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

ld — Load Doubleword

Category: Memory · Form: DS · Opcode: 0xe8000000

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
ld	ld	—	Load Doubleword
ldu	ldu	—	Load Doubleword with Update
ldux	ldux	—	Load Doubleword with Update Indexed
ldx	ldx	—	Load Doubleword Indexed

Syntax

ld [RD], [ds]([RA0])
ldu [RD], [ds]([RA])
ldux [RD], [RA], [RB]
ldx [RD], [RA0], [RB]

Encoding

ld — form DS

• Opcode word: 0xe8000000
• Primary opcode (bits 0–5): 58
• Extended opcode: —
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode
6–10	RT	destination GPR (or RS)
11–15	RA	source GPR (0 ⇒ literal 0)
16–29	DS	14-bit signed word-scaled displacement
30–31	XO	extended opcode

ldu — form DS

• Opcode word: 0xe8000001
• Primary opcode (bits 0–5): 58
• Extended opcode: —
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode
6–10	RT	destination GPR (or RS)
11–15	RA	source GPR (0 ⇒ literal 0)
16–29	DS	14-bit signed word-scaled displacement
30–31	XO	extended opcode

ldux — form X

• Opcode word: 0x7c00006a
• Primary opcode (bits 0–5): 31
• Extended opcode: 53
• 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

ldx — form X

• Opcode word: 0x7c00002a
• Primary opcode (bits 0–5): 31
• Extended opcode: 21
• 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	ld: read; ldx: read	Source GPR; when the encoded register number is 0 the operand is the literal 64-bit zero, not r0.
ds	ld: read; ldu: read	14-bit signed word-aligned displacement (DS << 2).
RD	ld: write; ldu: write; ldux: write; ldx: write	Destination GPR.
RA	ldu: read; ldu: write; ldux: read; ldux: write	Source GPR (r0–r31).
RB	ldux: read; ldx: read	Source GPR.

Register Effects

ld

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

ldu

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

ldux

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

ldx

• 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(ds || 0b00)
RT <- MEM(EA, 8)

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

ld

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="ld"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:347
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:36
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:380
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1096-1101

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::ld => {
            let ea = if instr.ra() == 0 { 0u64 } else { ctx.gpr[instr.ra()] };
            let ea = ea.wrapping_add(instr.ds() as i64 as u64) as u32;
            ctx.gpr[instr.rd()] = mem.read_u64(ea);
            ctx.pc += 4;
        }

ldu

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="ldu"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:367
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:36
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:381
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1126-1131

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::ldu => {
            let ea = ctx.gpr[instr.ra()].wrapping_add(instr.ds() as i64 as u64) as u32;
            ctx.gpr[instr.rd()] = mem.read_u64(ea);
            ctx.gpr[instr.ra()] = ea as u64;
            ctx.pc += 4;
        }

ldux

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="ldux"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:378
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:36
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:764
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1132-1137

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::ldux => {
            let ea = ctx.gpr[instr.ra()].wrapping_add(ctx.gpr[instr.rb()]) as u32;
            ctx.gpr[instr.rd()] = mem.read_u64(ea);
            ctx.gpr[instr.ra()] = ea as u64;
            ctx.pc += 4;
        }

ldx

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="ldx"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_memory.cc:389
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:36
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:755
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:1102-1107

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::ldx => {
            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_u64(ea);
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• DS-form, not D-form. The displacement is 14 bits scaled by 4 (EXTS(ds || 0b00)), giving a signed range of ±32 KiB in 4-byte steps. Bits 30–31 are the extended opcode used to distinguish ld (XO=0) from ldu (XO=1). The assembler accepts a normal byte displacement and verifies divisibility by 4.
• Big-endian read. The 64 bits at EA..EA+7 form the loaded value, most-significant byte first. Xenia-rs's mem.read_u64 returns the host-native value of that big-endian doubleword.
• No zero/sign-extension question. ld already fills the entire 64-bit register; there is no lda (load doubleword algebraic) — the doubleword is the architectural maximum.
• RA0 (non-update forms). RA = 0 in ld and ldx means base is literal zero. ld RT, 0x100(0) reads from absolute 0x100.
• Update-form invalid forms. ldu / ldux invoke "RA = 0" and "RA = RT" as invalid forms. AIX docs say results are undefined; xenia performs the read first, then writes back RA ← EA, which would silently destroy the loaded value if RA == RT.
• Alignment. Xenon does not enforce doubleword alignment for ld itself — unaligned 8-byte loads are tolerated. However, real POWER cores may take an alignment exception on some implementations; portable code keeps doublewords 8-byte aligned.
• 64-bit pointer / counter loads. Although Xbox 360 user code is 32-bit, kernel structures and TOC entries are doublewords; ld is the standard load for them.

Related Instructions

• lwz, lhz, lbz — narrower zero-extending loads.
• lwa, lha — sign-extending loads (no lda exists; ld already fills the register).
• ldbrx — byte-reversed doubleword load.
• ldarx / stdcx — load-reserve / store-conditional doubleword pair.
• std, stdu, stdx, stdux — corresponding stores.

IBM Reference

• AIX 7.3 — ld (Load Doubleword)
• AIX 7.3 — ldu / ldux / ldx