Instruction Syntax

Mnemonic	Format	Flags
lwax	rD,rA,rB	None

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	011111 (0x1F)
rD	6-10	Destination register
rA	11-15	Base register (can be 0)
rB	16-20	Index register
XO	21-30	0000000001 (1) - Extended opcode
Rc	31	Record bit (0)

Operation

if rA = 0 then EA ← (rB)
else EA ← (rA) + (rB)
rD ← EXTS(MEM(EA, 4))

A word (32 bits) is loaded from memory, sign-extended to 64 bits, and placed in register rD. The effective address is computed by adding the contents of registers rA and rB, or just rB if rA is 0.

Note: This indexed form is essential for array access and dynamic addressing with signed 32-bit data in 64-bit environments. The sign extension preserves the sign of signed word values, making it ideal for processing signed integer arrays and data structures. If rA=0, it is treated as the value 0, not the contents of register r0.

Affected Registers

• rD: Loaded with sign-extended word value
• rA: Used as base register (if not 0)
• rB: Used as index register

Examples

Example 1: Load signed word from array

# Load signed word from array using indexed addressing
lwax r3, r4, r5    # r3 = sign-extended word from address r4 + r5

Example 2: Load with zero base register

# Load from absolute address
lwax r6, r0, r7    # r6 = sign-extended word from address r7

Example 3: Array element access

# Access signed integer array element
slwi r8, r9, 2     # Multiply index by 4 (word size)
lwax r10, r11, r8  # Load signed word from array[r9]

Related Instructions

lwa, lwz, lwzx, lwaux