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

cmpi — Compare Immediate

Category: Integer ALU · Form: D · Opcode: 0x2c000000

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
cmpi	cmpi	—	Compare Immediate

Syntax

cmpi [CRFD], [L], [RA], [SIMM]

Encoding

cmpi — form D

• Opcode word: 0x2c000000
• Primary opcode (bits 0–5): 11
• 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
L	cmpi: read	Operand-length bit for compare instructions (0 ⇒ 32-bit, 1 ⇒ 64-bit).
RA	cmpi: read	Source GPR (r0–r31).
SIMM	cmpi: read	16-bit signed immediate. Sign-extended to 64 bits before use.
RD	cmpi: write	Destination GPR.
CRFD	cmpi: write	CR destination field (crf, 0–7).

Register Effects

cmpi

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

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

if L = 0 then a,b <- EXTS((RA)[32:63]), EXTS(SIMM)
else             a,b <- (RA), EXTS(SIMM)
CR[BF] <- signed_compare(a, b) || XER[SO]

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

cmpi

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="cmpi"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_alu.cc:552
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:13
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:335
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:824-849

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::cmpi => {
            let bf = instr.crfd();
            if instr.l() {
                // 64-bit compare. Compare directly so boundary i64 values
                // (e.g. ra=i64::MIN, imm=1) don't mis-sign through a
                // wrapped subtract.
                let ra = ctx.gpr[instr.ra()] as i64;
                let imm = instr.simm16() as i64;
                ctx.cr[bf] = crate::context::CrField {
                    lt: ra < imm,
                    gt: ra > imm,
                    eq: ra == imm,
                    so: ctx.xer_so != 0,
                };
            } else {
                let ra = ctx.gpr[instr.ra()] as i32;
                let imm = instr.simm16() as i32;
                ctx.cr[bf] = crate::context::CrField {
                    lt: ra < imm,
                    gt: ra > imm,
                    eq: ra == imm,
                    so: ctx.xer_so != 0,
                };
            }
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Immediate is sign-extended. SIMM is treated as a signed 16-bit value in the range [-32768, 32767] and sign-extended to the operand width. Use cmpli for unsigned comparisons against a 16-bit value.
• L bit selects width. L = 0 (the usual cmpwi) compares the low 32 bits of RA (sign-extended) against EXTS(SIMM); L = 1 (cmpdi) does a full 64-bit signed compare. Most Xbox 360 code uses cmpwi because pointers and counters are 32-bit ABI.
• Simplified mnemonics dominate disassembly. cmpwi crN, RA, SIMM ≡ cmpi crN, 0, RA, SIMM and cmpdi crN, RA, SIMM ≡ cmpi crN, 1, RA, SIMM. The default CR field is cr0 if omitted.
• BF is a CR field (0–7), not a bit. Same convention as cmp. Distinct standalone compares should target cr1..cr7 to avoid clobbering the implicit cr0 chain set up by Rc=1 arithmetic.
• SO is copied from XER[SO]. This makes overflow observable downstream of an addo. / mulo. etc. via bso/bns.
• Xenia-rs quirk. The interpreter recomputes EQ after the signed subtract, defending against the same 32-bit narrowing edge case noted in cmp. Functionally equivalent to spec.
• No register written other than the 4-bit CR field — there is no Rc or OE bit.

Related Instructions

• cmp — register-register signed compare.
• cmpli — unsigned compare against immediate (zero-extended).
• cmpl — unsigned register compare.
• cmpwi, cmpdi (simplified mnemonics) — select L=0 / L=1.
• mcrxr — clear sticky overflow before a fresh compare sequence.

IBM Reference

• AIX 7.3 — cmpi (Compare Immediate)
• AIX 7.3 — cmpwi / cmpdi (simplified mnemonics)