Lecture Practice Quiz 3                         

Be prepared to answer questions similar to the following;

Assuming that execution begins at address 000, what value will the accumulator (acc) 
contain when each of the machine language programs executes a HALT instruction. (If 
the answers are not obvious, execute the programs with the "sim1.c" simulator.)

1. In machine language, instructions can be used as data (usually a bad idea).

  000 1000
  001 3001
  002 0000
  000

2. The SIM memory words have no place for a + or - sign. However, numbers between 5000 
and 9999 can be interpreted as negative numbers. If we were using the signed 
interpretation, how should the number 9999 be interpreted?            

  000 1003
  001 4004
  002 0000
  003 0050
  004 0051
  000

3. Program 3.

  000 5002
  001 2012
  002 1012
  003 3012
  004 2012
  005 1012
  006 3012
  007 2012
  008 1012
  009 3012
  010 2012
  000

4. Even short programs can be very confusing.

  000 5002
  001 3000
  002 4002
  003 2004
  004 0000
  005 0000
  000

5. As program 2 illustrated, the number 9999 can play the role of -1 in the SIM1 
   machine language. The ADD instruction is implemented with the following code.
                                                                                      10

        case ADD:
           acc = acc + memory[digit234];
           if (acc > WORDLIMIT)               /* wrap if acc > 9999    */
               acc = acc - (WORDLIMIT + 1);   /* by subtracting 10,000 */
           break;

    As a result:

       1 + 9999 = 0
       2 + 9999 = 1
      20 + 9999 = 19
     500 + 9999 = 499

    a. What number would play the role of -10 (so that adding this number to, for 
       example, 50 would yield 40). (Hint - if you drove a new car backwards for a mile, 
       the odometer (if it is mechanical) would go backwards from 000000 to 999999. What 
       would happen if you went backwards for 10 miles?).

    b. Can you give a simple formula for taking the negative of any SIM1 number between
       1 and  4999.

    c. What happens if you apply the same formula to numbers between 5001 and 9999?

6. Write assembly language programs equivalent to four machine language programs above.