Im attempting to put together a basic C Virtual machine that will read a binary file from an assembler program and use that file to execute commands based on the binary data in that file. Im at the point where i need to decode values into its hex format and then use these to determine opcode and execution operations. This is the part im confused on how to work around this? I have the registers and instructions defined but im wondering how i am to use the Fetch function to read the "instruction bytes" from memory and then decode to populate OP1 and/or OP2 in order to determine what command is to be done, what has me confused is with a command like pop push and return for within this example they all have the same starting opcode of 7 and differ only in the latter 2 bytes of push being 40 pop being 80 and return being 00. How can i pass all the 4-8 bytes of a command into fetch and be able to determine the command to be executed?
MAIN.C
#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
unsigned char memory[1028];
typedef enum{
halt,
add,
and,
divide,
mult,
sub,
or,
popushret,
interrupt,
addimmediate,
branchifequal,
branchifless,
jump,
call,
load,
store
} Instructions;
typedef enum{
R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, R14, R15, R16,
PC,
OP1, OP2,
Result,
CIP,
} Registers;
void decode( ) { // Read only from the array of bytes from fetch. Populate the OP1 and OP2 registers if appropriate. Fetch any additional memory needed to complete the instruction.
}
void fetch(int memory){ // Read instruction bytes from memory.
printf("%02x\n", memory);
}
int loads(char *filename){
FILE *file = fopen(filename, "rb");
ssize_t read;
if(file == NULL){
exit(-1);
}
while(fread(&memory, sizeof(unsigned char), 1, file) == 1){
fetch(*memory);
}
fclose(file);
exit(0);
}
int main(int argc, char** argv){
if (argc <= 1){
printf("No file found\n");
return -1;
}
char *filename = argv[1];
loads(filename);
}
INPUT.txt
74
40
77
40
11
23
OUTPUT.TXT
Desired output from the above is to push 4 to stack, push 7 to stack, pop 7 into R2 and 4 into R1 and the and add both into R3 and store in result
