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Communication with Assembler Programs

Calling a C Function from Assembler

A function written in C can be called from an assembler function as long as general register 12 addresses the CRAB when the C function is called. If the function is a library function, the calling (assembler) function must use the CENTRY and CEXIT macros to preserve the DSA chain. Most library functions depend upon being called from a normal C framework, that is, with general register 12 addressing the CRAB and general register 13 addressing a C DSA.

Calling a C Library Function from Assembler shows the sumint function expanded to call printf to write the result to stdout . Note that the printf parameter list is created in the DSA and that the CENTRY parameter DSA now specifies a non-zero DSA size. This version of sumint is called from the C main program in Sample C main Program. 



Calling a C Library Function from Assembler



         EJECT
         PRINT ON,GEN
SUMINT@  CSECT
         CREGS USING
         SPACE
SUMINT   CENTRY INDEP=NO,DSA=DSALEN
*---------------------------------------------------------------------*
* Make sure we actually got a plist address on the call.              *
*---------------------------------------------------------------------*
         SR    R3,R3              Clear R3 for sum'ing
         LTR   1,1                Is there a plist?
         BZ    DONE               Nope, just leave w/R3=0!
         SPACE
*---------------------------------------------------------------------*
* Sum integers passed via VL-format parameter list.                     *
*---------------------------------------------------------------------*
         SR    R3,R3              Clear R3 for summing
NEXTADD  DS    0H
         L     R4,0(R1)           Load pointer
         A     R3,0(R4)           Add integer to sum
         TM    0(R1),X'80'        End of VL-Plist? <---Note This Check
         BNZ   DONE               Yes, finish up and exit
         LA    R1,4(R1)           No, bump to next argument
         B     NEXTADD            Start again
DONE     DS    0H                 Yes, prepare to return
         SPACE
*---------------------------------------------------------------------*
* Call printf to display the sum of integers                          *
*---------------------------------------------------------------------*
         ST    R3,SUMINTS         SUM of int's to Parmlist
         MVC   FMTPTR,=A(FORMAT)  Move format pointer to PRINTF
*                                 Parmlist
         L     R15,=V(PRINTF)     R15 -> PRINTF
         LA    R1,PARMLIST        R1  -> Parmlist Address
         BALR  R14,R15            Call PRINTF
         SPACE
*---------------------------------------------------------------------*
* Exit with the sum of the integers provided to CEXIT in R3.          *
*---------------------------------------------------------------------*
         CEXIT  RC=(R3),INDEP=NO
         EJECT
*---------------------------------------------------------------------*
* Constants                                                           *
*---------------------------------------------------------------------*
         LTORG                         Area for Literal Pool
FORMAT   DC    X'15'                   New Line Before Output
         DC    C'Assembler Sum: %d'
         DC    X'1500'                 New line with NULL terminator
*---------------------------------------------------------------------*
* Working Storage                                                     *
*---------------------------------------------------------------------*
         COPY  DSA                     Required for CENTRY/CEXIT
PARMLIST DS    0D
FMTPTR   DS    A                       Address of printf parmlist
SUMINTS  DS    F                       Sum of int's
DSALEN   EQU   *-DSA                   Length of DSA
*---------------------------------------------------------------------*
* Dsects                                                              *
*---------------------------------------------------------------------*
         COPY  CRAB                    Required for CENTRY/CEXIT
         END   SUMINT


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