IBM Mainframe Forum

by **Shady** » Fri Jul 06, 2012 10:23 pm

Hello,

I'm relative new to assembler. I bought some books for reading and also coded a bit... The code isn't a problem in this post. At the moment the program works fine. I just want to advance it and I need an info.

I've written a program for a collegue. He wrotes a metal C program and needs a WTO in his C program. OK, so I wrote a very little program for him. OK. Works fine. Now he also want to call that program from job control with EXEC PGM=...,PARM='PARM TEXT'.
I know that the first halfword from the parameter is the length of the content of PARM=. In the WTO for the metal C program is no prefix with the length. There I just loop through the parameter and search for the end of the parameter.

So my question: How can I differ wheather the PARM comes from a call from a program (like Metal C) or it comes from batch with PARM=
Is there a control block or sth. where I can find something to identify the source?

Regards

by **steve-myers** » Sat Jul 07, 2012 2:19 am

See this topic.

In concept, a program should not care, or even know, if it is called from JCL or another program. There are no 100% certain methods for a program to determine if it is called by JCL or another program. Some possibilities:

Compare the save area address used when your program is entered with the address in TCBFSA in the current TCB. If they match, AND the current TCB address matches the address in TCBJSTA in the current TCB, your program probably - though not certainly - was called from JCL. It still could have been called from another program using the XCTL macro, but ...
Obtain the current request block address from TCBRBP in the current TCB. Compare the 24-bit "next" RB address with the address of the current TCB. If they are equal, your program was probably called from JCL.

I wrote the following program to test these ideas

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TESTJCL CSECT DEFINE PROGRAM CSECT
PUSH PRINT
PRINT NOGEN
CVT DSECT=YES DEFINE CVT DATA AREA
IKJTCB , DEFINE TVB DATA AREA
IHARB DSECT=YES DEFINE RB DATA AREA
POP PRINT
TESTJCL CSECT RETURN TO PROGRAM CSECT
USING *,12 ESTASBLISH ADDRESSABILITY
SAVE (14,12),,* SAVE REGISTERS
LR 12,15 COPY ENTRY POINT ADDRESS TO REG 12
* WE DO NOT HAVE TO OBTAIN A NEW SAVE AREA
L 2,CVTPTR LOAD CVT ADDRESS
L 2,CVTTCBP-CVTMAP(,2) LOAD ADDR OF THE TCB POINTER WORDS
L 2,4(,2) LOAD ADDR OF THE CURRENT TCB
N 2,=A(X'FFFFFF') ISOLATE THE 24-BIT TCB ADDRESS
L 3,TCBRBP-TCB(,2) LOAD ADDR OF THE CURRENT RB
N 3,=A(X'FFFFFF') ISOLATE THE 24-BIT RB ADDRESS
L 4,TCBFSA-TCB(,2) LOAD ADDR OF THE FIRST SAVE AREA
N 4,=A(X'FFFFFF') ISOLATE THE 24-BITE ADDRESS
L 5,TCBJSTCB-TCB(,2) LOAD ADDR OF THE JOB STEP TCB
N 5,=A(X'FFFFFF') ISOLATE THE 24-BIT TCB ADDRESS
CR 2,5 TEST IF CURRENT TCB = THE JOB STEP ->
TCB
BNE NOTJCL BR IF NOT
WTO 'CURRENT TCB IS THE JOBSTEP TCB',ROUTCDE=11,DESC=7
CR 4,13 TEST IF SAVE AREA = FIRST SAVE AREA
BNE FSAERR BR IF NOT
WTO 'SAVE AREA PONTER = TCBFSA',ROUTCDE=11,DESC=7
B FSAOK
FSAERR WTO 'SAVE AREA POINTER ¬= TCBFSA',ROUTCDE=11,DESC=7
FSAOK CLM 2,B'0111',RBLINK-RBBASIC+1(3) TEST RBLINK
BNE RBERROR BR IF NOT
WTO 'RBLINK POINTS TO TCB',ROUTCDE=11,DESC=7
B EXIT
RBERROR WTO 'RBLINK DOES NOT POINT TO TCB',ROUTCDE=11,DESC=7
B EXIT
NOTJCL WTO 'TCB IS NOT THE JOB STEP TCB',ROUTCDE=11,DESC=7
EXIT RETURN (14,12),T,RC=0
DC 0D'0'
LTORG ,
DC 0D'0'
END TESTJCL

I assembled the program, then used this JCL to run it.

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//A EXEC PGM=LOADER,PARM='MAP'
//SYSLOUT DD SYSOUT=*
//SYSLIN DD DISP=(SHR,PASS),DSN=&SYSUID..TESTJCL.OBJ
//B EXEC PGM=IEWL,PARM=MAP
//SYSPRINT DD SYSOUT=*
//SYSLMOD DD DISP=(,PASS),UNIT=SYSDA,SPACE=(TRK,(1,1,1)),DSN=&&G(G)
//SYSLIN DD DISP=(SHR,PASS),DSN=&SYSUID..TESTJCL.OBJ
//C EXEC PGM=*.B.SYSLMOD

The LOADER program loads the object deck directly into storage and then runs it as a subtask, so it should not be considered as called from JCL. Step B prepares a load module from the object deck, and step C runs the program from JCL.

by **Shady** » Sat Jul 07, 2012 9:17 am

Thank you very much!
If I can find a free time frame I'll try to change my prog next week.

Again: Thank you.

by **steve-myers** » Sun Jul 08, 2012 8:16 am

A few additional thoughts.

When a program is called using full standard linkage, register 1 points to a parameter list. A parameter list contains one or more words that point to parameters. The high order bit in the last word of a standard parameter list is on. When a program is called from JCL, the parameter list contains 1 word, and the high order bit is on. Your program can test this: if the high order bit of the first word is not on, it was not called from JCL. Of course, if the parameter list contains just 1 word, that does not mean it was called from JCL. This code mimics JCL.

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CALL PGM,PARMTEXT,VL
...
PARMTEXT DC H'3',CL3'JCL'

The VL parameter in the CALL macro directs the macro to insert the high order bit in the parameter list.

In truth, few self respecting Assembler programmers would tolerate the expansion of that CALL macro: it is -

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+ CNOP 0,4
+ B *+8
+IHB0001B DC V(PGM)
+ LA 1,IHB0003
+ B IHB0003A
+IHB0003 DS 0F
+ DC A(PARMTEXT+X'80000000')
+IHB0003A EQU *
+ L 15,IHB0001B
+ BALR 14,15

The reason: it is horribly inefficient. A branch around an inline data area is slow, and there are two of these branches. A self respecting Assembler programmer does something like this -

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L 15,=V(PGM)
CALL (15),MF=(E,PARMLIST)
...
PARMLIST CALL ,PARMTEXT,VL,MF=L
PARMTEXT DC H'3',C'JCL'

The first CALL macro becomes -

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CALL (15),MF=(E,PARMLIST)
+ DS 0H
+ LA 1,PARMLIST
+ BALR 14,15

The second CALL macro becomes -

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PARMLIST CALL ,PARMTEXT,VL,MF=L
+PARMLIST DS 0F
+ DC A(PARMTEXT+X'80000000')

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