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HW2: Simple Multiplication in MIPS

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HW2: Simple Multiplication in MIPS
This assignment will help you become more comfortable coding in MIPS including the use of
comparison and branch/jump instructions.
Deliverables: Your program in a file named username.asm
NOTE: You should use good coding standard including comments in this program (if anything
assembly language programs require more descriptive comments than other programs you’ve
written). It’s often helpful to follow the pattern in the start.asm file, where you add a comment with
the more readable pseudocode at the end of every line of assembly, plus occasional longer comments
between lines of assembly.
Your Task:
Write a program in MIPS to multiply 2 signed integers using only the following subset of MIPS
instructions:
– any of the load and store instructions (e.g. lw, sw, li)
– any of the add and subtract instructions (e.g. add, addi, sub)
– any of the branch/jump instructions (e.g. beq, bne, j)
– the comparison instructions slt and slti
Note that you may NOT use instructions mult or div, or any others not listed above, rather you will use
addition and loops to do the multiplication. You do not need to worry about overflow, but make sure
to consider any other edge cases if necessary, your program should be able to multiply any 2 integers
that don’t result in overflow.
I have provided the initial program start.asm, you must use this or functionally identical code to load
the 2 numbers to be multiplied from the given locations in memory. You may not change the memory
locations of the operands or the result. This program also includes an example of how to initialize
memory to contain specific values as soon the program is built (assembled), so you don’t have to edit
the memory by hand before running the program. Try building start.asm, then look at the data
segment in the execute screen – it should look something like this
It will probably be easiest to test if you uncheck the hexadecimal values box so that you can see the
values in memory as standard decimal integers. Note that the first 2 slots in memory now have the
values 5 and 6. When the MIPS assembler sees the label .data, it stores the values listed after the label
into sequential words (32-bit or 4-byte chunks of memory) starting at the beginning of the data
section (address 0x10010000). You can also use keywords to store other sizes of data, such as a single
byte or a 64 bit value, but the default is the 32 bit word size. Now run the program, you should see an
11 appear in the 3rd slot in memory.
Replace the line that adds the 2 operand values with your multiplication algorithm. Start by creating a
loop that can multiply 2 positive integers successfully, then add the ability to handle negative integers
as well. Don’t forget about the $zero register – it is useful not only for comparisons but also for things
like converting between negative and positive versions of a number (0-(-x) = x). Test multiplying
different numbers and be sure the correct result is appearing in the 3rd slot of the data section of
memory. The grader will only look at that value in memory, so getting the correct result into a
register is not enough.
Save your program as username.asm to submit. Try to make the code as efficient as possible. This
means minimizing the total number of instructions (especially those inside a loop), and doing as little
as possible branching/jumping to as near as possible places in the code.
Don’t forget to include comments! You will receive no partial credit for test cases that fail if you do
not include clear, readable comments to make debugging easier. Also adding at least the pseudo-code
in-line comments AS YOU GO will be invaluably helpful to you in writing your own programs!
Advanced – Detect Overflow (optional, just for fun)
This task is completely optional, and just for your own learning/practice, it will earn no extra grade
points.
Add the ability to detect overflow in your multiplication program. If you do find overflow, change the
result of the multiplication to 0 and set the 4th value in memory to be all 1’s (0xffffffff). There are many
methods of detecting overflow in multiplication, most will require looking at instructions in MIPS that
we have not discussed. Try to find any detection method that consistently works for both positive and
negative operands, then try to find a more efficient method.

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