了解stack machine运行时的绝佳代码

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/* OPERATIONS: Internal Representation */ 
enum code_ops { HALT, STORE, JMP_FALSE, GOTO, DATA, LD_INT, 
                LD_VAR, READ_INT, WRITE_INT, LT, EQ, GT, 
                ADD, SUB, MULT, DIV, PWR }; 

/* OPERATIONS: External Representation */ 
char *op_name[] = {"halt", "store", "jmp_false", "goto", 
                   "data", "ld_int", "ld_var", "in_int", 
                   "out_int", "lt", "eq", "gt", 
                   "add", "sub", "mult", "div", "pwr" }; 

struct instruction { enum code_ops op; int arg; };
struct instruction code[999]; 
int stack[999];
int pc = 0; 
truct instruction ir; 
int ar = 0; 
int top = 0;
void fetch_execute_cycle() 
{ do { /* Fetch */ ir = code[pc++];
       /* Decode & Execute */ 
       switch (ir.op) { 
          case HALT      : printf( "halt\n" ); break; 

          case READ_INT  : printf( "Input: " );
                             scanf( "%ld", &stack[ar+ir.arg] );     break;
          case WRITE_INT : printf( "Output: %d\n", stack[top--] );  break;
          case STORE     : stack[ir.arg] = stack[top--];            break;
          case JMP_FALSE : if ( stack[top--] == 0 ) pc = ir.arg;
                                                                    break;
          case GOTO      : pc = ir.arg;                             break;
          case DATA      : top = top + ir.arg;                      break;
          case LD_INT    : stack[++top] = ir.arg;                   break;
          case LD_VAR    : stack[++top] = stack[ar+ir.arg];         break;
          case LT        : if ( stack[top-1] < stack[top] )
                                stack[--top] = 1;
                             else stack[--top] = 0;                 break;
          case EQ   : if ( stack[top-1] == stack[top] ) stack[--top] = 1; else stack[--top] = 0;        break; 
          case GT   : if ( stack[top-1] > stack[top] )  stack[--top] = 1; else stack[--top] = 0; top--; break; 
          case ADD  : stack[top-1] = stack[top-1] + stack[top]; top--; break; 
          case SUB  : stack[top-1] = stack[top-1] - stack[top]; top--; break; 
          case MULT : stack[top-1] = stack[top-1] * stack[top]; top--; break; 
          case DIV  : stack[top-1] = stack[top-1] / stack[top]; top--; break; 
          case PWR  : stack[top-1] = stack[top-1] * stack[top]; top--; break; 
          default   : printf( "%sInternal Error: Memory Dump\n" ); break; } }
  while (ir.op != HALT); }