BCTR is the RR version of BCT. It is used to support counted loops - loops in which the number of iterations is known before entry to the loop body. Operand 1 is a register which contains a count - the number of iterations the loop should perform. Each time the BCTR is executed, the operand 1 register is decremented by 1. If the result is not zero, a branch is taken to the address stored in the register denoted by operand 2. If the result is zero, no branch is taken and execution continues with the instruction following the BCTR. Here is an example of how BCTR can be used to create a counted loop.

LA R10,LOOP PUT TARGET ADDRESS IN R10

LA R8,3 LOOP 3 TIMES

LOOP EQU *

... (LOOP BODY GOES HERE)

BCTR R8,R10 BRANCH TO “LOOP” IF NOT ZERO

MVC X,Y

First the address of the loop body ( the target address) is loaded into register 10. Then the number of iterations, 3, is put in R8. The loop body is executed. The BCTR decrements R8 to 2, and since the result is not zero, a branch is taken to the address in R10 which is LOOP. The loop body is executed again and the BCTR decrements R8 to 1. Since the result was not zero, a branch is taken to LOOP. The third execution of the loop body occurs and BCTR decrements R8 to 0. No branch is taken in this case, and execution falls through the loop and continues with the MVC instruction.

BCTR is sometimes coded with R0 specified in operand 2 to indicate that no branch should be taken. Execution continues with the next instruction.

BCTR R5,R0

In this case, R5 is decremented by 1. Since R0 was coded as the second operand, execution continues with the instruction following the branch. Coding BCTR in this way provides an easy and efficient way to subtract 1 from a register.

Some Unrelated BCTR’s

R4 = X’00000008’

R5 = A(HERE)

R6 = X’00000001’

R7 = X’00000002’

BCTR R4,R5 R4 = X’00000007’, BRANCH TO “HERE”

BCTR R4,R0 R4 = X’00000007’, DON’T BRANCH, FALL THROUGH

BCTR R6,R5 R6 = X’00000000’, DON’T BRANCH

BCTR R7,R5 R7 = X’00000001’, BRANCH TO “HERE”