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

lha — Load Half Word Algebraic

Category: Memory · Form: D · Opcode: 0xa8000000

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
lha	lha	—	Load Half Word Algebraic
lhau	lhau	—	Load Half Word Algebraic with Update
lhaux	lhaux	—	Load Half Word Algebraic with Update Indexed
lhax	lhax	—	Load Half Word Algebraic Indexed

Syntax

lha [RD], [d]([RA0])
lhau [RD], [d]([RA])
lhaux [RD], [RA], [RB]
lhax [RD], [RA0], [RB]

Encoding

lha — form D

• Opcode word: 0xa8000000
• Primary opcode (bits 0–5): 42
• Extended opcode: —
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode
6–10	RT	destination GPR (or RS when storing)
11–15	RA	source GPR (0 ⇒ literal 0 for RA0 forms)
16–31	D/SI/UI	16-bit signed or unsigned immediate

lhau — form D

• Opcode word: 0xac000000
• Primary opcode (bits 0–5): 43
• Extended opcode: —
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode
6–10	RT	destination GPR (or RS when storing)
11–15	RA	source GPR (0 ⇒ literal 0 for RA0 forms)
16–31	D/SI/UI	16-bit signed or unsigned immediate

lhaux — form X

• Opcode word: 0x7c0002ee
• Primary opcode (bits 0–5): 31
• Extended opcode: 375
• 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

lhax — form X

• Opcode word: 0x7c0002ae
• Primary opcode (bits 0–5): 31
• Extended opcode: 343
• 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
RA0	lha: read; lhax: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
d	lha: read; lhau: read	16-bit signed displacement (d) added to the base address register.
RD	lha: write; lhau: write; lhaux: write; lhax: write	Destination GPR.
RA	lhau: read; lhau: write; lhaux: read; lhaux: write	Source GPR (r0–r31).
RB	lhaux: read; lhax: read	Source GPR.

Register Effects

lha

• Reads (always): RA0, d
• Reads (conditional): none
• Writes (always): RD
• Writes (conditional): none

lhau

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

lhaux

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

lhax

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

Status-Register Effects

No condition-register or status-register effects.

Operation (pseudocode)

EA <- (RA|0) + EXTS(d)
RT <- SEXT16_to_64(MEM(EA, 2))

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

lha

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="lha"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:128
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:40
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:365
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1066-1071

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::lha => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(instr.d() as i64 as u64) as u32;
            ctx.gpr[instr.rd()] = mem.read_u16(ea) as i16 as i32 as u32 as u64;
            ctx.pc += 4;
        }

lhau

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="lhau"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:149
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:40
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:366
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1084-1089

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::lhau => {
            let ea = ctx.gpr[instr.ra()].wrapping_add(instr.d() as i64 as u64) as u32;
            ctx.gpr[instr.rd()] = mem.read_u16(ea) as i16 as i32 as u32 as u64;
            ctx.gpr[instr.ra()] = ea as u64;
            ctx.pc += 4;
        }

lhaux

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="lhaux"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:162
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:40
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:805
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1090-1095

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::lhaux => {
            let ea = ctx.gpr[instr.ra()].wrapping_add(ctx.gpr[instr.rb()]) as u32;
            ctx.gpr[instr.rd()] = mem.read_u16(ea) as i16 as i32 as u32 as u64;
            ctx.gpr[instr.ra()] = ea as u64;
            ctx.pc += 4;
        }

lhax

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="lhax"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:173
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:40
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:801
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1072-1077

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::lhax => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(ctx.gpr[instr.rb()]) as u32;
            ctx.gpr[instr.rd()] = mem.read_u16(ea) as i16 as i32 as u32 as u64;
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Sign-extending half-word load. Reads 2 bytes big-endian, treats them as a signed 16-bit integer, sign-extends to 64 bits. Compare with lhz, which zero-extends. Xenia's snapshot does the cast chain u16 -> i16 -> i64 -> u64 to obtain the canonical sign-extended bit pattern.
• Big-endian read. Byte at EA is the most-significant 8 bits of the half; byte at EA+1 is the least-significant. On little-endian hosts mem.read_u16 returns the big-endian word in host-native form already.
• RA0 (non-update forms). RA = 0 in lha and lhax selects literal zero — useful for absolute-address access patterns.
• Update-form invalid forms. lhau / lhaux invoke RA = 0 and RA = RT as invalid forms; xenia performs the load before writing back RA ← EA, so an RA = RT collision silently destroys the loaded value.
• No alignment requirement. Xenon executes unaligned half-word loads without a fault.
• Common in audio / graphics code. lha is the standard load for signed 16-bit PCM samples and signed 16-bit packed vertex deltas.
• Use lha rather than lhz + extsh. Both produce the same result, but lha is one fused instruction and the compiler will pick it whenever the source type is int16_t / short.

Related Instructions

• lhz, lhzu, lhzx, lhzux — zero-extending counterparts.
• lwa, lwax, lwaux — sign-extending word loads (32→64).
• lbz — byte load (no sign-extending byte load exists; use lbz + extsb).
• lhbrx — byte-reversed half-word load (zero-extending).
• sth, sthu, sthx, sthux — corresponding stores.

IBM Reference

• AIX 7.3 — lha (Load Half Algebraic)
• AIX 7.3 — lhau / lhax / lhaux