Instruction Syntax

Mnemonic	Format	Description
mtctr	rS	Move to Count Register

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	011111 (0x1F)
rS	6-10	Source register
Reserved	11-15	00000
Reserved	16-20	00000
Reserved	21-29	000000000
Reserved	30	0
SPR	31-31	Special Purpose Register (9 = CTR)

Operation

CTR ← (rS)

The contents of register rS are copied into the Count Register (CTR).

Affected Registers

Count Register (CTR)

(always)

• Updated with the value from rS

Examples

Basic Move to Count Register

mtctr r3     # Copy value from r3 to CTR

Function Pointer Setup

# Set up function pointer call
lis r3, function_addr@ha
addi r3, r3, function_addr@l
mtctr r3          # Load function address into CTR
bctrl             # Call function

Loop Counter Setup

# Set up loop counter
li r3, 10         # Load loop count
mtctr r3          # Set CTR to loop count
loop:
    # loop body
    bdnz loop     # Decrement CTR and branch if not zero

Jump Table Implementation

# Jump table using CTR
lis r3, jump_table@ha
addi r3, r3, jump_table@l
slwi r4, r4, 2    # Multiply index by 4
lwzx r5, r3, r4   # Load jump address
mtctr r5          # Load address into CTR
bctr              # Jump to address

Dynamic Code Execution

# Execute code at runtime address
lis r3, dynamic_code@ha
addi r3, r3, dynamic_code@l
mtctr r3          # Set CTR to code address
bctr              # Execute the code

Related Instructions

mfctr, bctr, bcctr, bdnz, bdnzt, bdnzf