Schedule Syllabus Project

MIPS breakpoints

This lab explores how breakpoints work. Since some assembly is required, we will use MARS to simulate MIPS, since most of you are familiar with it.

Complete the question sheet as you complete the lab.

Preperation

Check your repos for mips-breakpoints. Inside will be several files:

• music.asm : The music file from the installation instructions
• exception.s : An exception handler with support for breakpoints
• bp_test.asm : A small piece of code for testing breakpoints
• breakpoints.s : A template file for setting breakpoints

For this project, you will need these settings in MARS:

• Unset Settings > Assemble all files in directory
• Unset Settings > Initialize Program counter to global 'main' if defined
• Set Settings > Self-modifying code
• Set Settings > Permit extended (pseudo) instructions and formats
• Use Settings > Exception Handler ...
  • Select the exception.s file in the handler directory.

First tests

• Open bp_test.asm. Look over the code.

• Open breakpoints.s. Let's add a breakpoint. Insert this code:

  la   $a0 main
  addi $a0 $a0 4
  jal  set_bp

  This should set a breakpoint at the second instruction past main. Note that there is a lot of startup code and main should be located at 0x004000b8.

• Assemble the code in bp_test.asm. Run the code.

• The simulator should print a message that the breakpoint was hit. Verify that the breakpoint was at the correct instruction location.

• Add breakpoints for lines 11, 18, and 25. Note that pseudo instructions might change where these are in memory!

  • Run the code

Breakpoint code

• Open exception.s.
• The breakpoint code is a bit complex. There are three main breakpoint sections: installation, handle breakpoint hits, and restore breakpoints.
• Lines ~483-520 is the code that installs breakpoints. This backups the original instructions, replaces the instruction with the break instruction, and marks the breakpoint as active.
  • How many breakpoints does the code support?
  • What data is saved for each breakpoint?
  • Describe a way to support an effectively unlimited number of breakpoints.
• Lines ~238-436 is the main breakpoint handler. It prints a breakpoint message, inserts another breakpoint after this one, repalces the causing breakpoint with the original instruction, and retries.
  • You can make execution pause on breakpoints using the code near line 276.
  • Most of the code is taken up by creating the restore breakpoint. Why is this so complex?
  • Can software or hardware changes make this less complex?
• Lines ~217-236 is the restore breakpoint handler. This happens after the main breakpoint. This handler puts the original breakpoint back, so it can be hit again if the code reruns.
  • Aside from repairing the first breakpoint, what other main action does this code do?

Music code

• Close all other code files.
• Open music.asm
• Run it without breakpoints first and observe the results in the I/O pane and the sound.
• Use breakpoint.s again and set a breakpoint at the first la $a0, stanza1 in main.
  • It might be helpful to enable the pause feature of the breakpoint handler.
  • What has already happened in the program?
• Remove the other breakpoint and set a breakpoint at the first line of playNotes.
  • How many times is the breakpoint hit?
• Remove the other breakpoint and set a breakpoint at the first lines of both playNote and playMapNote.
  • If you're using pauses, you will definitely want to disable them for this one.
  • Which of these two functions is called first?