Before Jump

For many years a program on z architecture looked like

MYPROG  CSECT
        LR  12,15
        USING MYPROG,12
        B     OVER
        DC    C'*MYPROG  *'
OVER    DS   0H
...   

       LA 3,MYAUTOSTORAGE

       USING  MYDATA,3
       L  4,FIELD2 
MYDATA DSECT 
FIELD1 DS F
FIELD2 DS F 

To reference a field you have to do it off a register. For example

• B OVER is converted to BR 0x10 off register 12
• L 4,FIELD2 is converted to Load offset 4 from register 3

Each base register could cover 4KB, so for large programs, or large data you needed multiple registers.

Post Jump

A significant leap in the instructions was the introduction of the “… relative” instruction. For example the Jump (or as it is properly know the BRANCH RELATIVE ON CONDITION) is jump the number of half words from the start of the instruction

OFFSET  
0000 MYPROG  CSECT 
0000         LR  12,15 
0000         USING MYPROG,12 
0002         J   OVER     (A7F4 0007)
0006         DC    C'*MYPROG  *' 
0010 OVER    DS   0H F

The Jump instruction A7M4 xxxx) has condition M (F is always branch) and xxxx in this case if 0007. The address of the instruction is 0002, so jump to h2 + 2 * 7 = 0010 — which is the label for OVER.

This is a faster instruction than the B OVER, because it has to do less work. It can jump to +- 65534 bytes, and does not use any base registers. Not using a base register means that more registers can be used to access data.

But ..

I was writing some assembler code within a C program, and hit a few snags. The C code does not make a base register available to access fields.

With code like

MYLABEL  DC C'HELLO'
         LA  R2,MYLABEL

With base registers, this code will work and create the LA instruction with the offset of MYLABEL from the base register.

However, because there is no base register and USING, the LA instruction fails.

I found two ways of doing it when using relative instructions

Branch and save

          BRAS 2,OVERWTOL
SWTO      WTO TEXT=label,MF=L 
OVERWTOL  DS  0H         

This jumps to the label OVERWTOL, and saves the “return address” in register 2. Register 2 points to label SWTO.

Load address relative

OFFSET
0004        SWTO      WTO TEXT=label,MF=L 
0074        LARL  2,SWTO 

Which becomes load address with value -38 half words from the start of the instruction, so “offset” FFFFFFC8

This is half the solution

I was using the WTO macro to write message on the console.

   WTO   text=(Mydata),MF=(E,(4)) 
...
MYDATA   DC H'5',CL8'HELLO' 

This failed because there was no base register.

I had to use

         LARL   2,MYDATA   
         WTO   text=((2),MF=(E,(4)) 
...
MYDATA   DC H'5',CL8'HELLO' 

and this worked successfully.