Instruction Syntax

Mnemonic	Format	Flags
bctr		LK = 0
bctrl		LK = 1

Note: BCTR is a derived form of BCCTR with BO=20 and BI=0, meaning it always branches (unconditional).

Instruction Encoding

Field	Bits	Description
Primary Opcode	0-5	010011 (0x13)
BO	6-10	Branch Option (20 = always branch)
BI	11-15	Branch Input (0 = not used)
LK	31	Link (0 = no link, 1 = link)

Operation

if (LK == 0) then
    NIA ← CTR
else
    NIA ← CTR
    LR ← CIA + 4

Branch to the address contained in the Count Register (CTR). If LK=1, the address of the instruction following the BCTR is placed into LR.

Affected Registers

Link Register (LR)

(if LK = 1)

• Updated with the address of the instruction following BCTR

Next Instruction Address (NIA)

• Set to the value in CTR

Examples

Function Pointer Call

# Call function through function pointer
mtctr r3          # Load function address into CTR
bctrl             # Branch to function and link

Switch Statement Implementation

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

Dynamic Code Execution

# Execute dynamically generated code
lis r3, code_addr@ha
addi r3, r3, code_addr@l
mtctr r3          # Set CTR to code address
bctr              # Execute the code

Return from Function Pointer

# Return from function called via function pointer
mflr r3           # Save return address
mtctr r3          # Load return address into CTR
bctr              # Return to caller

Related Instructions

bcctr, bclr, blr, mtctr, mfctr, b