Converting an assembly jump command to its hex value in C - c

Question

Im attempting to create an assembler in c which just reads the instructions from an input file and then translates them to their machine/hex code. So far so good until i hit the jump instruction, while i have it so that it translates the hex properly to in the case of jump 24 to c0 00 00 18 im unsure of how to check those 00's properly without hard coding it in. How can i do this? Should i be using shifts?
PR1.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char *ltrim(char *s) {
while (*s == ' ' || *s == '\t') s++;
return s;
}
char getRegister(char *text) {
if (*text == 'r' || *text=='R') text++;
return atoi(text);
}
char getHex(char *text){
int number = (int)strtol(text, NULL, 16);
char reservednum[5];
if(number <= 0xFFFF){
sprintf(&reservednum[0], "%04x", number);
}
return atoi(reservednum);
}
int assembleLine(char *text, unsigned char* bytes) {
text = ltrim(text);
char *keyWord = strtok(text," ");
if (strcmp("add",keyWord) == 0) {
bytes[0] = 0x10;
bytes[0] |= getRegister(strtok(NULL," "));
bytes[1] = getRegister(strtok(NULL," ")) << 4 | getRegister(strtok(NULL," "));
return 2;
}
else if(strcmp("subtract", keyWord) == 0){
bytes[0] = 0x50;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = getRegister(strtok(NULL, " ")) << 4 | getRegister(strtok(NULL, " "));
return 2;
}
else if(strcmp("and", keyWord) == 0){
bytes[0] = 0x20;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = getRegister(strtok(NULL, " ")) << 4 | getRegister(strtok(NULL, " "));
return 2;
}
else if(strcmp("divide", keyWord) == 0){
bytes[0] = 0x30;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = getRegister(strtok(NULL, " ")) << 4 | getRegister(strtok(NULL, " "));
return 2;
}
else if(strcmp("multiply", keyWord) == 0){
bytes[0] = 0x40;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = getRegister(strtok(NULL, " ")) << 4 | getRegister(strtok(NULL, " "));
return 2;
}
else if(strcmp("or", keyWord) == 0){
bytes[0] = 0x60;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = getRegister(strtok(NULL, " ")) << 4 | getRegister(strtok(NULL, " "));
return 2;
}
else if(strcmp("halt", keyWord) == 0){
bytes[0] = 0x00;
bytes[1] = 0x00;
return 2;
}
else if(strcmp("return", keyWord) == 0){
bytes[0] = 0x70;
bytes[1] = 0x00;
return 2;
}
else if(strcmp("addimmediate", keyWord) == 0){
bytes[0] = 0x90;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = getHex(strtok(NULL, " "));
return 2;
}
else if(strcmp("interrupt", keyWord) == 0){
bytes[0] = 0x80;
bytes[0] |= getHex(strtok(NULL, " "));
bytes[1] = 0x00;
return 2;
}
else if(strcmp("push", keyWord) == 0){
bytes[0] = 0x70;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = 0x40;
return 2;
}
else if(strcmp("pop", keyWord) == 0){
bytes[0] = 0x70;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = 0x80;
return 2;
}
else if(strcmp("load", keyWord) == 0){
bytes[0] = 0xE0;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = getRegister(strtok(NULL, " ")) << 4 | getHex(strtok(NULL, " "));
return 2;
}
else if(strcmp("store", keyWord) == 0){
bytes[0] = 0xF0;
bytes[0] |= getRegister(strtok(NULL, " "));
bytes[1] = getRegister(strtok(NULL, " ")) << 4 | getHex(strtok(NULL, " "));
return 2;
}
else if(strcmp("jump", keyWord) == 0){
bytes[0] = 0xC0;
bytes[1] = 0x00;
bytes[2] = 0x00;
bytes[3] = getHex(strtok(NULL, " "));
return 4;
}
}
int main(int argc, char **argv) {
FILE *src = fopen(argv[1],"r");
FILE *dst = fopen(argv[2],"w");
while (!feof(src)) {
unsigned char bytes[4];
char line[1000];
if (NULL != fgets(line, 1000, src)) {
printf ("read: %s\n",line);
int byteCount = assembleLine(line,bytes);
fwrite(bytes,byteCount,1,dst);
}
}
fclose(src);
fclose(dst);
return 0;
}