From now on, code style will be graded.

Your programs are getting bigger and the grader(s) need to be able to read them without clawing their eyeballs out. See the style guide for examples of good code style.

You are not required to use “Java-style” indentation, and comments are not required but are encouraged. At the very least, you code should be indented as shown in the “Indentation” section. Remember: tab key, not space bar.

Failure to use proper style can cause you to lose points from now on.

In this lab, you’ll be using arrays, functions, and for loops. You’ll also get a taste of that “top-down design” I talked about in the lecture on functions.

What will this program do? It will let you place tiles on a tilemap.


You don’t know what a tilemap is? Oh, okay.

What the heck is a tilemap?

Think of real tiles you see on floors or walls. Let’s just restrict it to square tiles in a grid pattern, like the image on the right. You can put patterns on the tiles, and you can then put multiple tiles together to create an image.

In computer graphics, a tilemap is a grid of square images. Each grid square can be one of multiple different images, identified by number (e.g. “the tile at row 6 column 4 is image 9”).

This is a pretty old-fashioned way of doing graphics on computers, but it has some nice advantages:

In old computers this was used originally to put text onscreen: one character per tile. But pretty soon, people started using this to put graphics onscreen, and through the 1980s, 1990s, and even into the 2000s, many home computers and video game consoles had support for tilemap-based graphics. This style of graphics lives on in video games even today, but in software form. (Ever play Mario Maker?)

0. Getting Started

Here is the include file for this lab. Just like lab 2:


.include "lab3_include.asm"

	running: .word 1

.global main

I’ve made it even easier to print strings for this lab. All you have to do now is:

print_str "hello, world!\n"

or whatever. You can put this anywhere, and it will not mess up any registers. So you can use it for “debug prints” as much as you like. (The lstr macro from lab 2 is still available too, if you want it.)

1. main and load_graphics

In class we talked about “top-down design” and “stubbing out” functions. Well that’s what we’re gonna do. We’re going to write our main function to call a bunch of other functions, then stub those functions out, and we’ll end up with a program that does nothing but has a bunch of things that we can work on in order.


void main() {
    // set up everything

    // main loop
    do {
    } while(running != 0);

    // do syscall 10 here!!

The pseudocode for main is on the right. Write your main function following it. Notes:

If you try to assemble your code now, it won’t work. That’s because you’re calling multiple functions that don’t exist yet. Let’s fix that.

Stubbing out functions

To get your program assembling, let’s stub out the functions that don’t exist.

Never put new functions before main. main should always be the first function you see.

  1. After main, let’s stub out load_graphics. You start a function by making a comment to visually separate it from the previous function, like the # ----- below. Then, you write the name as a label; and finally it ends with jr ra. So:

         # this is the end of main
         # for the love of god don't copy and paste this comment into main, I'm
         # just putting it here for illustrative purposes
         li v0, 10
     # ---------------------------------------------------
     jr ra
  2. Do the same thing again (as in, copy and paste what you just wrote and change the labels) for the load_map, check_input, and draw_cursor functions. You should now have something like:

     # ---------------------------------------------------
     jr ra
     # ---------------------------------------------------
     jr ra
     # ---------------------------------------------------
     # ...etc...
  3. Assemble. It should now work.
  4. Open the display, connect it, and run. It should do… nothing.
    • Well, it’s doing something. It’s looping infinitely.
    • You have to hit the stop button to stop the program.

Important: If your program stopped on its own and you saw -- program is finished running -- in the messages, your do-while loop is not working right. Fix it now, before you continue. (You can put a test print like print_str "a" in the loop to make sure it prints that over and over. Remove that print when you’ve confirmed it works.)


Right now, the images we want to put on the display are in the data segment. Go look in lab3_include.asm for the tilemap_gfx and sprite_gfx arrays starting around line 117. There they are!

But the display can’t see those. We have to copy the graphics to specific locations in the display’s memory so it can see them. There are two functions that do that: display_load_tm_gfx to load tilemap (tm) graphics, and display_load_sprite_gfx to load sprite graphics. (We’ll talk about sprites.)

Here’s what you need to do in your load_graphics function. “Inside” load_graphics means “between the load_graphics: label and the jr ra.”

  1. call display_load_tm_gfx(tilemap_gfx, 0, 4)
    • that’s a function call with three arguments.
    • you have to put the values in the argument registers before the call. So it breaks down like:
     a0 = tilemap_gfx; // use la for this
     a1 = 0;
     a2 = 4;
  2. call display_load_sprite_gfx(sprite_gfx, 0, 1) the same way

Alright, now assemble and run. You should see…

Nothing? You see nothing? Just a black screen? Hmmmmm. That’s not right! Hit the pause button and look at what instruction your program is running:

And even if you try hitting the step button, it just stays there.

This is what happens when you try to go two function calls deep without pushing and popping ra. You will get stuck in load_graphics and never be able to return to main because you lost the return address to main!

So one last thing you have to do:

And now, when you assemble and run, you should see a grassy field like the image to the right. (Yes, it’s grass. I’m a programmer, not an artist.)

So from now on every function you write (except main) should look like this:

push ra
    # code goes in between push and pop
pop ra
jr ra

2. draw_cursor

Next we will draw the cursor, a little square frame to indicate which tile we are editing. We will draw it using a sprite, which is a movable image that is drawn on top of the tilemap.

The way we put sprites onscreen is by storing values into the display_spr_table array. This is an array of bytes, where each sprite has 4 bytes of information. The bytes of the sprite table in memory look like this:

The “offset” at the bottom is what you add to the address of display_spr_table to get to that byte. You’ll be seeing that offset in the store instructions below.

Each sprite has a position (X, Y), a tile number (which image to draw), and flags which are options for the sprite. We won’t get into the flags too much in this lab, but setting flags to any number that ends in 1 will enable the sprite, causing it to appear onscreen. All the sprites start off disabled, and we only need one sprite for this program.

Here’s what you need to do:

  1. In the .data segment at the top of your program, declare two variables, cursor_x and cursor_y as .words, both initialized to 0.
  2. Then in draw_cursor, if you didn’t add the push ra and pop ra to draw_cursor already, do it now.
  3. Use la to put the address of display_spr_table into some tttttttemporary register. (I’m using t2 below but you can use whatever t register you want. Just don’t use a, or v, or s registers for this. They’re not aemporary or vemporary or semporary. They’re temporary.)
  4. Set the sprite’s X position by:
    • loading cursor_x into another temporary register
    • multiplying it by 8 without using li or mult omg
    • storing that value like so:

        sb t0, 0(t2) # where t0 is cursor_x * 8, and t2 is the array address
    • The syntax 0(t2) looks super weird, but it means “the address t2 + 0.” Yes. It’s addition. It’s a dumb syntax. But just look at the diagram above - offset 0 means the X coordinate of sprite 0.
  5. Do the same thing with cursor_y - load it, multiply it by 8, but store into 1(t2)
    • you should reuse the same temporary register for the Y value as you used for the X!
    • Again, 1(t2) means t2 + 1 - that’s the offset of sprite 0’s Y coordinate.
  6. Store a zero byte into 2(t2) to pick tile 0. (Do you have a register that holds zero?)
  7. Finally, store the constant 0x41 into 3(t2). REUSE THE REGISTER
    • We’ll come back to this in the future, but this will make it visible and color it red.

And when you run your program now, you should see a red box in the top-left!

If you don’t… well, here are your options:

3. check_input part 1

Let’s make your program interactive. But we won’t be using the input syscalls anymore. Instead, we will be using the display’s built-in input facilities. It has both keyboard and mouse (trackpad) support, but we won’t look at the mouse until next lab.

Keyboard input works is through the display_key_pressed variable. The way you do it is a little odd:

  1. You store a number indicating the key you want to check into display_key_pressed.
    • This tells the display which key you are interested in.
  2. You then load a value from display_key_pressed. Yes, the same “variable.”
    • The loaded value is either 0 to mean “not pressed” or 1 to mean “pressed.”
  3. Repeat for any other keys you want to check.

It works like this because it’s a memory-mapped input/output (MMIO) location. We’re not interacting with memory, we’re interacting with the display through the store and load.

Exiting the program

In check_input, we want to check if the user hit the “escape” key, and if so, end the program. We will end the program not by using syscall 10, but by setting running = 0, which will cause the main loop to terminate when we get back there. (This is how most programs actually exit.) So:

  1. In check_input, load the constant KEY_ESCAPE into t0, because t0 is like your dominant hand.
    • Do not write li t0, 27. Write li t0, KEY_ESCAPE. NAMES!!!!!!!!
  2. Use sw to store t0 into display_key_pressed.
  3. Use lw to load t0 from display_key_pressed. yes. reuse the same register.
  4. If t0 != 0
    • Set the running variable to 0.
      • Look at how running is declared to know which kind of store to use.
      • You can do this in one line.
    • This is not an if-else, just a simple if.

Now, run your program. It should stay running (the stop button should be green like this).

Then, click on the display to interact with it (like it says) and hit the escape key on your keyboard. Your program should stop and print -- program is finished running --. Woo! Interactivity!

If your program stops when you run it without you pressing escape, you probably got your if wrong. Remember:

Moving the cursor

Moving the cursor is similar. You are going to have multiple ifs in a row. These aren’t if-elses, just ifs. Here’s what you need to write after the if that you just wrote for escape, and guess what: you can write all of this code with only the t0 register. DO IT. GET USED TO IT!

Now test your program. When you click on the display, you should now be able to move the cursor around with the arrow keys on your keyboard. Tapping the key just moves it one tile, and holding the key moves it continuously. It should behave like the GIF below. If it doesn’t, you know what to do by now.

But there is one issue: if you move the cursor off any of the sides of the screen, it disappears. (If you hold it long enough, it comes back, but that’s not what we want.)

Limiting the cursor’s movement

The way we’ll solve this is with modular arithmetic. We’ll make it so the cursor “wraps around” when it goes off any of the sides. Doing this is extremely easy:

4. check_input part 2 and place_tile

Now we’ll make it possible to place tiles into the tilemap. First, copy and paste these lines into your .data segment:

.eqv KEYS_LEN 4
keys_to_check: .word KEY_Z      KEY_X     KEY_C      KEY_V

These are two parallel arrays. Parallel arrays are one way of representing what we would use classes for in Java. Instead of having an “object” with multiple fields, we have multiple arrays, and each index in those arrays “go together” to describe one thing. So:

Now, here’s what you have to do in check_input, at the end of the function (after checking for left, right, up, down):

  1. Make a for loop using s0 as the loop counter that loops from s0 = 0 to s0 < KEYS_LEN.
    • PUT. THE CONSTANT. IN. THE BRANCH. blt s0, KEYS_LEN, _loop
  2. Inside that loop, here’s what you need to do (notes follow):

     display_key_held = keys_to_check[i];
     if(display_key_held) {
         // 3 arguments... what registers do they go in?
         // when do you do that, before or after the jal?
         place_tile(cursor_x, cursor_y, keys_to_tiles[i]);
    • keys_to_check[i] - i is represented by s0 here. What type of array is keys_to_check? So what do you have to do with the index, and which load instruction do you use?
    • display_key_held is similar to display_key_pressed, except it tells you if the key is being held down. It works just like display_key_pressed though - store the key you want to check into it, then load a value from it, 1 means held, 0 means not held.
    • you have to index keys_to_tiles[i] similarly to keys_to_check.
    • place_tile is another function that you are about to make!

The tilemap table

The way we change the tilemap is by storing values into the display_tm_table array. This is an array of bytes, where each grid square of the tilemap has 2 bytes of information. The tilemap is two-dimensional, and is 32 squares wide and 32 squares tall. The way this is represented in memory is like so:

Each tile is 2 bytes of data, so to move “right” by 1 column, you move over by 2 bytes.

Each row is 64 bytes, so to move “down” by 1 row, you move over by 64 bytes.

This gives us a formula for calculating the offset from the beginning of display_tm_table of any tile coordinate (X, Y), where X is the column and Y is the row: offset = Y * 64 + X * 2.

Okay. That’s the theory out of the way, and why we’re about to do what we’re about to do.


  1. Make the place_tile function. It’s a separate function. Do not put it INSIDE check_input.
    • place_tile takes 3 arguments. a0, a1 are the tile X and Y respectively, and a2 is the type of tile to store. You don’t have to “do” anything to get the arguments. You just write place_tile assuming the arguments are already in a0, a1, and a2.
    • What you can do though is document it with comments, like
     # a0 = x
     # a1 = y
     # a2 = tile type
     push ra

  2. Inside place_tile:
    • calculate the offset using the formula from the previous section.
      • X and Y are your a0 and a1!
    • store a byte into display_tm_table at that offset

That should be it. Now to try it out:

You’re breaking the law

One final thing: go back to check_input and look at your for loop. See how you’re using s0?

You didn’t ask for permission to do that. 😱

In order to “ask permission” to use an s register in any function EXCEPT main, you must push it at the beginning of the function and pop it at the end. So change the start and end of check_input to look like:

push ra
push s0


pop s0 # <------- notice! BACKWARDS from the pushes!
pop ra
jr ra

Yeah it seems like nothing changed when you run the program, and it didn’t, this time. All that was happening was that you were getting lucky before.

Lucky and correct are not the same thing.

Small tangent: declarative design

The way we did the key checking in the previous part might have seemed a little odd to you. Why the heck did we have arrays of keys and tiles instead of just writing simpler ifs? Well, we invested a little bit more complexity up front (having to write the loop) for a more maintainable and modifiable program. See, these arrays:

.eqv KEYS_LEN 4
keys_to_check: .word KEY_Z      KEY_X     KEY_C      KEY_V

You can kind of just look at them and see what they do. Z puts a grass tile, X puts sand, etc.

Now, if we want to change the behavior of the code, we don’t need to touch the code at all. We just change these arrays. Want to use different keys? Want to swap which keys place which tiles? Want to add more keys to place other kinds of tiles? Just change the arrays, and the code doesn’t have to change!

This is a declarative way of writing code. Instead of hard-coding which keys place which tiles in check_input, we have more flexible code that reads what it’s supposed to do from the arrays. We simply “declare” what we want the code to do in the arrays. Abstraction.

5. load_map and char_to_tile_type

Okay, time to take the training wheels off and let you go figure these out.

First, copy and paste this array into your .data segment:

	map_data: .ascii
		"######          "
		"#    #          "
		"#..........     "
		"#..........     "
		"#    #   ..     "
		"######   ..     "
		"         ..     "
		"         ..     "
		"         ..     "
		"         ..   .."
		"         ..  .~~"
		"         ....~~~"
		"        .~~~~~~~"
		"       .~~~~~~~~"
		"       .~~~~~~~~"
		"       .~~~~~~~~"

This is actually a 16x16 array of bytes. Right? ;)

Now to write load_map:

void load_map() {
    // use s registers for row and col.
    // push and pop them at the beginning and end of this function.
    for(row = 0; row < 16; row++) {
        for(col = 0; col < 16; col++) {
            place_tile(col, row, char_to_tile_type(map_data[(row * 16) + col]));

Remember: write the code from the outside in, not top-to-bottom.

Also don’t get overwhelmed by that line inside the loop. Break it down. Think about the order of operations. It’s something like…

a0 = map_data[(row * 16) + col]; // byte array.
a0 = col;
a1 = row;
a2 = v0; // return value from char_to_tile_type

Finally, what is char_to_tile_type? It’s this:

int char_to_tile_type(int ch) {
    switch(ch) {
        case ' ': return TILE_GRASS; // do NOT jr ra here.
        case '.': return TILE_SAND;  // ditto; see below
        case '#': return TILE_BRICK;
        case '~': return TILE_WATER;
            System.out.print("invalid character!\n");
            System.exit(0); // syscall 10

Remember that to return a value from a function, you just put that value into v0. But then to actually exit the function, you need to jump to the pop ra line. If you don’t, the stack will get imbalanced and you’ll have a bad time!

Done correctly, you should now see the map to the right loaded when your program starts. You can still move the cursor around and edit it. And you can edit the strings in the map_data array to edit what shows up when the program starts!


To submit:

  1. On Canvas, go to “Assignments” and click on this lab.
  2. Click “Start Assignment.”
  3. Under “File Upload,” click the “Browse” button and choose your .asm file.
  4. Click “Submit Assignment.”

If you need to resubmit, that’s fine, just click “New Attempt” on the assignment page and upload it again.

Try This: Knowing what you know about functions now, go back to your lab 2 code. Move the various commands into their own functions and jal to them in each of the cases in main. Then, try making a draw_line function, and have both the line and rectangle commands use it. The program should still behave exactly the same way, but the code should be shorter and more readable!

Going further?

If you want to mess around with this more, go right ahead. Just don’t submit this stuff. Do this after submitting.

If you want to add another type of tile, here’s what you have to do:

  1. In lab3_include.asm, add another tile’s graphics to the end of the array following the pattern of the existing tiles.
    • Each byte is 1 pixel. Each tile is 8x8 pixels.
    • Colors 0x40 through 0x4F are for backwards compatibility with the old display and their meanings are given in the COLOR_ constants above.
    • There are actually 256 colors available:
      • colors 0x01 to 0x3F are a full, albeit small, palette
      • colors 0x50 to 0x7F are all black.
      • colors 0x80 to 0xFF are a gradual greyscale from full black to full white.
  2. In load_graphics, increase the last argument (a2) to display_load_tm_gfx to 5.
    • or 6, or 7, or however many tiles you have.
  3. Increase the KEYS_LEN constant, and add another entry to both keys_to_check and keys_to_tiles.
    • You’ll need this, which has the constants for all the keys that the display can handle. You can copy and paste the keys you care about into your program.

If you want to do more stuff… let me know and I’ll tell you how :D