Return to callingconvent CVS log

Up to [MW Coherent from dump] / coherent / a / usr / man / MULTI

coherent

calling conventions   Technical Information   calling conventions



The following presents the calling conventions for COHERENT.

The design  of the calling  conventions had to  take into account
the fact  that C  does not require  that the number  of arguments
passed  to a  function be  the  same as  the number  of arguments
specified in  the function's declaration.  Routines  with a vari-
able number  of arguments are  not uncommon; for  example, printf
and scanf can take  a variable number of arguments.  Another con-
sideration was the availability of register variables.

Therefore,  COHERENT uses  the following  calling  sequence.  The
function arguments  are pushed onto the stack  from the first, or
rightmost, through the last, or leftmost.  lloonnggs are pushed high-
half  first; this  makes the  word order  compatible with  the dddd
instruction.  The  function is then called with  a near call.  An
add  instruction after  the call removes  the arguments  from the
stack.

For example, the function call


        int a;
        long b;
        char c;

        foo()
        {
                example(a, b, c);
        }


generates the code


        movb     al,c
        cbw
        push     ax
        push     b+2
        push     b
        push     a
        call     example_
        add      sp,8


Note that  an underbar  character `_'   has been appended  to the
function  name.  This  serves two purposes.   First, it  makes it
harder to accidentally  call routines written in other languages.
Second,  it means  that two  routines with the  same name  can be
called from C and another language in identical fashions.

The  parameters and  local variables in  the called  function are
referenced as offsets  from the bp register.  The arguments begin
at  offset 8  and continue toward  higher addresses,  whereas the
local variables begin at  offset -2 and continue toward lower ad-
dresses.


COHERENT Lexicon                                           Page 1




calling conventions   Technical Information   calling conventions




The sp  register points  the local  variable with the  lowest ad-
dress.  Thus,  when eexxaammppllee_ is  reached in  the above model, the
stack frame resembles the following:


     High      ZDDDDDDDDDDDDDDDDDDDDDDD?
               3 c (widened to a word) 3
               CDDDDDDDDDDDDDDDDDDDDDDD4
               3    high half of b     3
               CDDDDDDDDDDDDDDDDDDDDDDD4
               3     low half of b     3
               CDDDDDDDDDDDDDDDDDDDDDDD4
               3          a            3
     Low       @DDDDDDDDDDDDDDDDDDDDDDDY


Functions  return iinntts  in the  ax register,  lloonnggs in  the dx:ax
register pair, pointers in  the ax register and ddoouubbllees in ffppaacc_.
The following program


     example(a, b, c)
     int a, b, c;
     {
         return (a * b - c);
     }


when compiled  with the -VVAASSMM option,  produces the following as-
sembly-language code:


     .shri
     .globl example_



example_:
     push      si
     push      di
     push      bp
     mov       bp, sp
     mov       ax, 10(bp)
     imul      8(bp)
     sub       ax, 12(bp)
     pop       bp
     pop       di
     pop       si
     ret


The runtime startup initializes the registers cs, ds, es, and ss,
and the segment  registers remain unchanged.  Other registers may
be overwritten.


COHERENT Lexicon                                           Page 2




calling conventions   Technical Information   calling conventions




COHERENT pushes function arguments as follows.


          cchhaarr   Widened to iinntt, then pushed
          iinntt    Pushed in machine word order
          lloonngg   Pushed high order word, then low-order word
          ffllooaatt  Widened to ddoouubbllee, then pushed
          ddoouubbllee Pushed high order, then low order
          ssttrruucctt Pushed in memory order
          uunniioonn  Pushed in memory order


Functions return values as follows:


          cchhaarr   In al
          iinntt    In ax
          lloonngg   In dx:ax
          ffllooaatt  Same as ddoouubbllee
          ddoouubbllee In ffppaacc_
          ssttrruucctt Pointer in ax
          uunniioonn  Pointer in ax
          pointerIn ax


A  function that  returns a  struct or  union actually  returns a
pointer; the code generated for the function call block-moves the
result to its destination Functions that return a float or double
return it in the global double ffppaacc_.

For example, consider the call


     example(i, l, c, cp);


where i is an int, l is a long, c is a char, cp is a pointer to a
char, and  example declares two automatic  iinntts.  After execution
of the  call and the prologue of example,  the stack contains the
following 11 words:


     High      ZDDDDDDDDDDDDDDDDDD?
               3        cp        3
               CDDDDDDDDDDDDDDDDDD4
               3         c        3
               CDDDDDDDDDDDDDDDDDD4
               3  high word of l  3
               CDDDDDDDDDDDDDDDDDD4
               3   low word of l  3
               CDDDDDDDDDDDDDDDDDD4
               3         i        3
               CDDDDDDDDDDDDDDDDDD4
               3  return address  3


COHERENT Lexicon                                           Page 3




calling conventions   Technical Information   calling conventions



               CDDDDDDDDDDDDDDDDDD4
               3     saved SI     3
               CDDDDDDDDDDDDDDDDDD4
               3     saved DI     3
               CDDDDDDDDDDDDDDDDDD4
               3     saved BP     3
               CDDDDDDDDDDDDDDDDDD4
               3 space for auto 1 3
               CDDDDDDDDDDDDDDDDDD4
               3 space for auto 2 3
     Low       @DDDDDDDDDDDDDDDDDDY


The following example performs a simple function call:


main()
{
     example(1, 2);/* call sample routine */
}

example(p1, p2)
{
     int a, b;

     a = 3;
     b = 4;
}


When the  function eexxaammppllee is about to  return, the stack appears
as follows:


High ZDDDDDDDDDDDDDDDDD?
     3       2         3^  ppaarrmm 22    1100(bbpp)
     CCDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD44
     33       11         33^  parm 1     8(bp)
     CDDDDDDDDDDDDDDDDD4
     3 Return Address: 3
     3    2 words in   3
     3   LARGE model,  3
     3 1 in SMALL model3              6(bp)
     CDDDDDDDDDDDDDDDDD4
     3     main's SI   3              4(bp)
     CDDDDDDDDDDDDDDDDD4
     3     main's DI   3              2(bp)
     CDDDDDDDDDDDDDDDDD4
     3     main's BP   3               (bp)
     CDDDDDDDDDDDDDDDDD4
     3       3         3^ aa          -22(bbpp)
     CCDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD44
LLooww  33       44         33^ SP b       -4(bp)
     @DDDDDDDDDDDDDDDDDY



COHERENT Lexicon                                           Page 4




calling conventions   Technical Information   calling conventions




***** See Also *****

C language, technical information





















































COHERENT Lexicon                                           Page 5

This archive runs on limited infrastructure. Preserving old code on modern bandwidth. Automated agents are requested to crawl responsibly.