Goals

This lab will give you practice running and debugging assembly programs using the MARS simulator.

Intro to Assembly and MARS

The following exercises use a MIPS simulator called MARS, which provides a rich debugging GUI. You can run MARS on your home computer by downloading the jar file from the Internet or from shtech.org. You will need Java J2SE 1.5.0 (or later) SDK installed on your computer, which can be obtained from Sun. If your home computer is a Mac, you can also follow the instructions here to install MARS as an app in one step.

Assembly Basics:

• Assembly programs go in text files with a .s extention.
• Programs must contain a label "main:" (similar to the main() function in C programs).
• Programs must end with a "addi $v0,$0,10" followed by a "syscall". main() is special and must transfer control back to the operating system when it is done rather than just returning.
• Labels end with a colon (:).
• Comments start with a pound sign (#).
• You CANNOT put more than one instruction per line.

Exercises

Exercise 1: Familiarizing yourself with MARS

Getting started:

1. Run MARS.
2. Load fibonacci.s using File-->Open.
3. View and edit your code in the "Edit" tab. Notice the code highlighting and 'completion suggestion' features.
4. When ready, assemble your code using Run-->Assemble (or press F3).
5. This will take you automatically to the "Execute" tab, which is where you can run and debug your program.
6. Step through the program using Run-->Step (or press F7).
7. You should take the time to familiarize yourself with everything in the Run menu (and the keyboard shortcuts).

For this exercise, we calculate the Fibonacci numbers using fib[0] = 0; fib[1] = 1; fib[n] = fib[n-1] + fib[n-2].
Follow the steps below and record your answers to the questions. The Help menu (F1) may come in handy.

1. What do the .data, .word, .text directives mean (i.e. what do you use them for)?
2. How do you set a breakpoint in MARS? Set a breakpoint on line 14 and run to it. What is the instruction address? Has line 14 executed yet?
3. Once at a breakpoint, how do you continue to execute your code? How do you step through your code? Run the code to completion.
4. Find the "Run I/O" window. What number did the program output? Which fib number is this?
5. At what address is n stored in memory? Try finding this by (1) looking at the Data Segment and (2) looking at the machine code (Code column in the Text Segment).
6. Without using the "Edit" tab, have the program calculate the 13th fib number by manually modifying this memory location before execution. You may find it helpful to uncheck the "Hexadecimal Values" box at the bottom of the Data Segment.
7. How do you view and modify the contents of a register? Reset the simulation (Run-->Reset or F12) and now calculate the 13th fib number by (1) breaking at a well-chosen spot, (2) modifying a single register, and then (3) unsetting the breakpoint.
8. Lines 19 and 21 use the syscall instruction. What is it and how do you use it? (Hint: look in Help)

Check-off

Exercise 2: A short MIPS program

Write a piece of MIPS code from scratch that, given values in $s0 and $s1, accomplishes the following:

$t0 = $s0
$t1 = $s1
$t2 = $t0 + $t1
$t3 = $t1 + $t2
...
$t7 = $t5 + $t6

In other words, for each register from $t2 to $t7, store the sum of the previous two $t# register values. The $s0 and $s1 registers contain the initial values. Set the values of $s0 and $s1 manually with MARS instead of in your code. Finally, have your code print out the final value of $t7 as an integer (Hint: syscall).

Save your code in a file called lab3_ex2.s. Don't forget the "main:" label and to end your program with a "syscall 10"!

Check-off

Exercise 3: Compiling from C to MIPS

For this exercise we need to use a cross-compiler for MIPS that allows us to compile programs for the MIPS architecture on your x86 machine. Go to https://sourcery.mentor.com/GNUToolchain/release3133 to download the code (or from shtech.org for the Linux version - 394MiB). This is a 32bit program - if you are using a 64bit Ubuntu it will tell you which packages you need to install. You might need to add the bin folder (most likely ~/mgc/embedded/codebench/bin) to your PATH.

Compile The file lab3_ex3.c into MIPS code using the command:

$ mips-linux-gnu-gcc -O2 -fno-delayed-branch -march=mips32r2 -S lab3_ex3.c

The -O2 option (capital letter "O" and 2) turns on a level of optimization. The -S option generates assembly code. Don't worry about the delayed branch option for now; we will revisit this topic again when we talk about pipelining. The above command should generate assembly language output for the C code. Please note that you will NOT be able to run this code through MARS.

Find the assembly code for the loop that copies sources values to destination values. Then find where the source and dest pointers you see in lab3_ex3.c are originally stored in the assembly file. Finally, explain how these pointers are manipulated through the loop.

Check-off