S01E03 – Making Willy Walk

Displaying Miner Willy

So, we want to make Miner Willy move then? Well, we can’t do that unless he’s shown on the screen. In our previous episode, we created our game loop, and so we need to make an adjustment to that loop in order to call our display routine for Miner Willy.

repeat
    gosub @DisplayMinerWilly
    gosub @WaitVBL
until false

For our subroutine, we will keep it simple for the moment and just place Miner Willy at a specific location on our screen.

@DisplayMinerWilly
    sprite 1,100,100,1
    return

We have already learned about the label and the return command in previous episodes, but what else are we doing here? We’ve introduced a new command:

Let’s transpile our program, load it into STOS, and run it. We should simply get Miner Willy, frame 1, at position 100,100 on the screen. It works! But now we need to make him move.

A Moving Willy

We need to define some form of mechanism to make Miner Willy move. In the ZX Spectrum version of Manic Miner, this was done purely with keyboard input. We could do the same for our game, but for ease, I’ve decided to use the joystick to control our hero. We therefore need to read some input from the joystick, which STOS has some handy routines to deal with.

Before we can do anything in terms of joystick input, we need to do some setup so we can control the position of Miner Willy. We always need to know his current x and y positions on the screen so that we can increment them as required for walking and jumping. We need to do this in a series of variables, which we will need to initialise before trying to display Miner Willy. There’s also some additional setup that we will do too but won’t use for the moment; this is to cater for the fact that we are not going to use either the sprite movement or animation routines that are built into STOS.

Let’s create a new subroutine for initialising everything we need for Willy.

@InitMinerWilly
        MWD = 0 : MWWS = 1 : MWWC = 2 : MWWCC = MWWC : MWXPOS = 16 : MWYPOS = 80 : MWSPD = 0

There are a fair few variables there! I’ve tried to use logical naming conventions for them, but let’s go through them one by one and describe what they are used for:

We now need to make a call to our initialisation routine and also update our sprite command so that we are not forcing Willy to always be at position 100,100 on the screen. So we do that after we have loaded our assets. We may need to move this later on, but it serves it’s purpose here for the moment.

gosub @LoadAssets
gosub @InitMinerWilly

Now, we need to update our sprite command so that we are using the newly created MWXPOS and MWYPOS variables.

@DisplayMinerWilly
    sprite 1,MWXPOS,MWYPOS,1
    return

OK, let’s transpile our program, load it into STOS, and see what we have. You should now see that Miner Willy has moved positions. He’s further to the left and higher up the screen. That means that our sprite positioning with MWXPOS and MWYPOS is working, and we can begin to manipulate those with the joystick input.

Joystick Input

To get Miner Willy to move, we need to read the joystick data stream. STOS provides joystick input commands, so we will use them. Let’s go ahead and create a new function that we can call whenever we want to read the joystick.

@ReadJoystick
    JL = jleft : JR = jright : JF = fire : JSTCK = (JL or JR)
    return

Let’s breakdown the calls that are being used:

The above is simple enough, but why are we storing the state of the joysticks in variables? Well, we do this for a couple of reasons. Essentially, the input from the joystick is a continual stream of data being thrown at STOS. If we used the direct joystick commands throughout our source code, we could end up with some strange results as the joystick states may be different whilst our code is executing. For example, making Miner Willy do something at a point in which he shouldn’t be doing it. In addition to this, we would force STOS to perform additional reads and therefore consume more processor time at each call. By storing the values within variables, we only read the joystick once per VBL and therefore save a bunch of processing power.

Now we need to make sure that we call the routine to get the values from the joystick. We must do this within the game loop, so add the following code:

repeat 
    gosub @ReadJoystick 
    gosub @DisplayMinerWilly 
    gosub @WaitVBL 
until false

Applying Movement to Willy

We’ve got Miner Willy all set up now, and we have our variables available for changing his X and Y positions on the screen. We’ve also started to feed joystick input into our JL, JR and JF variables. Now that we’ve got that in place, we need to make Miner Willy move. Earlier, we created the MWSPD variable to hold the current speed at which Miner Willy is walking. It is this variable that we manipulate in order to signal that movement is to occur. We do this within the @ReadJoystick section of code.

@ReadJoystick
    JL = jleft : JR = jright : JF = fire : JSTCK = (JL or JR)
    if not(JSTCK) then MWSPD = 0 : else MWSPD = MWWS : if JL then MWD = -1 : else MWD = 0
    return

Let’s break down that new line of code that we have just added in:

We now know in which direction Miner Willy is facing (MWD) and at what speed he is moving (MWSPD). This information can now be used to update the MWXPOS variable accordingly. We’ll add this code into the @DisplayMinerWilly section.

@DisplayMinerWilly
    if MWD = 0 then MWXPOS = MWXPOS + MWSPD : else MWXPOS = MWXPOS - MWSPD
    sprite 1,MWXPOS,MWYPOS,1
    return

Transpile the code and see what happens. You should have a Miner Willy that now moves left and right on the screen using the joystick.

That’s all for this episode, folks. In the next episode, we will look at animating Miner Willy using a very interesting technique!

In the meantime, you can find the various episode source codes and assets on the Manic Miner Github page.