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

negx — Negate

Category: Integer ALU · Form: XO · Opcode: 0x7c0000d0

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
neg	negx	—	Negate
nego	negx	OE=1	Negate
neg.	negx	Rc=1	Negate
nego.	negx	OE=1, Rc=1	Negate

Syntax

neg[OE][Rc] [RD], [RA]

Encoding

negx — form XO

• Opcode word: 0x7c0000d0
• Primary opcode (bits 0–5): 31
• Extended opcode: 104
• 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	negx: read	Source GPR (r0–r31).
RD	negx: write	Destination GPR.
CR	negx: 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	negx: write (conditional)	Overflow-enable bit. When 1, the instruction updates XER[OV] and stickies XER[SO] on signed overflow.

Register Effects

negx

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

Status-Register Effects

• negx: 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) + 1

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

negx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="negx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_alu.cc:406
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:59
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:866
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:342-356

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::negx => {
            // PPCBUG-006: 32-bit ABI. `(!ra).wrapping_add(1)` on u64 always
            // sets upper 32 bits — every neg poisoned the GPR. neg_ov also
            // checks at 64-bit INT_MIN; should be 32-bit INT_MIN.
            let ra32 = ctx.gpr[instr.ra()] as u32;
            let result32 = (!ra32).wrapping_add(1);
            ctx.gpr[instr.rd()] = result32 as u64;
            if instr.oe() {
                overflow::apply(ctx, ra32 == 0x8000_0000);
            }
            if instr.rc_bit() {
                ctx.update_cr_signed(0, result32 as i32 as i64);
            }
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Two's-complement negate. RT ← ~RA + 1, equivalent to 0 − RA. A specialisation of subfx where RB is implicit zero.
• INT64_MIN is its own negation. neg(0x8000000000000000) = 0x8000000000000000 — the only fixed point. nego should set XER[OV] in this case (it is the canonical signed-overflow trigger), but xenia-rs does not implement OE (interpreter.rs:201 has no oe() branch).
• RB field unused. Set to 0 by assemblers; ignored.
• Rc=1 CR0 update truncates to 32 bits in xenia-rs. interpreter.rs:204. Important: neg. of a 64-bit value with high bits set will give a CR0 that doesn't match spec (which compares the full 64-bit ~RA + 1 to zero).
• No carry produced. Use subfic RT, RA, 0 (RT ← 0 − RA with carry) when you need the borrow.
• Latency: single cycle. Negate is the cheapest XO-form ALU operation (cheaper than subf despite being a special case, because there's no RB operand fetch).

Related Instructions

• subfx — generalisation: neg RT, RA ≡ subf RT, RA, 0 (but the latter requires materialising 0 in a register).
• subfic — RT ← SIMM − RA with XER[CA]; subfic RT, RA, 0 produces a borrow.
• addx, addmex, addzex — for chained negation (multi-word two's complement).
• not (simplified) — bit-wise complement via nor RA, RS, RS; distinct from negate.

IBM Reference

• AIX 7.3 — neg (Negate)