Instruction Syntax

Mnemonic	Format	Flags
srwi	rA,rS,n	None

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	010101 (0x15)
rS	6-10	Source register
rA	11-15	Destination register
n	16-20	Shift count (0-31)
MB	21-25	00000 (Begin bit)
ME	26-30	00000 (End bit)

Operation

rA ← (rS) >> n

SRWI shifts the value in source register rS right by n bit positions and stores the result in destination register rA.

1. Takes the value from source register rS
2. Shifts it right by n bit positions (logical shift)
3. Stores the result in destination register rA

Note: SRWI is a derived form of the RLWINM instruction with MB=0 and ME=31-n.

Affected Registers

General Purpose Registers (GPRs)

• rA (Destination register) - Result of right shift operation

Examples

Basic Right Shift

# Shift r3 right by 2 bits
srwi r4, r3, 2     # r4 = r3 >> 2 (divide by 4)

# Shift r5 right by 8 bits
srwi r6, r5, 8     # r6 = r5 >> 8 (divide by 256)

Division by Powers of 2

# Divide by 2
srwi r3, r4, 1     # r3 = r4 / 2

# Divide by 4
srwi r3, r4, 2     # r3 = r4 / 4

# Divide by 8
srwi r3, r4, 3     # r3 = r4 / 8

# Divide by 16
srwi r3, r4, 4     # r3 = r4 / 16

Bit Field Extraction

# Extract upper 8 bits
srwi r3, r4, 24    # r3 = r4 >> 24 (bits 24-31)

# Extract middle 8 bits
srwi r3, r4, 8     # r3 = r4 >> 8 (bits 8-15)
andi r3, r3, 0xFF  # Mask to 8 bits

Address Calculation

# Convert byte offset to word offset
srwi r3, r4, 2     # r3 = r4 / 4 (byte offset to word offset)

# Extract page number from address
srwi r3, r4, 12    # r3 = r4 / 4096 (page number)

Data Packing/Unpacking

# Unpack 16-bit value from upper half
srwi r3, r4, 16    # r3 = r4 >> 16 (upper 16 bits)

# Extract bit field
srwi r3, r4, 5     # r3 = r4 >> 5
andi r3, r3, 0x1F  # Mask to 5 bits

Flag Testing

# Test if bit 7 is set
srwi r3, r4, 7     # Shift bit 7 to position 0
andi r3, r3, 1     # Extract bit 0
cmpwi r3, 0        # Compare with zero
bne bit_set        # Branch if bit was set

Related Instructions

slwi (Shift Left Word Immediate), rlwinm (Rotate Left Word Immediate then AND with Mask), srw (Shift Right Word), slw (Shift Left Word)