xenia-rs/migration/project-root/ppc-manual/memory/stvx.md

stvx — Store Vector Indexed

Category: Memory · Form: X · Opcode: 0x7c0001ce

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
stvx	stvx	—	Store Vector Indexed
stvx128	stvx128	—	Store Vector Indexed 128

Syntax

stvx [VS], [RA0], [RB]
stvx128 [VS], [RA0], [RB]

Encoding

stvx — form X

• Opcode word: 0x7c0001ce
• Primary opcode (bits 0–5): 31
• Extended opcode: 231
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode
6–10	RT/FRT/VRT	destination
11–15	RA/FRA/VRA	source A
16–20	RB/FRB/VRB	source B
21–30	XO	extended opcode (10 bits)
31	Rc	record-form flag

stvx128 — form VX128_1

• Opcode word: 0x100001c3
• Primary opcode (bits 0–5): 4
• Extended opcode: 451
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (4)
6–10	VD128l	destination low 5 bits
11–15	RA	address register
16–20	RB	offset register
21–27	XO	extended opcode
28–29	VD128h	destination high 2 bits
30–31	—	reserved

Operands

Field	Role	Description
VS	stvx: read; stvx128: read	Source vector register (alias for VD on stores).
RA0	stvx: read; stvx128: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
RB	stvx: read; stvx128: read	Source GPR.

Register Effects

stvx

• Reads (always): VS, RA0, RB
• Reads (conditional): none
• Writes (always): none
• Writes (conditional): none

stvx128

• Reads (always): VS, RA0, RB
• Reads (conditional): none
• Writes (always): none
• Writes (conditional): none

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

EA <- ((RA|0) + (RB)) & ~0xF           ; align to 16
MEM(EA, 16) <- byteswap(VS)

C Translation Example

/* stvx VS, RA, RB — 16-byte aligned store of a vector register    */
uint64_t base = (insn.RA == 0) ? 0 : r[insn.RA];
uint32_t ea   = (uint32_t)((base + r[insn.RB]) & ~(uint64_t)0xF);
mem_write_vec128_be(ea, v[insn.VS]);

Implementation References

stvx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="stvx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:193
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:79
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:791
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1849-1859

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::stvx => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = (ea.wrapping_add(ctx.gpr[instr.rb()]) & !0xF) as u32;
            // PPCBUG-511: stvx was missing invalidate_for_write.
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() { t.invalidate_for_write(ea); }
            }
            let bytes = ctx.vr[instr.rs()].as_bytes();
            for i in 0..16 { mem.write_u8(ea + i as u32, bytes[i]); }
            ctx.pc += 4;
        }

stvx128

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="stvx128"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:196
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:79
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:417
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1860-1870

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::stvx128 => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = (ea.wrapping_add(ctx.gpr[instr.rb()]) & !0xF) as u32;
            // PPCBUG-511: stvx128 was missing invalidate_for_write.
            if let Some(t) = ctx.reservation_table.as_ref().filter(|t| t.is_enabled()) {
                if t.has_active_reservers() { t.invalidate_for_write(ea); }
            }
            let bytes = ctx.vr[instr.vs128()].as_bytes();
            for i in 0..16 { mem.write_u8(ea + i as u32, bytes[i]); }
            ctx.pc += 4;
        }

Extended Pseudocode

EA <- ((RA|0) + (RB)) & ~0xF                   ; force 16-byte alignment
MEM(EA, 16) <- byte_order_adjusted(VS)         ; lane 0 at EA, lane 15 at EA+15

Special Cases & Edge Conditions

• Alignment is forced, not checked. The low four bits of the effective address are cleared before the store — alignment violations silently corrupt adjacent data rather than trap. This differs from scalar stw (no alignment enforcement) and from stvewx (which stores only one element and keeps the exact EA).
• Big-endian lane layout. Vector lane 0 (the most-significant bytes of the 128-bit register) lives at the lowest address; lane 15 at EA + 15. On little-endian hosts the whole 16-byte block is byte-swapped at the memory boundary so the PowerPC-visible layout is preserved. Xenia's helper mem_write_vec128_be handles this.
• RA0 semantics. When RA = 0 the base is the literal zero — just like scalar loads/stores. Combined with the alignment mask this lets stvx VS, 0, RB store to address RB & ~0xF.
• No update form. Unlike scalar stores, VMX stores have no u variant that post-writes the base. Use stvxl for the cache-hint variant (suggests "last" — the line is not expected to be reused soon).
• VMX128 sibling (stvx128). Identical semantics; the only difference is the operand encoding. VMX128 uses a 7-bit register index split across three non-contiguous bit fields (VS128l ‖ VS128h) so it can address v0..v127 instead of the 32-register Altivec space. All alignment, byte-order and RA0 rules are the same.
• Read-before-write. The 16-byte write occurs as one conceptual store; subsequent loads from the same address observe the complete new value. There's no split-transaction window visible to software.

Related Instructions

• lvx, lvx128 — the load counterparts.
• stvxl, stvxl128 — cache-hint "last-use" variants.
• stvebx / stvehx / stvewx — store single element (byte / half / word) at the exact (unaligned) address.
• stvlx / stvrx — store-left / store-right for unaligned vector I/O.
• dcbz — zero a cache line; often paired with stvx in block-fill idioms.

IBM Reference

• AIX 7.3 — stvx (Store Vector Indexed)
• PowerISA Book II (Altivec / VMX). Xbox 360 VMX128 is Microsoft-documented in the XDK; xenia's ppc-instructions.xml captures the deltas.