Base register in assembler (2)

Dealing with programs bigger than 4K

This time I will discuss how to make a program bigger than 4096 bytes which is the limitation of one base register.

PGM5     CSECT                             PGM5     RMODE ANY                                 ENTRY PGM5                                STM   14,12,12(13)                         BASR  11,0                        START    DS    0H                                  USING START,11                            MVC   WTOTEXT,=CL70'HELLO WORLD'         B     WTO                         WTOTEXT_LGD DC Y(L'WTOTEXT)                WTOTEXT  DS    CL70                                DC    4096C' ' ⇐ This is my way to increase the the size WTO      DS    0H                                  WTO   TEXT=WTOTEXT_LGD,ROUTCDE=11         LM    14,12,12(13)                 RETURN   RETURN RC=0                               LTORG                                     END                               PGM5     BINDER                                    END

PGM5 is bigger than 4096 bytes


When I assemble the source above, I get a serious problem. Experienced programmers will recognize it but not with joy. We've all tried to solve this problem for hours on end after normal work hours.

000000                00000 01136     2 PGM5     CSECT                                                               3 PGM5     RMODE ANY                                                           4          ENTRY PGM5                     000000 90EC D008            00008     5          STM   14,12,12(13)              000004 0DB0                           6          BASR  11,0                     000006                                7 START    DS    0H                                       R:B  00006           8          USING START,11                 000006 0000 0000 0000 00000 00000     9          MVC   WTOTEXT,=CL70'HELLO WORL ** ASMA034E Operand =CL70'HELLO WORLD' beyond active USING range by 235 bytes   ** ASMA435I Record 9 in EXJEL.EXJEL005.JOB07575.D0000101.? on volume:           00000C 0000 0000            00000    10          B     WTO                      ** ASMA034E Operand WTO beyond active USING range by 83 bytes                   ** ASMA435I Record 10 in EXJEL.EXJEL005.JOB07575.D0000101.? on volume:          000010 0046                          11 WTOTEXT_LGD DC Y(L'WTOTEXT)             000012                               12 WTOTEXT  DS    CL70                     000058 4040404040404040              13          DC    4096C' '                 001058                               14 WTO      DS    0H                       001058 A715 003A            010CC    15          WTO   TEXT=WTOTEXT_LGD,ROUTCDE 0010E0 98EC D008            00008    54          LM    14,12,12(13)              0010E4 41F0 0000            00000    55 RETURN   RETURN RC=0                    0010F0                               60          LTORG                          0010F0 C8C5D3D3D640E6D6              61                =CL70'HELLO WORLD'                                           62          END

I will give you two ways to solve it, a quick and dirty and a more structured approach.

Several base registers

We are essentially in need for one more base register and we can actually add one more so we can span over 8K instead of 4K. I add register 10 as a new register so the program has 10 and 11 as its base registers. Register 10 addresses the first 4096 bytes and register 11 the rest.

I will explain how to do that by explaining each line. We start at line 5

5. Load current address into register 10
6. Just a label that is not used
7. Then we load register 10 into register 11 and adds 4095 (X’FFF’) to register 11
8. Add one to register 11 because one base register can span over 4096 bytes
9. The USING tells the assembler to use both registers as base register.

PGM5     CSECT                              PGM5     RMODE ANY                                  ENTRY PGM5                                 STM   14,12,12(13)                         BASR  10,0                         START    DS    0H                                   LA    11,X'FFF'(,10)                       LA    11,1(,11)                            USING START,10,11                          MVC   WTOTEXT,=CL70'HELLO WORLD'           B     WTO                          WTOTEXT_LGD DC Y(L'WTOTEXT)                 WTOTEXT  DS    CL70                                 DC    4096C' '                     WTO      DS    0H                                   WTO   TEXT=WTOTEXT_LGD,ROUTCDE=11         LM    14,12,12(13)                 RETURN   RETURN RC=0                                LTORG                                      END                                PGM5     BINDER                                     END

Source of PGM5 that has two base registers

If you are now in the first half of the program and you branch to the other half, the branch will use register 11 and visa versa. Take a look at location counter 000014. The WTO label is in the second half of the program so register 11 is used.

Loc  Object Code    Addr1 Addr2  Stmt   Source Statement                                                           1          PRINT NOGEN                    000000                00000 0113E     2 PGM5     CSECT                                                               3 PGM5     RMODE ANY                                                           4          ENTRY PGM5                     000000 90EC D00C            0000C     5          STM   14,12,12(13)             000004 0DA0                           6          BASR  10,0                     000006                                7 START    DS    0H                       000006 41B0 AFFF            00FFF     8          LA    11,X'FFF'(,10)           00000A 41B0 B001            00001     9          LA    11,1(,11)                               R:AB  00006          10          USING START,10,11              00000E D245 A014 B0F2 0001A 010F8    11          MVC   WTOTEXT,=CL70'HELLO WORL 000014 47F0 B05A            01060    12          B     WTO                      000018 0046                          13 WTOTEXT_LGD DC Y(L'WTOTEXT)             00001A                               14 WTOTEXT  DS    CL70                     000060 4040404040404040              15          DC    4096C' '                 001060                               16 WTO      DS    0H                       001060 A715 003A            010D4    17          WTO   TEXT=WTOTEXT_LGD,ROUTCDE 0010E8 98EC D00C            0000C    56          LM    14,12,12(13)             0010EC 41F0 0000            00000    57 RETURN   RETURN RC=0                    0010F8                               62          LTORG                          0010F8 C8C5D3D3D640E6D6              63                =CL70'HELLO WORLD'                                           64          END

Assembler list of PGM5 with two base registers

Separating your program into subrutines

If you are a clever programmer you will probably always separate your assembler program into subroutines. That makes it easier to understand and maintain. I will show you an example where I separated the above program into two CSECTs. You do not need to code the CSECT but I like it because it shows that I am serious about it. You are now not allowed to branch between CSECTs. You would not do that anyway but an ignorant programmer, that might want to maintain your program, would try and you have now prohibited him from destroying your architecture.

I use register 11 in both CSECTs. I can do that because I save all registers on entry to CSECT WTO_HW and reload them right before returning to the caller. WTO_HW has actually become a small program which the Binder (Linkage Editor) links together with PGM5 into a load module. At the end of each CSECT I “DROP” register 11 as base register. Again that is in order to tell the assembler that the USING goes till here and no further. Again a good practise.

PGM5     CSECT                               PGM5     RMODE ANY                           PGM5     AMODE 31                                    ENTRY PGM5                                  STM   14,12,12(13)                          BASR  11,0                                  USING *,11                                  LA    1,SAVEAREA                            ST    1,8(,13)                              ST    13,4(,1)                              LR    13,1                                  L     15,=A(WTO_HW)                         BASR  14,15                                 L     1,4(,13)                              LM    14,12,12(1)                   RETURN   RETURN RC=0                         SAVEAREA DS    18F                                   LTORG                                       DC    4096C' '                              DROP  11                            ******************************************** WTO_HW   CSECT                                       STM   14,12,12(13)                          BASR  11,0                                  USING *,11                                  MVC   WTOTEXT,=CL70'HELLO WORLD'            WTO   TEXT=WTOTEXT_LGD,ROUTCDE=11           LM    14,12,12(13)                          BR    14                            WTOTEXT_LGD DC Y(L'WTOTEXT)                  WTOTEXT  DS    CL70                                  LTORG                                       DROP  11                                    END                                 PGM5     BINDER                                      END

PGM5 is now well structured

Loc  Object Code    Addr1 Addr2  Stmt   Source Statement                                                           1          PRINT NOGEN                    000000                00000 01074     2 PGM5     CSECT                                                               3 PGM5     RMODE ANY                                                           4 PGM5     AMODE 31                                                            5          ENTRY PGM5                     000000 90EC D00C            0000C     6          STM   14,12,12(13)             000004 0DB0                           7          BASR  11,0                                     R:B  00006           8          USING *,11                     000006 4110 B022            00028     9          LA    1,SAVEAREA               00000A 5010 D008            00008    10          ST    1,8(,13)                 00000E 50D0 1004            00004    11          ST    13,4(,1)                 000012 18D1                          12          LR    13,1                     000014 58F0 B06A            00070    13          L     15,=A(WTO_HW)            000018 0DEF                          14          BASR  14,15                    00001A 5810 D004            00004    15          L     1,4(,13)                 00001E 98EC 100C            0000C    16          LM    14,12,12(1)              000022 41F0 0000            00000    17 RETURN   RETURN RC=0                    000028                               22 SAVEAREA DS    18F                      000070                               23          LTORG                          000070 00001078                      24                =A(WTO_HW)               000074 4040404040404040              25          DC    4096C' '                                                     26          DROP  11                                                           27 *************************************** 001078                01078 0012E    28 WTO_HW   CSECT                          001078 90EC D00C            0000C    29          STM   14,12,12(13)             00107C 0DB0                          30          BASR  11,0                                     R:B  0107E          31          USING *,11                     00107E D245 B096 B0E2 01114 01160    32          MVC   WTOTEXT,=CL70'HELLO WORLD' 001084 A715 003A            010F8    33          WTO   TEXT=WTOTEXT_LGD,ROUTCDE =11 00110C 98EC D00C            0000C    72          LM    14,12,12(13)             001110 07FE                          73          BR    14                       001112 0046                          74 WTOTEXT_LGD DC Y(L'WTOTEXT)             001114                               75 WTOTEXT  DS    CL70                     001160                               76          LTORG                          001160 C8C5D3D3D640E6D6              77                =CL70'HELLO WORLD'                                           78          DROP  11                                                           79          END

The assembler list of PGM5.

Location 00107E shows that register 11 is a new base register with a new value.

By the way. Please, never use large work areas or constants in your program. It will give you problems with your base registers and your load module will be huge and take up space in the loadlib. Getmain your work area or, if it is constants, either use an assemble table or read them from an external medium.

Next time, I will show a more modern way to solve the problem.