A demo from class; a "pico" fetch/execute virtual machine written in just over 5 minutes.

picovm.c

#include <stdio.h>

int code[] = {0x73, 0x75, 0x1A, 0x00};
int stack[] = {0, 0, 0};

#define A 0
#define B 1
#define C 2

void printStack () {
  printf("A %02X, B %02X, C %02X\n", stack[A], stack[B], stack[C]);
}

int main () {
  int pc = 0;
  
  while (code[pc] != 0x00) {
    /* Grab the nibbles. */
    int upper = code[pc] >> 4;
    int lower = code[pc] & 0x0F;
      
    /* Increment the program counter. */
    pc++;
      
    switch (upper) {
      /* Load a constant. */
      case 7:
        stack[C] = stack[B];
        stack[B] = stack[A];
        stack[A] = lower;
        break;
          
      /* An operation. I cheat, because
         we only support ADD in this example. 
      */
      case 1:
        stack[A] = stack[A] + stack[B];
        break; 
    }
    
    /* Show the stack every time we fetch and
       execute an instruction. 
    */
    printStack();
  }
  
  /* Make C happy. */
  return 0;
}