bc

Branch Conditional

bc BO,BI,target_addr (AA=0 LK=0)
bc BO,BI,target_addr (AA=1 LK=0)
bc BO,BI,target_addr (AA=0 LK=1)
bc BO,BI,target_addr (AA=1 LK=1)

Branches conditionally based on the Count Register (CTR) and/or a bit in the Condition Register (CR).

Details

Branches conditionally based on the state of the Count Register (CTR) and/or a bit in the Condition Register (CR), as controlled by the BO field. The AA field determines absolute vs. relative addressing; LK=1 saves the return address in LR. This is the fundamental conditional branch instruction.

Pseudocode Operation

ctr_ok ← (BO[2] = 1) | (CTR ≠ 0 ⊕ BO[3])
if BO[2] = 0 then CTR ← CTR - 1
cr_ok ← (BO[0] = 1) | (CR[BI] = BO[1])
if ctr_ok & cr_ok then
  if AA = 0 then NIA ← CIA + (BD || 0b00) else NIA ← (BD || 0b00)
if LK = 1 then LR ← CIA + 4

Programming Note

The bc instruction branches to a target address based on the condition bits in the Condition Register (CR). Ensure that the branch condition and target address are correctly set. The instruction operates at user privilege level, but care must be taken with conditional logic to avoid unintended execution paths.

Extended Mnemonics

Extended Mnemonic Equivalent Instruction

Example

bc 12, 2, label

// Branch if CR bit 2 is set (beq).

Encoding

Binary Layout
10
6
BO
21
BI
16
AA
1
LK
0
target_addr
2
 
Format B-form
Opcode 0x40000000
Extension Base
Registers Altered LR, CTR

Operands

  • BO
    Branch Options (5 bits)
  • BI
    CR Bit Index (5 bits)
  • BD
    14-bit Signed Displacement
  • target_addr
    Target address