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Runtime Environment - Interfacing C with Assembly Language
sembler directive. This defines the symbol as external and allows the
linker to resolve references to it.
Similarly, to access a C function or object from assembly, declare the C
object with .global, thus creating an undefined external reference that
the linker must resolve.
5.4.1.1 An Example of
an
Assembly Language Function
Example 5-1 illustrates a C function called main which calls an assembly
language function called aSIn-func. The aSIn-func function has one argu-
ment which is a pointer to an integer.
aSIn-func calls another C function
called c-func with one argument which is a global variable named gvar.
aSIn-func takes the value returned from c_func and stores it in the integer
pointed to by its single argument.
Example 5-1. An Assembly Language Function
extern int aSIn-func(); /* declare external asm function */
int gvar;
/* define global variable
*/
main()
{
int i, j;
i
=
aSIn-func(&i);
FP
STK
.set
.set
.global
. global
. global
-asIn-func:
MMTM
MOVE
MOVE
MOVE
MOVE
CALLA
MOVE
MOVE
MMFM
RETS
(a) C Program
A13
A14
-gvar
-c_func
-asIn-func
SP,A7,FP
STK,FP
*FP(-32),A7,1
STK -*SP 1
@-g~ar,*STK+,l
-c_func
A8,*A7,1
*SP(96),STK,1
SP,A7,FP
2
frame pointer
program stack pointer
declare global variable
declare C function
declare this function
save registers on SP
set up FP
get argument
function call: save STK
push argument
call function
result in A8
restore caller's STK
(pop arguments)
restore saved registers
return & pop caller's STK
(b) Assembly Language Program
In the C program in Example 5-1, the
extern
declaration of asmfunc is op-
tional, since the function returns an int. Like C functions, assembly functions
need only be declared if they return non-integers.
In the assembly language code in Example 5-1, note the underscores on all
the C symbol names when used in the assembly code.
5-13
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