Lab 2
Debugging a Machine Code Program

Objectives

• To gain practice debugging machine code using the LC-3 debugger.

What you should do

• Download a copy of lab2file and save it in your directory that contains the LC-3 Simulator. Open the file using LC3Edit (use Save As... to rename the file to lab2.bin). This file contains the (buggy) machine code for a program that is supposed to add 10 (decimal) to every integer in an array. The array should start at location x4000 and the resulting values should be placed in an array that starts at location x5000. The array of integers is terminated by a word containing x0000. Your task is to find and fix the bugs in the program.

• Using LC3Edit, create the object file lab2.obj. Start up the LC-3 Simulator and open the object file.

• Put some data into memory beginning at location x4000 (these will be the integers in your array). The data should terminate with a word that contains the value zero.

• Put a breakpoint at the TRAP HALT instruction. Run the program. When the program executes correctly, each of your data values will have been increased by 10 and stored at consecutive locations beginning at location x5000. Unfortunately, the program's not working yet.

• Now you're on your own. (Hint: there are three bugs in the program.) Use the tools available to you - remember these debugging features from Lab 1:
  • Breakpoints A breakpoint is just what the name implies - a point at which you break out of the currently-executing program and are brought into the debugger. You can then check out the state of various registers and memory locations to make sure they contain the values they should contain at that point. If you reach a breakpoint and you find something amiss, you know that there is a bug in your program in one of the instructions that executed prior to the breakpoint. If on the other hand, everything looks OK, you can narrow down your search for bugs in the instructions that execute after the breakpoint. Branch instructions are usually good places to put breakpoints; that allows you to stop after each iteration of a loop, for example, and make sure the loop control variables and other registers and variables contain the correct values. To set a breakpoint, double-click on the little gray square to the left of an instruction (the breakpoint takes you out of the program before executing that instruction). To remove a breakpoint, double-click in that same place again (instead of a gray square you will now be double-clicking on a red stop sign). To continue execution after a breakpoint, click the Run Program button (the second button from the left).

  • Single-step Single-stepping through a program is useful when you have isolated the bug to a section of your program, but you don't know exactly which instruction is causing the problem. Single-stepping allows you to execute your program one instruction at a time. After the execution of each instruction you can examine the registers and memory to make sure they contain what you expect them to contain. To single-step through a program, click the step-over button (the third button from the left on the toolbar) each time you want to execute an instruction. (We will use the step-into button later, when we write programs that contain subroutines.)

  • Refer to "LC-3 Simulator Lab Manual" found at the authors' website for more information about using the debugger.

• If you don't find all the bugs, that's OK (try to finish this exercise on your own time). Turn in your .bin file lab2.bin using web-based turnin.

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