Machine language simulation

Hi,

This is an exercise from the deitel book. First i got it to work. Then later it has a bug. There is something wrong with the looping. But i don't know how to explain, sorry. By looking at the code is there something wrong with it?

thnx in advance.

Code:

---

/* Simpletron - Computer simulation */

#include <stdio.h>

int main()
{
  int memory[ 100 ] = { 0 };
  int accumulator = 0;
  int operationCode;
  int operand;
  int instructionCounter = 0;
  int instructionRegister;
  int temp;

  printf( "***            Welcome to Simpletron!        ***\n" );
  printf( "*** Please enter your program one instruction ***\n" );
  printf( "*** (or data word) at a time. I will type the ***\n" );
  printf( "*** location number and a question mark (?).  ***\n" );
  printf( "*** You then type the word for that location. ***\n" );
  printf( "*** Type the sentinel -99999 to stop entering ***\n" );
  printf( "*** your program.                            ***\n\n" );

  while ( memory[ instructionCounter ]  != -99999 ) {
      printf( "%2d ? ", instructionCounter );
      scanf( "%d", &temp );
      if ( temp >= -9999 && temp <= 9999 )
        memory[ instructionCounter ] = temp;
      else if ( temp == -99999 ) {
        memory[ instructionCounter ] = temp;
        break;
      }
      instructionCounter++;
  }
  printf( "\n" );

  printf( "*** Program loading completed ***\n" );
  printf( "*** Progam execution begins ***\n\n" );

  instructionCounter = 0;

  while ( instructionRegister != -99999 ) {
      instructionRegister = memory[ instructionCounter ];
      operationCode = instructionRegister / 100;
      operand = instructionRegister % 100;

      printf( "\ninstructionRegister = %d, operationCode = %d, operand = %d\n", instructionRegister, operationCode, operand );

      switch ( operationCode ) {
        case 10:
            scanf( "%d\n", &memory[ operand ] );
            printf( "scaned for the first time\n" );
            break;
        case 11:
            printf( "%d\n", memory[ operand ] );
            break;
        case 20:
            accumulator = memory[ operand ];
            break;
        case 21:
            memory[ operand ] = accumulator;
            break;
        case 30:
            accumulator += memory[ operand ];
            break;
        case 31:
            accumulator -= memory[ operand ];
            break;
        case 32:
            accumulator /= memory[ operand ];
            break;
        case 33:
            accumulator *= memory [operand ];
            break;
        case 40:
            instructionCounter = operand;
            continue;
        case 41:
            if ( accumulator < 0 ) {
              instructionCounter = operand;
              continue;
            }
            break;
        case 42:
            if ( accumulator == 0 ) {
              instructionCounter = operand;
              continue;
            }
            break;
        case 43:
            printf( "\n\n*** Simpletron execution terminated ***\n\n" );
            instructionRegister = -99999;
            continue;
        default: /* invalid operation code encountered */
            printf( "\nSimpletron execution abnormally terminated ***\n" );
            instructionRegister = -99999;
            continue;
      }
      instructionCounter++;
  }

  printf( "\n\n*** END *** \n" );

  getch();
  return 0;

---

A sample program which adds two numbers is:

1007
1008
2007
3008
2109
1109
4300
0000
0000
0000
-99999

hey, instead of having a continue after the branchnegative, and branchZero statements, put a break in it, and add increment the instructionCounter in an else statement. That way, if it gets to those statements, it will increment to the next instruction if it doesn't have to brach the trasnfer of controll. That should do it, let me know if it works

dude, are you in my class or something ???

The only thing I noticed is this:

Code:

---

>        case 10:
>            scanf( "%d\n", &memory[ operand ] );
Leave off the newline here:
            scanf( "%d", &memory[ operand ] );
>            printf( "scaned for the first time\n" );
>            break;

---

I ran it, and after that change, it seemed to work good. Using your data, it did the addition.

The only other thing I noticed is instructionRegister isn't initialized the first time thru the loop. But of course what are the odds of it being -99999?

>But of course what are the odds of it being -99999?
And what are the odds of that one odd time being the most critical time the program is run? ;) This program also fails to include conio.h when it uses getch at the end.

-Prelude

>And what are the odds of that one odd time being the most critical time the program is run?

Using Murphy's Law, pretty good ? :D