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

mulhdx — Multiply High Doubleword

Category: Integer ALU · Form: XO · Opcode: 0x7c000092

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
mulhd	mulhdx	—	Multiply High Doubleword
mulhd.	mulhdx	Rc=1	Multiply High Doubleword

Syntax

mulhd[Rc] [RD], [RA], [RB]

Encoding

mulhdx — form XO

• Opcode word: 0x7c000092
• Primary opcode (bits 0–5): 31
• Extended opcode: 73
• 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	mulhdx: read	Source GPR (r0–r31).
RB	mulhdx: read	Source GPR.
RD	mulhdx: write	Destination GPR.
CR	mulhdx: write (conditional)	Condition-register update. When Rc=1, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result.

Register Effects

mulhdx

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

Status-Register Effects

• mulhdx: CR0 ← signed-compare(result, 0) with SO ← XER[SO], when Rc=1.

Operation (pseudocode)

RT <- ((RA) * (RB))[0:63]                 ; high 64 of signed 64×64

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

mulhdx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="mulhdx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_alu.cc:297
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:57
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:864
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:445-453

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::mulhdx => {
            let ra = ctx.gpr[instr.ra()] as i64 as i128;
            let rb = ctx.gpr[instr.rb()] as i64 as i128;
            ctx.gpr[instr.rd()] = (ra.wrapping_mul(rb) >> 64) as u64;
            if instr.rc_bit() {
                ctx.update_cr_signed(0, ctx.gpr[instr.rd()] as i64);
            }
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Returns the high 64 bits of a signed 64×64 product. Pair with mulldx (which returns the low 64 bits) to obtain the full 128-bit product. Both must be issued separately; PowerPC has no fused multiply-double-wide instruction.
• No OE bit. This XO-form instruction has no overflow-enable variant — there is no "high half overflow" because the high half is always defined.
• Xenia widens to i128 natively. interpreter.rs:275 does the multiply in 128 bits then extracts the high 64. The i64 as i128 casts ensure signed extension on both sides.
• Rc=1 CR0 update is correctly 64-bit. interpreter.rs:278 uses as i64 directly. CR0 reflects the sign of the high half: LT if the product is negative, GT if positive and large enough to overflow into the high half, EQ if the product fits in 64 bits signed (so the high half is the sign-extension of the low half — but xenia's check uses raw signed-zero compare, which equates only when the high half is exactly zero, i.e. the product is in [0, 2^63)).
• Use mulhdux for the unsigned high half. The two instructions differ in whether the operands are sign- or zero-extended before the multiply.
• Slow. 64-bit multiply is multi-cycle on Xenon; combining mulhd with mulld for a full 128-bit product roughly doubles the cost.

Related Instructions

• mulldx — low 64 bits of the same signed product.
• mulhdux — high 64 bits, unsigned interpretation.
• mullwx, mulhwx, mulhwux — 32-bit family.
• divdx, divdux — 64-bit division.

IBM Reference

• AIX 7.3 — mulhd (Multiply High Doubleword)