Instruction Syntax

Mnemonic	Format	Flags
mtspr	SPR,rS	None

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	011111 (0x1F)
SPR	6-20	Special Purpose Register number
XO	21-30	0101000111 (0x287)
Reserved	31	Should be zero

Operation

SPR[SPR] ← rS

The contents of general-purpose register rS are copied to the specified Special Purpose Register (SPR).

Note: This instruction provides access to various system registers. Common SPR numbers include 8 (LR - Link Register), 9 (CTR - Count Register), and others for processor-specific functionality.

Affected Registers

Special Purpose Registers (SPRs)

(always)

• SPR - Destination Special Purpose Register (loaded with rS contents)

General Purpose Registers (GPRs)

(read only)

• rS - Source register (read only)

Examples

Write Link Register

# Write to Link Register (SPR 8)
mtspr 8, r3            # Copy r3 to LR
# r3 now contains return address

Write Count Register

# Write to Count Register (SPR 9)
mtspr 9, r4            # Copy r4 to CTR
# r4 now contains loop counter value

Save Return Address

# Save return address before function call
mfspr r5, 8            # Save current LR
bl some_function       # Function call
mtspr 8, r5            # Restore original LR

Set Performance Counter

# Set performance monitoring counter
li r6, 1000            # Load counter value
mtspr 952, r6          # Set PMC1 (SPR 952)
# Performance counter now set to 1000

Related Instructions

mfspr, mtmsr, mfmsr