In this lab, you’ll be practicing the things we learned about this week: variables, arrays, and simple conditional flow control.

Make a new file in MARS and name it with your username like abc123_lab2.asm. Then, use this tiny code skeleton to get started:

.data
# put variables here.

.text # don't forget this!
.globl main
main:
	# put code here.

1. Making variables

In the data segment, make three variables:

• a byte variable named small, initialized to 200
• a half variable named medium, initialized to 400
• a word variable named large, initialized to 0

Follow the pattern I showed in the slides.

Never write a program all at once and then compile/assemble it. Write a little at a time and let the assembler/compiler help you along the way. Then test it! Make sure

2. Accessing those variables

Now, in main, write some code to do the following:

1. load the value of small into a register
   • you don’t need to keep this value around long, so a t register is the right choice here.
2. load the value of medium into another register
3. multiply those values together
4. store the product into large
   • you should be able to see its value in the “Data Segment” window after you run your program.
5. print the product out as well
   • it should be 80000, but see below if it’s not.

Hints

• Use the right kind of load/store for each step.
  • The assembler will happily let you use lw on a byte, sh on a word etc.
  • It’s your responsibility to use the right loads and stores.
• The move a, b instruction lets you copy the value from b into a.
  • Sometimes you have a value in the “wrong” register and need to get it somewhere else.
  • Like when you have to use the print syscall and it wants the value to be in a0!

How does 200 * 400 = -22400???

Did this happen to you? Hmmm. Why could that be happening…

Try unchecking the ‘hexadecimal values’ option in the Data Segment window:

Then, have a look at the registers on the right.

Do you notice anything weird about the values you loaded from small and medium?

Remember sign versus zero extension? And the two versions of the loads? Well, you gotta use the right one ;)

3. Arrays and loops

Go back to the data segment and add the following:

.eqv NUM_ITEMS 5
values:	.word 0:NUM_ITEMS

.eqv makes a named constant. Naming constants is good practice.

The values array is an array of words, and it is NUM_ITEMS items long, and filled with 0s.

Making a for loop

A for loop is really a while loop. If you write:

for(int i = 0; i < 10; i++) {
	...
}

this is the same as:

int i = 0;
while(i < 10) {
	...
	i++;
}

Okay? Keep this pattern in mind.

In main after the code you already wrote, start with this code:

ask_loop_top:  # while(...)

ask_loop_body: # {

	j ask_loop_top
ask_loop_exit: # }

This is the basic form of a loop: a label for the top (where the condition is checked), a label for the body (the code inside), a j to go back to the top, and a label for the code after the loop.

Now, you need to turn this into the asm equivalent of for(i = 0; i < NUM_ITEMS; i++).

Hints

• Use s0 as the register to represent i.
  • We’ll learn about the s registers soon. Just trust me ;)
• Look at that pattern above. Where should you set i to 0?
  • Is that code gonna be inside the loop?
    • (no)
• How do you increment i?
  • where should you increment it?
• Look at the slides where I talk about a simple conditional loop.
  • We didn’t get to it in class on Wednesday, but we will on Monday.
• You can use NUM_ITEMS as the number to compare s0 to.
  • NAMED CONSTANTS GOOD.

Done correctly, this loop should run five times and then stop. You can use the step button to step through it one instruction at a time to make sure this happens.

Having problems?

If it never loops, or if it never stops looping, have a look at the contents of s0 as you step through it and see what is happening.

Being able to debug a program is a super important skill. Try to figure it out instead of asking for help as soon as it doesn’t work correctly. (But also, it’s okay if you have no idea and ask for help instead of turning in a broken program. :D)

4. Making the loop do something

Make sure the loop loops five times properly before trying this part. Seriously.

Inside the loop (after ask_loop_body:), write some code to:

1. input a number from the user
   • syscall 5 is the one you want for this.
2. store that number (which is in v0 after the syscall) into the correct item in the array.
   • you will have to calculate the array address like we learned in class.
   • go to the examples page and have a look at arrays.asm for an example array address calculation.

Hints

• The way syscall 5 works is like this:

        li	v0, 5
        syscall # asks the user for a number
        # now, whatever the user typed in is in v0 instead of the 5 we put there.
  

• When syscall 5 runs, you’ll be typing in the window at the bottom.
  • Hit enter after you type the number.
  • You can try “Settings > Popup dialog for input syscalls” if you don’t like that.
• There are three parts of the array address calculation:
  • la
  • which register represents i?
  • it’s an array of words.

If you did it correctly, after you run your program and type in 5 numbers, you should see those 5 numbers in the “Data Segment” window where your array is. I typed in 1, 2, 3, 4, 5 and got this:

Submitting

Make sure your file is named username_lab2.asm, like jfb42_lab2.asm.

Submit here.

Drag your asm file into your browser to upload. If you can see your file, you uploaded it correctly!

You can also re-upload if you made a mistake and need to fix it.