To interface an assembly routine with a high level language program
means to be able to call functions back and forth. And to be able to do so
requires knowledge of certain behavior of the HLL when calling functions.
This behavior of calling functions is called the calling conventions of the
language. Two prevalent calling conventions are the C calling convention and
the Pascal calling convention.
What is the naming convention
C prepends an underscore to every function or variable name while Pascal
translates the name to all uppercase. C++ has a weird name mangling
scheme that is compiler dependent. To avoid it C++ can be forced to use C
style naming with extern “C” directive.
How are parameters passed to the routine
In C parameters are pushed in reverse order with the rightmost being
pushed first. While in Pascal they are pushed in proper order with the
leftmost being pushed first.
Which registers must be preserved
Both standards preserve EBX, ESI, EDI, EBP, ESP, DS, ES, and SS.
Which registers are used as scratch
Both standards do not preserve or guarantee the value of EAX, ECX, EDX,
FS, GS, EFLAGS, and any other registers.
Which register holds the return value
Both C and Pascal return upto 32bit large values in EAX and upto 64bit
large values in EDX:EAX.
Who is responsible for removing the parameters
In C the caller removes the parameter while in Pascal the callee removes
them. The C scheme has reasons pertaining to its provision for variable
number of arguments.
CALLING C FROM ASSEMBLY
For example we take a function divide declared in C as follows.
int divide( int dividend, int divisor );
To call this function from assembly we have to write.
push dword [mydivisor]
