program for interpretation of a simple instruction set - c

Question

I have a problem I need to solve and I have no freaking idea how to do it. If someone would be willing to help I would very much appreciate it. I know I'm asking for a lot, but I really need it.
Create a program for interpretation of a simple instruction set consisting of the instructions: MVI, MOV, AND, OR, NOT, LESS, LEQ, GRE, GEQ, JMP, PRN, SUM, SUB, PRB, SL and SR, described in this document. Your task is to make a program that takes as an input a binary representation of a list of instructions, and as an output it prints the corresponding result (after the execution of the instructions). The input can contain all the instructions except SUB and PRB that you do not have to implement. Conversion from binary system to any other numeral system should not be made, except at the moment when you need to find the line that should be executed next when the condition is satisfied (GRE, GEQ, LESS, LEQ, JMP), but the comparison of the numbers in the condition should be made based on the binary representatio/pn. All data are represented in SM binary system. There are eight 16-bit registers available enumerated from 0 to 7.
#include<stdio.h>
#define MAX 1000
char registers[8][16];
void MVI(int reg, char *value) {
// code here
}
void MOV(int reg1, int reg2) {
// code here
}
void AND(int reg1, int reg2, int reg3) {
// code here
}
void OR(int reg1, int reg2, int reg3) {
// code here
}
void NOT(int reg1, int reg2) {
// code here
}
void PRN(int reg) {
// code here
}
void SUM(int reg1, int reg2, int reg3) {
// code here
}
void SL(int reg) {
// code here
}
void SR(int reg) {
// code here
}
int main() {
int i,j,k;
int N = 0; // number of lines in the input
char c;
char lines[MAX][16];
while (1) {
scanf("%c", &c);
if (c == '\n') {
break;
}
lines[N][0] = c;
for (i=1;i<16;i++) {
scanf("%c", &lines[N][i]);
}
N++;
scanf("%c", &c);
}
for (i = 0; i < 8; i++) {
for (j = 0; j < 16; j++) {
registers[i][j] = '0';
}
}
// code here
return 0;
}

Answer 1

I think the big piece you need is dispatching the functions based on the source line. There are a number of ways you can do this, but a useful piece for all of them is strstr(a,b)==a which will check if the string a begins with the contents of the string b.
You can do a chain of if-else blocks.
if (strstr(line[i], "SUM")==line[i]){
SUM(...);
} else if (strstr(line[i], "AND")==line[i]) {
AND(...);
}
Or you can precompile the user program by scanning for the opcodes when you read the source and store them as single-byte small codes. You would want the uppercase identifiers to be enum values, and use the lowercase versions for the function names. Then the chain is simpler.
if (line[i][0] == SUM) {
SUM(...);
} else if (line[i][0] == AND) {
AND(...);
}
But, with small integer codes, there are even better ways. A switch.
switch(line[i][0]){
case SUM: sum(...); break;
case AND: and(...); break;
}
A function table. But this is where you have be clever. A function must always be called with arguments of the correct type, but function-pointers allow you to bypass the compiler's ability
to check that this is so. So for this method, all functions should have the same arguments since
they are all called by a single function-call line.
void (*optab[])(...) = { sum, and, ... };
optab[ line[i][0] ](...); // calls sum() or and() by using the opcode in the array lookup