Self-Modifying Programs

A program that changes itself is often called a self-modifying program.

Experiment 4.5

This experiment investigates the concept of self-modifying programs.

Step 1. Start the simulation program and clear the simulated machines main memory. Then, place the program below in main memory.

Address Contents Address Contents Address Contents

00 23 04 B0 08 C0

01 08 05 06 09 00

02 33 06 C0 0A C0

03 05 07 00 0B 00

Step 2. Execute the machine languages program just entered and record the results.

(In particular, what is the value of the program counter when execution stops?)

Step 3. Translate into English the machine language program in Step 1 and explain how the program counter obtained the value observed in Step 2.

Step 4. Clear the simulated machines memory and place the following program in it.

Address Content

00, 01 2100 Initialize

02, 03 2270 counters.

04, 05 3109 Set origin

06, 07 320B and destination.

08, 09 1000 Now move

0A, 0B 3000 one byte.

0C, 0D 2001 Increment

0E, 0F 5101 addresses.

10, 11 5202

12, 13 2333 Do it again

14, 15 4010 if more

16, 17 B31A bytes to be

18, 19 B004 moved.

1A, 1B 2070 Adjust values

1C, 1D 3071 that are

1E, 1F 2079 location

20, 21 3075 dependent.

22, 23 207B

24, 25 3077

26, 27 208A

28, 29 3087

2A, 2B 2074

2C, 2D 3089

2E, 2F 20C0 Make sure

30, 31 30A4 program

32, 33 2000 will halt.

34, 35 30A5

36, 37 B070 Make the big jump!

Step 5. Execute this machine language program by clicking the Step button. Describe the changes that have appearing in the machines main memory.

Step 6. Continue executing and halting the program while observing the changes being made in memory and the program counter. When the program finally terminates, explain what the program did.

Step 7. Study the original program and explain the role of the instructions at locations 12 through 19.

Explain the role of the instructions in locations 1A through 2D.

Explain the role of the instructions in locations 2E through 33.