Description
This lab is about familiarizing you with the MARS MIPS simulator software, and getting you started on your FUN ASSEMBLY LANGUAGE JOURNEY.
1. Getting started
There is a new version of MARS, MARS_2191_b.jar . If you already downloaded MARS, please redownload this new version and delete the old one.
You must use the modified version of MARS available on the software page.
Please make sure you have the newest version of the JRE installed.
You should be able to double-click the JAR file to run it. You will see this:
Setting things up (important!)
In the Settings menu, make sure the following things are checked (enabled):
Show Labels Window (symbol table)
Clear Run I/O upon assembling
Initialize Program Counter to global ‘main’ if defined
Leave the other settings unchanged.
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Hello, nothing
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Make a new file, and save it with the name username_lab1.asm , where username is your username. (I would put jfb42_lab1.asm .)
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In MIPS, comments start with a # sign. At the top of the file, put your name and username in a comment.
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Just like in Java or C, our assembly programs start at a main function. Type these lines into your file:
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.globl main
main:
That is not a typo: it is .globl , not `.global.
main: is a label. Labels name parts of your code. Whatever you write after the label will be the code of the main function.
The .globl directive is only needed for main . Most of your functions won’t need it.
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Instead of “compiling,” we “assemble” an asm program. When you
assemble the program with 
, it’ll switch to the Execute tab.
5. Now run with 
. In the “Run I/O” at the bottom of the screen, it’ll say
— program is finished running (dropped off bottom) —
It’s a completely empty program that does nothing. Yay!
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Hello, a number
Now we want to do the equivalent of System.out.print(1234); .
1. Don’t copy and paste this, type it in after your main: line:
li a0, 1234
li v0, 1
syscall
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If you assemble and run again, you’ll see the number 1234 printed in the “Run I/O”.
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Try assembling, and then stepping through instruction-by-instruction with
this button: 
. Watch the values of the registers as you do so.
What the heck is going on?
A single line of high-level code will usually become multiple lines of assembly.
li stands for “load immediate”, and it puts a constant value into a register.
a0 is an argument register. When you put things in argument registers, you know that a function call is about to happen.
“Immediate” is an
assembly term for “a
constant that is written inside the instruction.”
syscall stands for “system call” and you’ll learn about those in 449.
The syscall instruction doesn’t let you specify which function to call, so instead, we tell MARS which system call we want by putting the number of the syscall in the v0 register.
Hit F1 to open MARS help. Then click “Syscalls”. This tells you about what syscalls are, which ones are available, what their numbers are, and their arguments and return values.
We put 1 into v0 . Which syscall is 1?
This is kind of a weird choice, since the v registers are usually used for return values. I’m not sure why they did this.
4. Making your own function
1. Before your .globl main line, type the following:
# ——————————–
print_int:
jr ra
# ——————————–
You now have another label, print_int , which we’ll be able to use as a function. But what is 
jr ra 
?
In a higher level language, you would write a function like:
void print_int() {
// …some code…
}
I like to put those comment lines to visually separate functions, since assembly is so freeform.
When the execution reaches the closing brace, the function is done, right? This is because your high level language puts a return; for you before the closing brace.
jr ra is the “return” instruction: it returns to the function that called this one. So when you make a function, write the jr ra first so you won’t forget, and then write all the code between the label and the jr ra .
If you don’t put jr ra at the end of your function, bad things will
happen. You will get infinite loops and functions running when they weren’t called and your program ending for no reason and it’s just bad ok???
2. Now put the following between the print_int: and jr ra lines:
li v0, 1
syscall
So your print_int function will have 3 instructions.
print_int takes one argument, which goes into a0 . Write the instruction to put 5678 into a0 .
4. After that, write:
jal print_int
This is a regular function call. jal is an instruction which calls the function you name.
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When you assemble and run your program, it should display:
12345678
If you get 
Error in : invalid program counter value: 0x00000000 
, don’t forget to turn on the “Initialize program counter to global main ” setting.
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Now change the previous syscall in main to call your print_int function with 1234 instead. Your program should still print 12345678 .
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Making print_int put a newline after the number
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Look at the “Syscalls” section of the MARS help. Find one that lets you print a single character. Read what register it expects its argument to be in.
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Inside print_int , between syscall and jr ra , call that “print a character” syscall with the newline character.
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Hint: you can put a single character in a register using single quotes, like li a0, ‘A’ . Do you remember how to write a newline character? It starts with \
3. Done correctly, your program will now output:
1234
5678
6. Printing a string
A string is an array of characters, and cannot possibly fit into a register. Instead, we deal with the string’s address – where, in memory, the string begins.
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To put a string into memory, we have to switch to the data segment. At the top of your program, do this:
.data
hello_message: .asciiz “Hello, world!”
.text
The .data and .text directives tell the assembler to switch to the data
and text (code) segments. Any time you switch to the data segment, you must switch back to the text segment to write more code.
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Try assembling, and look at the labels and memory views. The labels view shows your new hello_message label at address 0x10010000 .
In the memory view, make sure the “ASCII” option is checked, and you can see your string in memory starting at address 0x10010000 .
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Make a new function, print_string , which works like print_int . But instead of using syscall 1 to print an int, look up the syscall to print a string. print_string will also print a newline after printing the string, just like print_int did.
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In main , you want to call print_string , but to set up its argument a0 , you can’t use li . We want the string’s address to go into a0 , so we use a new instruction, la (load address):
la a0, hello_message
jal print_string
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Assemble and run. It should print the string on its own line, wherever you decided to call it in main . I put mine at the beginning:
Hello, world!
1234
5678
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Assemble and step through your program line-by-line, and watch the registers change. In particular, watch what happens on the la line you just added. See what value actually goes into a0 ?
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Nested function calls
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Make a new function called print_123 which does the assembly equivalent of:
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print_int(1);
print_int(2);
print_int(3);
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Run it. …Wait a second. Why did it print this and then stop?
1
2
3
Notice that the program is still running. In fact, it’s trapped in an infinite loop.
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Hit the stop button
to stop it. Now modify print_123 to look like this:
print_123:
push ra
#…the calls to print_int, but NOT the jr ra
pop
jr
ra
ra
That is, insert those push ra and pop ra instructions at the beginning and just before the end of the function, respectively.
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When you run it now, it should work perfectly. It should print 1, 2, 3, “Hello, world!”, 1234, 5678, and then stop.
Why did this happen? ra is like a “bookmark” that the CPU uses to get back to where it left off, when you return from a function. But when you 
jal print_int inside of print_123 , ra is replaced by a bookmark back to print_123 . So the jr ra at the end of print_123 just kept going back to the print_123 , forever. Pushing and popping ra inside print_123 lets us save the bookmark to main on the stack.
If that didn’t make any sense… that’s fine. We’ll get to it in class 😉
Make sure your file is named username_lab1.asm , like jfb42_lab1.asm .
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.
© 2016-2018 Jarrett Billingsley
