BASR is a RR instruction which is used to support linkage and internal subroutines. When executed, the address of the instruction which follows the BASR, a return address, is stored in the operand 1 register, and a branch is taken to the address in the operand 2 register. No branch is taken if R0 is specified as operand 2. In this case, execution continues with the instruction following the BASR. Consider the instruction sequence below

LA R15,SUB1 PUT TARGET ADDRESS IN R15

BASR R14,R15 SAVE “HERE” AS THE RETURN ADDRESS

HERE EQU * THE RETURN ADDRESS

MVC X,Y

...

SUB1 EQU *

... (SUBROUTINE CODE GOES HERE)

BR R14 BRANCH TO THE RETURN ADDRESS

First the address of the subroutine ( the target address) is loaded into register 15. When the BASR is executed, the address of the next instruction (MVC) is loaded into R14. Recall that the EQU directive does not generate object code, and so the address denoted by HERE is equivalent to the address of the MVC instruction. After the return address is loaded, a branch occurs to the address in R15 (SUB1). This begins execution of the code in the subroutine. At completion of the subroutine, an unconditional branch occurs to the address in R14. This causes execution to resume at the MVC instruction.

BASR can also be used in the format listed below.

BASR R12,R0

In this case, the address of the next instruction is loaded into R12, and no branch is taken since operand 2 was specified as R0. This form of a BASR is used in the topic called Base / Displacement Addressing. Briefly, R12 is declared to be a base register in a USING statement and the BASR loads R12 with the base address ( the address of the next instruction ).

BASR replaces an older instruction called “BALR”. Both of these instructions load the address of the next instruction into operand 1. The difference in their operation depends on the addressing mode that the machine is using:

In 24 bit mode: BALR loads bits 0 - 7 of operand 1 with linkage information ( instruction

length code, condition code, program mask )

BALR loads bits 8 - 31 of operand 1 with the 24 bit return address

BASR loads bits 0 - 7 with eight 0’s

BASR loads bits 8 - 31 of operand 1 with a 24 bit return address

In 31 bit mode BASR and BALR load bit 0 with a 1 indicating 31 bit mode addressing

BASR and BALR load bits 1 - 31 with a 31 bit return address

The information that was provided by BALR in bits 0 - 7, can now be obtained using the IPM ( Insert Program Mask ) instruction.

Some Unrelated BASR’s

BASR R4,R5 LOAD NEXT ADDRESS IN R4, BRANCH TO ADDRESS IN R5

BASR R4,R0 LOAD NEXT ADDRESS IN R4, DON’T BRANCH

BASR R3,R0 LOAD NEXT ADDRESS IN R3, DON’T BRANCH

1) Use BASR instead of BALR to avoid non-zero bits being placed in the high-order byte of the stored address.

2) Read about the use of BASR in the topic Base Displacement Addressing.