Project 1-1: MIPS Assembler
Project 1.1 Project 1.2
IMPORTANT INFO - PLEASE READ
The projects are part of your design project worth 2 credit points. As such they run in parallel to the actual course. So be aware that the due date for project and homework might be very close to each other! Start early and do not procrastinate.
So What Is This About?
In this part of the project, we will be writing an assembler that translates a subset of the MIPS instruction set to machine code. Our assembler is a two-pass assembler similar to the one described in lecture. However, we will only assemble the .text segment. At a high level, the functionality of our assembler can be divided as follows:
Pass 1: Reads the input (.s) file. Comments are stripped, pseudoinstructions are expanded, and the address of each label is recorded into the symbol table. Input validation of the labels and pseudoinstructions is performed here. The output is written to an intermediate (.int) file .
Pass 2: Reads the intermediate file and translates each instruction to machine code. Instruction syntax and arguments are validated at this step. The relocation table is generated, and the instructions, symbol table, and relocation table are written to an object (.out) file.
The Instruction Set
Please consult the MIPS Green Sheet for register numbers, instruction opcodes, and bitwise formats. Our asembler will support the following registers: $zero, $at, $v0, $a0 - $a3, $t0 - $t3, $s0 - $s3, $sp, and $ra. The name $0 can be used in lieu of $zero. Other register numbers (eg. $1, $2, etc.) are not supported.
We will have 18 instructions and 2 pseudoinstructions to assemble. The instructions are:
| Instruction | Format |
| Add Unsigned | addu $rd, $rs, $rt |
| Or | or $rd, $rs, $rt |
| Set Less Than | slt $rd, $rs, $rt |
| Set Less Than Unsigned | sltu $rd, $rs, $rt |
| Jump Register | jr $rs |
| Shift Left Logical | sll $rd, $rt, shamt |
| Add Immediate Unsigned | addiu $rt, $rs, immediate |
| Or Immediate | ori $rt, $rs, immediate |
| Load Upper Immediate | lui $rt, immediate |
| Load Byte | lb $rt, offset($rs) |
| Load Byte Unsigned | lbu $rt, offset($rs) |
| Load Word | lw $rt, offset($rs) |
| Store Byte | sb $rt, offset($rs) |
| Store Word | sw $rt, offset($rs) |
| Branch on Equal | beq $rs, $rt, label |
| Branch on Not Equal | bne $rs, $rt, label |
| Jump | j label |
| Jump and Link | jal label |
The pseudoinstructions are:
| Pseudoinstruction | Format |
| Load Immediate | li $rd, immediate |
| Branch on Greater Than or Equal | bge $rs, $rt, label |
| Move | move $rd, $rs |
Implementation Steps
Step 0: Obtaining the Files
Download framework first. You can compile you code by typing make. At first, you will get a bunch of -Wunused-variable and -Wunused-function warnings. The warnings tell you that variables/functions were declared, but were not used in your code. Don't worry, as you complete the assigment the warnings will go away.
Step 1: Building Blocks
Finish the implementation of translate_reg() and translate_num() in src/translation_utils.c. translate_reg() is incomplete, so you need to fill in the rest of the register translations. You can find register numbers on the MIPS Green Sheet. Unfortunately, there are no built-in switch statements for strings in C, so an if-else ladder is the way to compare multiple strings.
For translate_num(), you should use the library function strtol() (see documentation here). translate_num() should translate a numerical string (either decimal or hexadecimal) into a signed number, and then check to make sure that the result is within the bounds specified. If the string is invalid or outside of the bounds, return -1.
Step 2: SymbolTable
Implement a data structure to store symbol name-to-address mappings in src/tables.c. Multiple SymbolTables may be created at the same time, and each must resize to fit an arbitrary number of entries (so you should use dynamic memory allocation). You may design the data structure in any way you like, as long as you do not change the function definitions. A SymbolTable struct has been defined in src/tables.h, and you may use the existing implementation or create your own if that feels more intuitive. Feel free to declare additional helper methods. See src/tables.c for details.
In add_to_table, you cannot simply store the character pointer that was given, as it could point to a temporary array. You must store a copy of that string instead. You should use the helper functions defined in src/tables.c whenever appropriate.
You must make sure to free all memory that you allocate. See the Valgrind section under testing for more information.
Step 3: Instruction Translation
Implement translate_inst() in src/translate.c. The MIPS Green Sheet will again be helpful, and so will bitwise operations.
translate_inst() should translate instructions to hexadecimal. Note that the function is incomplete. You must first fix the funct fields, and then implement the rest of the function.You will find the translate_reg(), translate_num(), and write_inst_hex() functions, all defined in translate_utils.h helpful in this step. Some instructions may also require the symbol and/or relocation table, which are give to you by the symtbl and reltbl pointers. This step may require writing a lot of code, but the code should be similar in nature, and therefore not difficult. The more important issue is input validation -- you must make sure that all arguments given are valid. If an input is invalid, you should NOT write anything to output but return -1 instead.
Use your knowledge about MIPS instruction formats and think carefully about how inputs could be invalid. You are encouraged to use MARS as a resource. Do note that MARS has more pseudoinstruction expansions than our assembler, which means that instructions with invalid arguments for our assembler could be treated as a pseduoinstruction by MARS. Therefore, you should check the text section after assembling to make sure that the instruction has not been expanded by MARS .
If a branch offset cannot fit inside the immediate field, you should treat it as an error.
Step 4: Pseudoinstruction Expansion
Implement write_pass_one() in src/translate.c, which should perform pseudoinstruction expansion on the load immediate (li) and move instructions. The load immediate instruction normally gets expanded into an lui-ori pair. However, an optimization can be made when the immediate is small. If the immediate can fit inside the imm field of an addiu instruction, we will use an addiu instruction instead. Other assemblers may implement additional optimizations, but ours will not. For the move instruction, use the fewest number of instructions possible. Also, make sure that your pseudoinstruction expansions do not produce any unintended side effects. You will also be performing some error checking on the pseudoinstructions (see src/translate.c for details). If there is an error, do NOT write anything to the intermediate file, and return 0 to indicate that 0 lines have been written.
Step 5: Putting It All Together
Implement pass_one() and pass_two() in assembler.c. In the first pass, the assembler will strip comments, add labels to the symbol table, perform pseudoinstruction expansion, and write assembly code into an intermediate file. The second pass will read the intermediate file, translate the instructions into machine code using the symbol table, and write it to an output file. Afterwards, the symbol table and relocation table will be written to the output file as well, but that has been handled for you.
Before you begin, make sure you understand the documentation of fgets() and strtok(). It will be easier to implement pass_two() first. The comments in the function will give a more detailed outline of what to do, as well as what assumptions you may make. Your program should not exit if a line contains an error. Instead, keep track of whether any errors have occured, and if so, return -1 at the end. pass_one() should be structured similarly to pass_two(), except that you will also need to parse out comments and labels. You will find the skip_comment() and add_if_label() functions useful.
As an aside, our parser is much more lenient than an actual MIPS parser. Building a good parser is outside the scope of this course, but we encourage you to learn about finite state automata if you are interested.
Line Numbers and Byte Offsets
When parsing, you will need to keep track of two numbers, the line number of the input file and the byte offset of the current instruction. Line numbers start at 1, and include whitespace. The byte offset refers to how far away the current instruction is from the first instruction, and does NOT include whitespace. You can think of the byte offset as where each instruction will be if the instructions were loaded into memory starting at address 0. See below for an example.
The address of a label is the byte offset of the next instruction. In the example below, L1 has an address of 4 (since the next instruction is lw, whose address is 4) and L2 has an address of 8 (since the next instruction is ori, whose address is 8).
| Line # | Input File |
| 1 | addiu $t0 $a0 0 |
| 2 | L1: lw $t1 0($t0) |
| 3 | # This is a comment |
| 4 | L2: |
| 5 | ori $t1 $t1 0xABCD |
| 6 | addiu $t1 $t1 3 |
| 7 | |
| 8 | bne $t1 $a2 L2 |
| Output File | Byte Offset |
| addiu $t0 $a0 0 | 0 |
| lw $t1 0($t0) | 4 |
| ori $t1 $t1 0xABCD | 8 |
| addiu $t1 $t1 3 | 12 |
| bne $t1 $a2 label_2 | 16 |
Error Handling
If an input file contains an error, we only require that your program print the correct error messages. The contents of your .int and .out files do not matter.
There are two kinds of errors you can get: errors with instructions and errors with labels. Error checking of labels is done for you by add_if_label(). However, you will still need to record that an error has occurred so that pass_one() can return -1.
In pass_one(), errors with instructions can be raised by 1) write_pass_one() or 2) the instruction having too many arguments. In pass_two(), errors with instructions will only be raised by translate_inst(). Both write_pass_one() and translate_inst() should return a special value (0 and -1 respectively) in the event of an error. You will need to detect whether an instuction has too many arguments yourself in pass_one().
Whenever an error is encountered in either pass_one() or pass_two(), record that there is an error and move on. Do not exit the function prematurely. When the function exits, return -1.
For information about testing error message, please see the "Error Message Testing" section under "Running the Assembler".
Step 6: Testing
You are responsible for testing your code. While we have provided a few test cases, they are by no means comprehensive. Fortunately, you have a variety of testing tools at your service.
Valgrind
You should use Valgrind to check whether your code has any memory leaks. We have included a file, run-valgrind, which will run Valgrind on any executable of your choosing. If you get a permission denied error, try changing adding the execute permission to the file:
chmod u+x run-valgrind
Then you can run by typing:
./run-valgrind <whatever program you want to run>
For example, you wanted to see whether running ./assembler -p1 input/simple.s out/simple.int would cause any memory leaks, you should run ./run-valgrind ./assembler -p1 input/simple.s out/simple.int.
MARS
Since you're writing an assembler, why not refer to an existing assembler? MARS is a powerful reference for you to use, and you are encouraged to write your own MIPS files and assemble them using MARS.
Warning: in some cases the output of MARS will differ from the specifications of this project. You should always follow the specs. This is because MARS 1) supports more pseudoinstructions, 2) has slightly different pseudoinstruction expansion rules, and 3) acts as an assembler and linker. For example, MARS may expand an addiu with a 32-bit immediate into li followed by an addiu instruction. Not only will the machine code be different, but the addresses of any labels will also be different. Therefore, you should always examine the assembled instructions carefully when testing with MARS.
MARS also supports an assemble-only mode via the command-line (see documentation here). To save assembled output to a text file, run:
mars a dump .text HexText <output location> <input MIPS file>
Diff
diff is a utility for comparing the contents of files. Running the following command will print out the differences between file1 and file2:
diff <file1> <file2>
To see how to interpret diff results, click here. We have provided some sample input-output pairs (again, these are not comprehensive tests) located in the input and out/ref directories respectively. For example, to check the output of running simple.s on your assembler against the expected output, run:
./assembler input/simple.s out/simple.int out/simple.out diff out/simple.out out/ref/simple_ref.out
Running the Assembler
First, make sure your assembler executable is up to date by running make.
By default, the assembler runs two passes. The first pass reads an input file and translates it into an intermediate file. The second pass reads the intermediate file and translates it into an output file. To run both passes, type:
./assembler <input file> <intermediate file> <output file>
Alternatively, you can run only a single pass, which may be helpful while debugging. To run only the first pass, use the -p1 flag:
./assembler <-p1> <input file> <intermediate file>
To run only the second pass, use the -p2 flag. Note that when running pass two only, your symbol and relocation table will be empty since labels were stripped in pass_one(), so it may affect your branch instructions.
./assembler <-p2> <intermediate file> <output file>
When testing cases that should produce error messages, you may want to use the -log flag to log error messages to a text file. The -log flag should be followed with the location of the output file (WARNING: old contents will be overwritten!), and it can be used with any of the three modes above.
Error Message Testing
We have provided two tests for error messages, one for errors that should be raised during pass_one(), and one for errors that should be raised during pass_two(). To test for pass_one() errors, assemble input/p1_errors.s with the -p1 flag and verify that your output matches the expected output:
./assembler -p1 input/p1_errors.s out/p1_errors.int -log log/p1_errors.txt diff log/p1_errors.txt log/ref/p1_errors_ref.txt
To test for pass_two() errors, assemble input/p2_errors.s running both passes:
./assembler input/p2_errors.s out/p2_errors.int out/p2_errors.out -log log/p2_errors.txt diff log/p2_errors.txt log/ref/p2_errors_ref.txt
Your intermediate and output files (.int and .out files) do NOT need to match the reference output if the input file contains an error.
Notes regarding grading
- The Autolab will enforce a proper amount of comments again!
- Make sure you add proper comments - about one every four lines of code that you ADD. We will check this by hand.
- The Autolab will again use -Wpedantic -Wall -Wextra -Werror -std=c89. You may edit the Makefile if you need to - but the Autolab will replace the CFLAGS line. Your compilation step will have to use the CFLAGS!
Submission
You should submit an archive file names as framework.tar to Autolab. The archive must contain a folder named framework.
The directory tree of your submission should like the following:
|--- src
| |-- tables.c
| |-- tables.h
| |-- translate.c
| |-- translate.h
| |-- translate_utils.c
| |-- translate_utils.h
| |-- utils.c
| |-- utils.h
|--- assembler.c
|--- assembler.h
|--- Makefile
|--- run-valgrind
Autolab Results
tests 1-x stands for results for testcase x.
test 2-0 stands for the result for memory leak detection.