Subroutines

Subroutines

Internal Subroutine

subroutine that is located within the same CSECT as the calling routine

External Subroutine

Subroutine that is located outside of the calling routine
A separate CSECT

Internal Subroutines

On entrance to a subroutine, register 1 holds the address of a parameter list (if the subroutine gets passed any parameters)

On entrance to a subroutine, the initial value in any register that will be altered by the subroutine should be saved

Before exiting a subroutine, the initial value in any register that was altered by the subroutine should be restored

Subroutine Name: rtnName DS 0H

Setting up a parameter list:

must be a set of contiguous fullwords, each of which contains the address of a parameter to be passed
use address constants (adcons): label DC A(expr)

if expr is a non-negative integer, the generated fullword will contain the binary representation of the integer (same as doing F‘expr’)

if expr is a label or label+n, the generated fullword will contain the address of label or address of label+n

DC A(5) => 00000005

000100 SAVE DS F

DC A(SAVE) => 00000100

expr may be several non-negative integers or label separated by commas

DC A(5,SAVE) => 0000000500000100

A sample parameter list for a program with internal subroutines:

PARMLIST   DC   A(TABLE)
           DC   A(EOT)

To call a subroutine:

If the subroutine requires passed in parameters, put the address of the parameter list into register 1

Use the Branch and Link Instruction

RX Format: label BAL R,D(X,B)

LINK: The address of the instruction immediately following the BAL instruction is placed in R
BRANCH: Branches to D(X,B), which is usually rtnName

LA    1,PARMLIST
BAL   11,RTNNAME

To exit a subroutine:

Branch to the instruction immediately following the BAL instruction by executing a BR instruction with the register used in the calling routine BAL instruction

BR    11

Skeleton program with one internal subroutine:

MAIN     CSECT
         USING MAIN,R15
         STM   R0,R15,MAINSAVE
         ...
         BAL   R14,SUBRTN
         ...
         LM    R0,R15,MAINSAVE
         BR    R14
         LTORG
**** Storage for MAIN starts here ***
MAINSAVE DS    16F
         ...
*** Storage for MAIN ends here ***
SUBRTN   DS    0H                 SUBRTN starts here
         STM   R0,R15,SUBSAVE
         ...
         LM    R0,R15,SUBSAVE
         BR    R14
         LTORG
*** Storage for SUBRTN starts here ***
SUBSAVE  DS    16F
         ...
*** Storage for SUBRTN ends here ***
         END   MAIN