Instruction Syntax

Mnemonic	Format	Flags
stwu	rS,d(rA)	None

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	100101 (0x25)
rS	6-10	Source register
rA	11-15	Base register (updated)
SIMM	16-31	Signed immediate offset

Operation

EA ← (rA) + SIMM
(EA) ← (rS)
rA ← EA

STWU stores a word (32-bit value) to memory AND automatically updates the base register to point to the next location.

1. Calculates address = base register + offset
2. Stores the word from source register to that address
3. Updates the base register to point to the address where it just stored

Note: This is perfect for storing data in arrays or buffers because it automatically moves to the next location.

Affected Registers

General Purpose Registers (GPRs)

• rA (Base register) - Updated with effective address

Memory

• Word at effective address (EA)

Examples

Basic Store Word with Update

# Store word and automatically move to next location
stwu r1, 4(r3)    # Store r1 at address r3+4, then r3 = r3+4

Fill an Array

# Fill array with values 1, 2, 3, 4, 5
li r3, array        # Point to start of array
li r1, 1            # First value to store
stwu r1, 4(r3)      # Store 1, r3 now points to next location
li r1, 2            # Second value
stwu r1, 4(r3)      # Store 2, r3 moves to next location
li r1, 3            # Third value  
stwu r1, 4(r3)      # Store 3, r3 moves to next location
# ... and so on

Store Multiple Values

# Store calculated values in a buffer
li r3, buffer       # Point to buffer
li r4, 10           # Number of values to store
loop:
add r5, r6, r7      # Calculate some value
stwu r5, 4(r3)      # Store it, automatically move to next spot
addi r4, r4, -1     # Decrement counter
cmpwi r4, 0         # Are we done?
bne loop            # If not, continue

Related Instructions

stw, stwux, stwx