xenia-rs/migration/project-root/ppc-manual/vmx128/vmsum3fp128.md

vmsum3fp128 — Vector128 Multiply Sum 3-way Floating Point

Category: VMX128 · Form: VX128 · Opcode: 0x14000190

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
vmsum3fp128	vmsum3fp128	—	Vector128 Multiply Sum 3-way Floating Point

Syntax

vmsum3fp128 [VD], [VA], [VB]

Encoding

vmsum3fp128 — form VX128

• Opcode word: 0x14000190
• Primary opcode (bits 0–5): 5
• Extended opcode: 400
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (4 or 5)
6–10	VD128l	destination low 5 bits
11–15	VA128l	source A low 5 bits
16–20	VB128l	source B low 5 bits
21	VA128H	source A high bit
22	—	reserved
23–25	VC	optional VC / XO sub-field
26	VA128h	source A middle bit
27	—	reserved
28–29	VD128h	destination high 2 bits
30–31	VB128h	source B high 2 bits

Operands

Field	Role	Description
VA	vmsum3fp128: read	Source A vector register.
VB	vmsum3fp128: read	Source B vector register.
VD	vmsum3fp128: write	Destination vector register.

Register Effects

vmsum3fp128

• 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

vmsum3fp128

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="vmsum3fp128"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:1067
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:106
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:616
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:4513-4523

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::vmsum3fp128 => {
            // PPCBUG-436: flush per-product intermediates (not just the final sum).
            let a = ctx.vr[instr.va128()].as_f32x4();
            let b = ctx.vr[instr.vb128()].as_f32x4();
            let p0 = vmx::flush_denorm(a[0] * b[0]);
            let p1 = vmx::flush_denorm(a[1] * b[1]);
            let p2 = vmx::flush_denorm(a[2] * b[2]);
            let s = vmx::flush_denorm(p0 + p1 + p2);
            ctx.vr[instr.vd128()] = xenia_types::Vec128::from_f32x4(s, s, s, s);
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• 3-way float dot product. Computes s = VA[0]*VB[0] + VA[1]*VB[1] + VA[2]*VB[2] (ignoring lane 3 — the "w" component of a homogeneous vector) and broadcasts s to every lane of VD. Typical call site: 3D vector dot products where the w-component is padding.
• Scalar-result-splatted-across-lanes. Consuming code can then use any lane of VD as the dot-product result.
• Rounding. Xenia performs two adds in sequence (no fused triple-add in Rust). The order matches the spec but the summation order affects round-off by ~1 ulp. Games that need deterministic cross-host behaviour typically pre-scale their inputs.
• IEEE-754 binary32; VSCR[NJ] honoured.
• No VSCR[SAT], no FPSCR update.
• VMX128 register-fusion (7-bit IDs on VA, VB, VD).
• No IBM AIX entry — Xenon-only.
• No Rc, no XER.

Related Instructions

• vmsum4fp128 — 4-way dot-product (includes the w-lane).
• vmulfp128, vaddfp — the building blocks.
• vmaddcfp128, vmaddfp — fused MAC variants.
• vsumsws — integer sum-reduction analogue.

IBM Reference

• No IBM AIX entry — Xbox 360 VMX128 extension only.
• Xbox 360 XDK, Altivec-128 (VMX128) extensions. A 3-way dot product is a direct mirror of D3D9's float3 dot.
• IBM AltiVec Technology Programmer's Interface Manual, Chapter 5 — Floating-Point Arithmetic for the base float arithmetic semantics.