Example: monitoring and modifying registers and storage in C

The examples below use the following C program to demonstrate how to monitor and modify registers and storage.

int dbl(int j)            /* line 1 */
{                         /* line 2 */
  return 2*j;             /* line 3 */
}                         /* line 4 */
int main(void)
{
  int i;
  i = 10;
  return dbl(i);
}

If you compile the program above using the compiler options TEST(ALL),LIST, then your pseudo assembly listing will be similar to the listing shown below.

*  int dbl(int j)
           ST    r1,152(,r13)
*  {
           EX    r0,HOOK..PGM-ENTRY
*    return 2*j;
           EX    r0,HOOK..STMT
           L     r15,152(,r13)
           L     r15,0(,r15)
           SLL   r15,1
           B     @5L2
           DC    A@5L2-ep)
           NOPR
 @5L1      DS    0D
*  }
 @5L2      DS    0D
           EX    r0,HOOK..PGM-EXIT

To display a continuously updated view of the registers in the Monitor window, enter the following command:

MONITOR LIST REGISTERS

After a few steps, z/OS® Debugger halts on line 1 (the program entry hook, shown in the listing above). Another STEP takes you to line 3, and halts on the statement hook. The next STEP takes you to line 4, and halts on the program exit hook. As indicated by the pseudo assembly listing, only register 15 has changed during this STEP, and it contains the return value of the function. In the Monitor window, register 15 now has the value 0x00000014 (decimal 20), as expected.

You can change the value from 20 to 8 just before returning from dbl() by issuing the command:

%GPR15 = 8 ;