CS61C Fall 2014 Lab 4 - Memory Management and Assembly


The first part of this lab will let you practice working with dynamic memory allocation and introduce you to the Valgrind utility for checking memory leaks. The second part of the lab will give you practice running and debugging assembly programs using the MARS simulator.


Copy the lab files with

$ cp -r ~cs61c/labs/04/ ~/lab04 


Exercise 1: Memory Management

This exercise uses vector.h, vector-test.c, and vector.c, where we provide you with a framework for implementing a variable-length array. This exercise is designed to help familiarize you with C structs and memory management in C.

Your task is to fill in the functions vector_delete() and vector_set() in vector.c so that our test code vector-test.c runs without any memory management errors. Comments in the code describe how the functions should work. Look at the functions we've filled in to see how the data structures should be used. For consistency, it is assumed that all entries in the vector are 0 unless set by the user. Keep this in mind as malloc() does not zero out the memory it allocates.

Using Valgrind

To help you to find memory bugs, we have installed a copy of Valgrind Memcheck. Valgrind is ONLY on the lab machines in the Hive and the Orchard. This program will run an executable while keeping track of what registers and regions of memory contain allocated and/or initialized values. The program will run much slower (by a factor of about 10 to 50) so that this information can be collected, but Valgrind Memcheck can identify many memory errors automatically at the point at which they are produced. You will need to learn the basics of how to parse the Valgrind output.

You can test your code in the following two ways:

// 1) to check functionality:
$ make vector-test
$ ./vector-test

// 2) to check memory management using Valgrind:
$ make vector-memcheck

The Makefile calls Valgrind as follows:

$ valgrind --tool=memcheck --leak-check=full --track-origins=yes [OS SPECIFIC ARGS] ./<executable>

The --track-origins flag attempts to identify the sources of unitialized values. The --leak-check=full option tries to identify memory leaks. [OS SPECIFIC ARGS] are simply a set of arguments to Valgrind that differ across operating systems (in our case, Ubuntu (Linux)). If you are interested in learning more about these, see the Makefile.

The last line in the Valgrind output is the line that will indicate at a glance if things have gone wrong. Here's a sample output from a buggy program:

==47132== ERROR SUMMARY: 1200039 errors from 24 contexts (suppressed: 18 from 18)

If your program has errors, you can scroll up in the command line output to view details for each one. For our purposes, you can safely ignore all output that refers to suppressed errors. In a leak-free program, your output will look like this:

==44144== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 18 from 18)

Again, any number of suppressed errors is fine; they do not affect us.

Feel free to also use a debugger or add printf statements to vector.c and vector-test.c to debug your code.


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 by copying it from ~cs61c/bin/Mars4_3.jar on the instructional machines. 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.

You can run MARS in lab by typing 'mars &' at the command line. The ampersand is optional but will allow you to continue using that terminal window (on the Macs however, you'll need to run it in it's own terminal tab by pressing command-t first). To run the program remotely, you may either run via an instructional server (but NOT one of the Orchard machines), or through a local installation (recommended). When on an instructional server, you will need to be running an X-Server (like XMing), and enabling X11 tunneling.

Tip: Although it is possible, you should avoid running MARS remotely at all costs - it will be painfully slow to use and will overwhelm the servers if many students attempt to do so. It is in your best interest to setup/run a local copy of MARS.

Assembly Basics:

Exercise 2: Familiarizing yourself with MARS

Getting started:

  1. Run MARS.
  2. Load lab4_ex2.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 put in each section)?
  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)


Exercise 3: 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 lab4_ex3.s. Don't forget the "main:" label and to end your program with a "syscall 10"!


Exercise 4: Compiling from C to MIPS

For this exercise we need to use a program called mips-gcc (a cross-compiler for MIPS) that allows us to compile programs for the MIPS architecture on our x86 machines. If you are doing this lab remotely, you should SSH into one of the hive machines.

Compile The file lab4_ex4.c into MIPS code using the command:

$ mips-gcc -S -O2 -fno-delayed-branch -I/usr/include lab4_ex4.c -o lab4_ex4.s

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 lab4_ex4.c are originally stored in the assembly file. Finally, explain how these pointers are manipulated through the loop.