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Assignment 2: MIPS-C

Aims

· Understanding encoding of MIPS instructions

· Understanding semantics of MIPS instructions

· Generally building a concrete understanding of an example CPU

· Practicing C, including bit operations

The Assignment

mipsy is a MIPS emulator written in Rust.
You have extensively used mipsy in COMP1521 to run MIPS programs.

The input to mipsy is usually a file containing MIPS assembly instructions.

For example:

cat examples/42.s

main:

li $a0, 42 # printf("%d", 42);

li $v0, 1

syscall

li $a0, '\n' # printf("%c", '\n');

li $v0, 11

syscall

jr $ra

1521 mipsy examples/42.s

Your task in this assignment is to write mipsc a MIPS emulator written in C.

DON'T PANIC, mipsc is much simpler than mipsy.

mipsc only needs to emulate the small subset of MIPS instructions listed below.

These instructions has been chosen to be simpler to emulate, but still allow interesting programs to be run.

The input to mipsc is a file containing hexadecimal numbers representing the 32-bit instruction codes of MIPS instructions.

mipsc should read these instruction codes (code provided) and then emulate the instructions like this:

cat examples/42.hex

2004002a

20020001

0000000c

2004000a

2002000b

0000000c

dcc mipsc.c -o mipsc

./mipsc examples/42.hex

0: 0x2004002A addi $4, $0, 42

>>> $4 = 42

1: 0x20020001 addi $2, $0, 1

>>> $2 = 1

2: 0x0000000C syscall

>>> syscall 1

<<< 42

3: 0x2004000A addi $4, $0, 10

>>> $4 = 10

4: 0x2002000B addi $2, $0, 11

>>> $2 = 11

5: 0x0000000C syscall

>>> syscall 11

<<<

If the command-line argument -r is given (code provided) then only the output from syscalls should be shown, like this:

./mipsc -r examples/42.hex

Reference implementation

A reference implementation is available as 1521 mipsc. Use it to find the correct output for any input like this:

cat examples/square.hex

20100004

20110003

72108002

72318802

02302020

20020001

0000000c

2004000a

2002000b

0000000c

1521 mipsc examples/square.hex

0: 0x20100004 addi $16, $0, 4

>>> $16 = 4

1: 0x20110003 addi $17, $0, 3

>>> $17 = 3

2: 0x72108002 mul $16, $16, $16

>>> $16 = 16

3: 0x72318802 mul $17, $17, $17

>>> $17 = 9

4: 0x02302020 add $4, $17, $16

>>> $4 = 25

5: 0x20020001 addi $2, $0, 1

>>> $2 = 1

6: 0x0000000C syscall

>>> syscall 1

<<< 25

7: 0x2004000A addi $4, $0, 10

>>> $4 = 10

8: 0x2002000B addi $2, $0, 11

>>> $2 = 11

9: 0x0000000C syscall

>>> syscall 11

<<<

1521 mipsc -r examples/square.hex

Provision of a reference implementation is a common, efficient and effective method to provide or define an operational specification, and it's something you will likely need to work with after you leave UNSW.

Where any aspect of this assignment is undefined in this specification you should match the reference implementation's behaviour.

Discovering and matching the reference implementation's behaviour is deliberately part of the assignment.

If you discover what you believe to be a bug in the reference implementation, report it in the class forum. If it is a bug, we may fix the bug, or indicate that you do not need to match the reference implementation's behaviour in this case.

MIPS Instruction Subset

You need to implement only these 14 MIPS instructions:

Assembler	C	Bit Pattern
add $d, $s, $t	d = s + t	000000ssssstttttddddd00000100000
sub $d, $s, $t	d = s - t	000000ssssstttttddddd00000100010
slt $d, $s, $t	d = s < t	000000ssssstttttddddd00000101010
mfhi $d	d = HI	0000000000000000ddddd00000010000
mflo $d	d = LO	0000000000000000ddddd00000010010
mult $s, $t	HI,LO = s * t	000000sssssttttt0000000000011000
div $s, $t	HI = s % t; LO = s / t	000000sssssttttt0000000000011010
mul $d, $s, $t	d = s * t	011100ssssstttttddddd00000000010
beq $s, $t, I	if (s == t) PC += I	000100ssssstttttIIIIIIIIIIIIIIII
bne $s, $t, I	if (s != t) PC += I	000101ssssstttttIIIIIIIIIIIIIIII
addi $t, $s, I	t = s + I	001000ssssstttttIIIIIIIIIIIIIIII
ori $t, $s, I	t = s \| I	001101ssssstttttIIIIIIIIIIIIIIII
lui $t, I	t = I << 16	00111100000tttttIIIIIIIIIIIIIIII
syscall	syscall	00000000000000000000000000001100

The instruction bit pattern uniquely identifies each instruction:

· 0: Literal bit zero

· 1: Literal bit one

· I: Immediate (16-bit signed number)

· d, s, t: five-bit register number

System Calls

You only need to implement these 3 system calls.

Description	$v0	Pseudo-C
print integer	1	printf("%d", $a0)
exit	10	exit(0)
print character	11	printf("%c", $a0)

Syscall 1 should print $a0 as a signed 32-bit integer.

Syscall 11 should print the low byte (lowest 8 bits) of $a0.

Syscall 10 should exit the program.
See the section on halting for more details.

If an invalid syscall number is supplied an error message should be printed:

Match the reference implementation's message:

1521 mipsc examples/bad_syscall.hex

0: 0x20021092 addi $2, $0, 4242

>>> $2 = 4242

1: 0x0000000C syscall

>>> syscall 4242

Unknown system call: 4242

After printing the error message, the program should exit.
See the section on halting for more details.

Registers

All 32 registers, Plus Hi and Lo are set to be zero when execution begins.

The other 31 registers have no special meaning and can be used for any purpose.

The value of register $0 ($zero) is always 0.
Instructions that attempt to change it have no effect.

The values of registers $2 ($v0) and $4 ($a0) are used by the syscall instruction.

Hi and Lo are used by the MULT and DIV instructions.
And can only be accessed by the MFHI and MFLO instructions.

Halting

Execution halts if an exit syscall (syscall 10) is executed.

Execution halts if it reaches the location after the last instruction.

Execution halts if there is a branch to the location immediately after the last instruction.

Execution halts with an error message if there is a branch to any other location beyond the range of specified instructions:
Illegal branch to non-instruction: PC = ??

Execution halts after printing an error message,
eg: for an invalid syscall number or invalid instruction.

Converting MIPS Assembler Instruction codes

The command 1521 mips2hex will give you the hex codes for MIPS instructions.

cat examples/42.s

main:

li $a0, 42 # printf("%d", 42);

li $v0, 1

syscall

li $a0, '\n' # printf("%c", '\n');

li $v0, 11

syscall

jr $ra

1521 mips2hex examples/42.s

2004002a

20020001

0000000c

2004000a

2002000b

0000000c

1521 mips2hex has a couple of features that it make it easier for you to use MIPS programs as test inputs for mipsc.

For convenience, 1521 mips2hex translates some common pseudo-instructions, e.g. LI into the instruction codes for real instructions in the subset mipsc handles.

1521 mips2hex translates a few instructions outside the subset mipsc handles to the instruction codes for equivalent real instructions in the subset mipsc handles.

1521 mips2hex handles branch labels for you. It calculates the branch offset and outputs the instruction codes for the appropriate branch instructions.

Also for convenience 1521 mips2hex deletes the last instruction if it is JR because JR is not in the subset for this assignment and a last JR is often just main returning.

Note, mipsc does not need to implement pseudo-instructions or instructions outside the specified subset.

Examples

Some example MIPS programs are available in a zip file You will also need to do your own testing and construct your own examples using 1521 mips2hex.

Note the assembler for the example programs contains pseudo-instructions such as LI.

1521 mips2hex translates these pseudo-instructions to real instruction codes in the subset for this assignment.

Getting Started

Create a new directory for this assignment called mipsc, change to this directory, and fetch the provided examples: by running these commands:

mkdir -m 700 mipsc

cd mipsc

1521 fetch mipsc

unzip examples.zip

....

Or, if you're not working on CSE, you can download the examples.zip and starting code

You have been given starting code for this assignment in mipsc.c which already implements handling command line arguments and reading the hexadecimal instruction code into an array.

dcc mipsc.c -o mipsc

./mipsc examples/42.hex

0: 0x2004002A

1: 0x20020001

2: 0x0000000C

3: 0x2004000A

4: 0x2002000B

5: 0x0000000C

The code calls the function execute_instructions to simulate execution but the supplied code in execute_instructions only prints the instruction codes.

You need to change the code in execute_instructions to simulate the execution of the instructions.

You should add extra functions and #defines.

You may (but are not required to) create extra .c or .h files.

Assumptions and Clarifications

Like all good programmers, you should make as few assumptions as possible.

If in doubt, match the output of the reference implementation.

· You can assume mipsc.c is given a single file as a command line argument

· You do not have to implement MIPS instructions, system calls, or features that are not explicitly mentioned in the tables above.

· Your program should print an error message if given a hexadecimal number that does not correspond to an instruction in the above MIPS subset.

You can print this error message before executing the program or when execution reaches the invalid instruction code.

Match the reference implementation's message:

echo 12858AA > invalid.hex

cat invalid.hex

12858AA

1521 mipsc invalid.hex

Invalid instruction 0x012858AA at PC = 0

In the case of an invalid instruction, any output to stdout will be ignored.
As long as the given error message is printed to stderr, you program is correct.

· The reference implementation uses %08X to print invalid instruction codes.

· You will not be penalized if you implement extra MIPS instructions beyond the subset above and do not print an error message for them.

· Execution halts with an error message if there is a system call which is not in this subset. You can assume overflow does not occur during arithmetic or other operations.

· You can assume that division by zero should result in a quotient and a remainder of 0.

· You do not need to handle instructions that access memory such as LW or SW.

· You do not need to handle branch labels. 1521 mips2hex translates these into the relative offset which is part of the branch instruction code.

· You do not need to handle pseudo-instructions. 1521 mips2hex translates these into the appropriate real instructions.

Eg. LI is translated into ADDI.

· You do not need to handle the JR instruction. 1521 mips2hex deletes the last instruction if it is JR.

Any other occurences of JR would be invalid.

· The corresponding .hex files contains only real instructions in the assignment subset.

· For convenience mips2hex translates some pseudo-instructions and a few instructions outside the subset into instruction codes in the assignment subset.

This is mainly done to translate different types of branches into SLT plus BEQ or BNE.

· The reference implementation will print error messages stderr.
The provided code also uses stderr when appropriate.
Any additional error messages you add should also print to stderr.

Any messages that do not indicate an error should be printed to stdout.

· Your submitted code must be C only. You may call functions from the standard C library (e.g., functions from stdio.h, stdlib.h, string.h, etc.) and the mathematics library (math.h). You may use assert.h.

You may not submit code in other languages. You may not use system function, or other C functions to run external programs. You may not use functions from other libraries; in other words, you cannot use dcc's -l flag.

· Your program must not require extra compile options. It must compile with dcc *.c -o mipsc, and it will be run with dcc when marking. Run-time errors from illegal C will cause your code to fail automarking.

· If you need clarification on what you can and cannot use or do for this assignment, ask in the class forum.

· If your program writes out debugging output, it will fail automarking tests: make sure you disable debugging output before submission.

· You are required to submit intermediate versions of your assignment. See below for details.

Change Log

Version 1.0
(2022-10-31 12:00:00)

· Initial release

Version 1.1
(2022-10-31 20:00:00)

· (SPEC) Fix bit pattern for MFHI and MFLO, which had an extra 0 and were missing a d

· (SPEC) Fix the semantics of DIV, HI is remainder and LO is quotient, this was accidentally reversed

· (REFERENCE) Fix the implementation of DIV, HI is remainder and LO is quotient, this was accidentally reversed

Version 1.2
(2022-11-01 12:30:00)

· (SPEC) Add clarification that the program should halt when it tries to execute an invalid syscall

Version 1.3
(2022-11-02 03:30:00)

· (REFERENCE) Fix MULT incorrectly handling negative numbers

Assessment

Testing

When you think your program is working, you can use autotest to run some simple automated tests:

1521 autotest mipsc mipsc.c [optionally: any extra .c or .h files]

1521 autotest will not test everything.
Always do your own testing.

Automarking will be run by the lecturer after the submission deadline, using a superset of tests to those autotest runs for you.

Whilst we can detect when errors have occurred, it is often substantially harder to automatically explain what that error was. The errors from 1521 autotest may be less clear than in labs. You will need to do your own debugging and analysis.

Remember that you can always compaire your code to the reference implement.

dcc mipsc.c -o mipsc

./mipsc examples/42.hex | tee mipsc.out

0: 0x2004002A addi $4, $0, 42

>>> $4 = 42

1: 0x20020001 addi $2, $0, 1

>>> $2 = 1

2: 0x0000000C syscall

>>> syscall 1

<<< 42

3: 0x2004000A addi $4, $0, 10

>>> $4 = 10

4: 0x2002000B addi $2, $0, 11

>>> $2 = 11

5: 0x0000000C syscall

>>> syscall 11

<<<

1521 mipsc examples/42.hex | tee reference.out

0: 0x2004002A addi $4, $0, 42

>>> $4 = 42

1: 0x20020001 addi $2, $0, 1

>>> $2 = 1

2: 0x0000000C syscall

>>> syscall 1

<<< 42

3: 0x2004000A addi $4, $0, 10

>>> $4 = 10

4: 0x2002000B addi $2, $0, 11

>>> $2 = 11

5: 0x0000000C syscall

>>> syscall 11

<<<

diff -s mipsc.out reference.out

Files mipsc.out and reference.out are identical

Submission

When you are finished working on the assignment, you must submit your work by running give:

give cs1521 ass2_mipsc mipsc.c [optionally: any extra .c or .h files]

You must run give before Week 11 Monday 11:59:59 (midday) to obtain the marks for this assignment. Note that this is an individual exercise, the work you submit with give must be entirely your own.

You can run give multiple times.
Only your last submission will be marked.

If you are working at home, you may find it more convenient to upload your work via give's web interface.

You cannot obtain marks by emailing your code to tutors or lecturers.

You can check your latest submission on CSE servers with:

1521 classrun check ass2_mipsc

You can check the files you have submitted here.

Manual marking will be done by your tutor, who will mark for style and readability, as described in the Assessment section below. After your tutor has assessed your work, you can view your results here; The resulting mark will also be available via give's web interface.

Due Date

This assignment is due Week 11 Monday 11:59:59 (midday) (2022-11-21 11:59:00).

The UNSW standard late penalty for assessment is 5% per day for 5 days - this is implemented hourly for this assignment.

Your assignment mark will be reduced by 0.2% for each hour (or part thereof) late past the submission deadline.

For example, if an assignment worth 60% was submitted half an hour late, it would be awarded 59.8%, whereas if it was submitted past 10 hours late, it would be awarded 57.8%.

Beware - submissions 5 or more days late will receive zero marks. This again is the UNSW standard assessment policy.

Assessment Scheme

This assignment will contribute 15 marks to your final COMP1521 mark.

80% of the marks for assignment 2 will come from the performance of your code on a large series of tests.

20% of the marks for assignment 2 will come from hand marking. These marks will be awarded on the basis of clarity, commenting, elegance and style. In other words, you will be assessed on how easy it is for a human to read and understand your program.

An indicative assessment scheme for performance follows. The lecturer may vary the assessment scheme after inspecting the assignment submissions, but it is likely to be broadly similar to the following:

100% for performance	Implements all behaviour perfectly, following the spec and reference implementation exactly.
90% for performance	All instructions and syscalls mostly works with one or two minor bugs.
80% for performance	Most instructions mostly works with a few minor bugs.
65% for performance	Some instructions somewhat working
≤ 40% for performance	good progress, but not passing many/any autotests.

An indicative assessment scheme for style follows. The lecturer may vary the assessment scheme after inspecting the assignment submissions, but it is likely to be broadly similar to the following:

100% for style	perfect style
90% for style	great style, almost all style characteristics perfect.
80% for style	good style, one or two style characteristics not well done.
70% for style	good style, a few style characteristics not well done.
60% for style	ok style, an attempt at most style characteristics.
≤ 50% for style	an attempt at style.