Calling a C-function from assembler

//file c_function.h
#ifndef c_function_h
#define c_function_h

int add_int(int a, int b);

#endif

//file c_function.c
#include "c_function.h"

int add_int(int a, int b)
{
	return (a+b);
}

//file c_from_assem.a51
?PR?main?TEST SEGMENT CODE
?DT?main?TEST SEGMENT DATA
	EXTRN CODE (_add_int)
	EXTRN CODE (?C_STARTUP)
	RSEG ?DT?main?TEST
?main?BYTE:
	result:	DS	2

	RSEG ?PR?main?TEST
	USING 0
main:
	MOV	R7,#054H
	MOV	R6,#023H
	MOV	R5,#07BH
	MOV	R4,#00H
	LCALL	_add_int
	MOV 	result,R6
	MOV	result+01H, R7
LOOP:	AJMP	LOOP
	RET
END