB.w[i] (no borrow) and 0 otherwise. This is an inverted borrow — conventional borrow would be 1 on underflow, but Altivec's vsubcuw returns the opposite to match the XER[CA] convention used by scalar subfc / subfe.
• Complements vadduwm / vaddcuw for 256-bit (or wider) multi-precision arithmetic. After the lane subtract, chain the 4-bit borrow vector into the next word via vsubeuwm-style helpers (or manual software glue, since Altivec has no direct sube).
• No saturation, no flags, no VSCR effect. Despite being in the "carry" family, vsubcuw doesn't touch VSCR[SAT].
• Big-endian word lanes.
• No VMX128 sibling.

Related Instructions

• vsubuwm — the difference value (unsigned modulo).
• vaddcuw — the paired carry-out producer for unsigned-add.
• vadduwm — unsigned word add (modulo).

IBM Reference

• AIX 7.3 — vsubcuw (Vector Subtract and Write Carry-Out Unsigned Word)
• IBM AltiVec Technology Programmer's Interface Manual, Chapter 4 — Integer Arithmetic