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

lvlx — Load Vector Left Indexed

Category: Memory · Form: X · Opcode: 0x7c00040e

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
lvlx	lvlx	—	Load Vector Left Indexed
lvlx128	lvlx128	—	Load Vector Left Indexed 128

Syntax

lvlx [VD], [RA0], [RB]
lvlx128 [VD], [RA0], [RB]

Encoding

lvlx — form X

• Opcode word: 0x7c00040e
• Primary opcode (bits 0–5): 31
• Extended opcode: 519
• 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

lvlx128 — form VX128_1

• Opcode word: 0x10000403
• Primary opcode (bits 0–5): 4
• Extended opcode: 1027
• 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
RA0	lvlx: read; lvlx128: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
RB	lvlx: read; lvlx128: read	Source GPR.
VD	lvlx: write; lvlx128: write	Destination vector register.

Register Effects

lvlx

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

lvlx128

• Reads (always): RA0, RB
• 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

lvlx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="lvlx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:216
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:44
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:815
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:3083-3087

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::lvlx | PpcOpcode::lvlxl => {
            let ea = ea_indexed(ctx, instr);
            ctx.vr[instr.rd()] = crate::vmx::load_vector_left(mem, ea);
            ctx.pc += 4;
        }

lvlx128

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="lvlx128"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:219
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:44
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:420
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:3088-3092

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::lvlx128 | PpcOpcode::lvlxl128 => {
            let ea = ea_indexed(ctx, instr);
            ctx.vr[instr.vd128()] = crate::vmx::load_vector_left(mem, ea);
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Load-left half of an unaligned vector. lvlx reads (16 - (EA mod 16)) bytes starting at the exact EA and places them in the left (high-address-byte → low-lane) of the destination vector; the remaining lanes on the right are zero-filled. Combine with lvrx at EA + 15 to assemble a full unaligned vector across an alignment boundary.
• Companion idiom. lvlx VD, RA, RB ; lvrx Vtemp, RA, RB ; vor VD, VD, Vtemp produces the unaligned 16 bytes at EA regardless of alignment. This was the canonical unaligned-vector-read recipe before lvsl/vperm shuffles became the more common idiom.
• No alignment masking. Unlike lvx, the EA is not rounded down. EA mod 16 controls how the data is shifted into the destination.
• RA0 semantics. RA = 0 selects literal zero.
• Microsoft Xbox 360 specific. lvlx and lvrx are not in the standard Altivec specification — they are part of Microsoft's VMX128 / Cell BE-style extension, defined in PowerPC Cell and later VMX. The Xbox 360 Xenon supports them (decoder + xenia entry confirm).
• Implementation in xenia. The shared snapshot calls vmx::load_vector_left(mem, ea), which performs the unaligned partial-byte read and zero-fills the right side.
• VMX128 sibling (lvlx128). Same semantics; different operand encoding (7-bit register field, addressing v0..v127).
• lvlxl is the LRU-hint variant. Same data behaviour, hint ignored under emulation.

Related Instructions

• lvrx, lvrx128 — load-right partner; combine to read unaligned 16 bytes.
• lvlxl, lvlxl128 — LRU-hint variants.
• lvx, lvx128 — aligned load (the EA-masking sibling).
• stvlx, stvrx — symmetric unaligned stores.

IBM Reference

• AIX 7.3 — lvlx (Load Vector Left Indexed)
• PowerISA v2.07B Book I "Vector Facility"; Microsoft Xbox 360 XDK for VMX128 details.