README.md

Testing

Demonstration to show standlone assembly codes can be called the same way as assembly rountines are called from C programs.

Steps to run standlone assembly

• Sample code q1.asm
• Under .text section entry point should be mentioned.If the _start is not mentioned under .text then add it using global _start.
• start acts as a main function in assembly.
• mov parameters to rsi,rdi (default registers to pass arguments)
• call checkgreater.
• the ouput of the _checkGreater is done by write syscall

_print1:			    ;label to print 1 if x>y
	mov rax,1		    ;write syscall
	mov rdi,0
	mov rsi,msg1
	mov rdx,1
	syscall
	call _exit		   ;exit syscall to avoid segmentation fault
	ret 

• to return from jump instructions properly exit syscall is used.

_exit:      
       mov rax,60		;syscall to exit
       mov rdi,0
       syscall
       ret 

• instead of labels ,macros can also be used if you dont want to use registers as medium to pass arguments but macros can be called from C program. Calling macros from start as -checkGreater 4,3

%macro checkGreater 2		;macro definition name and no of parameters
	mov rax,%1		        ;load 1st parameter to rax
	cmp rax,%2		        ;compare with second parameter
	jg  _print1		        ;if greater jump to label print1
  jmp _print0		        ;else jump to label print2
%endmacro  			        ;end of macro

• Make file contents should be

#Makefile
all: q1.o q1
q1.o:q1.asm
	nasm -f elf64 -o q1.o q1.asm	
q1:q1.o
	ld q1.o -o q1
	./q1
	@echo 

Steps to call assembly routine from C

• Sample c code checkGreater.c
• Function declaration before main function as shown in line 1(mention return type,function name, parameters if any)
• calling the function in main as in line 2

#include <stdlib.h>
#include<inttypes.h>
void _checkGreater(int64_t,int64_t); // line1
int main(){
_checkGreater(42,53);      //line 2
return 0;
}

• Assembly code remains same as before.
• remove _start in text section and replace it with your function name(here _checkGreater)
• rest of the code remains the same.

Make file changes

#Makefile
all: q1.o checkGreater.o ans
q1.o:q1.asm
	nasm -f elf64 -o q1.o q1.asm	
q1:q1.o
	ld q1.o -o q1
	./q1
	@echo 
checkGreater.o:checkGreater.c
	gcc -c checkGreater.c -o checkGreater.o

ans:q1.o checkGreater.o
	gcc checkGreater.o q1.o -o ans
	./ans
	@echo	

Finally you tested standlone assembly programs can be called from C as well !.You can visit read nasm tutorial to know the syntax.