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

stvehx — Store Vector Element Half Word Indexed

Category: Memory · Form: X · Opcode: 0x7c00014e

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
stvehx	stvehx	—	Store Vector Element Half Word Indexed

Syntax

stvehx [VS], [RA0], [RB]

Encoding

stvehx — form X

• Opcode word: 0x7c00014e
• Primary opcode (bits 0–5): 31
• Extended opcode: 167
• 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

Operands

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

Register Effects

stvehx

• 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)

; 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

stvehx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="stvehx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:160
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:77
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:784
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1927-1941

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::stvehx => {
            // Store vS[slot] (1 halfword) at EA & ~1. slot = (EA & 0xF) >> 1.
            let base = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea_unaligned = base.wrapping_add(ctx.gpr[instr.rb()]) as u32;
            let ea = ea_unaligned & !0x1u32;
            // PPCBUG-512: stvehx 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 slot = ((ea_unaligned & 0xF) >> 1) as usize;
            let bytes = ctx.vr[instr.rs()].as_bytes();
            let h = ((bytes[slot * 2] as u16) << 8) | (bytes[slot * 2 + 1] as u16);
            mem.write_u16(ea, h);
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Single half-word element store. Architecturally stvehx writes exactly two bytes from half-word lane (EA mod 16) >> 1 of VS to address EA & ~1 (low bit forced to half-aligned). Other lanes are unaffected, and bytes outside the 2-byte window are unaffected.
• Xenia simplification — full 16-byte write. The xenia snapshot is shared with stvebx / stvewx: writes 16 bytes of the source vector at ea & ~0xF. This is stronger than the architectural 2-byte store — it overwrites 14 adjacent bytes that hardware would have left alone.
• EA forced half-aligned. Hardware drops the low bit; xenia's shared snapshot drops the low four bits.
• RA0 semantics. RA = 0 selects literal zero.
• No update form, no VMX128 sibling. No stvehux; no stvehx128.
• Big-endian half within the lane. The byte at the lower address is the most-significant byte of the half-word lane.
• Common idiom. Pair with vsplth to broadcast then store one half; rare in compiled code (compilers prefer sth).

Related Instructions

• stvebx, stvewx — single byte / word element stores.
• stvx, stvxl — full 16-byte aligned vector stores.
• stvlx, stvrx — store-left / store-right unaligned ops.
• lvehx — symmetric single-half load.

IBM Reference

• AIX 7.3 — stvehx (Store Vector Element Half Word Indexed)
• PowerISA v2.07B Book I "Vector Facility" § "Vector Load and Store".