Using Sprites

Assembler coding - Lesson 5

eZine lover (@eZine)

Published in

Assembler coding

· 24 Nov 2022

Firstly, you may wish to download 3 in 1 SPRITE EDITOR. A real cool editor package where you can create your own multi coloured sprites or fonts. The download file also contains other programs you may like. To run 3 in 1 editor type SYS 50000 after you load it.

Ok, lets now place a sprite on the screen and then get it moving. Here's roughly what we need to do:

Clear Screen
Set up Sprite pointer
Set up Sprite Colours & Mode
Set up X & Y position of Sprite
Move it Down
Move it Up

So lets write the code:

  10 LDA #147 
  20 JSR $FFD2 
  30 LDA #00 
  40 STA 53280 
  50 STA 53281 
  60 LDA #255 
  70 STA 53269 
  80 STA 53276 
  90 LDA #02 
  100 STA 53285 
  110 LDA #03 
  120 STA 53286 
  130 LDX #00 
  140 COLOUR LDA #04 
  150 STA 53287,X 
  160 INX 
  170 CPX #08 
  180 BNE COLOUR 
  190 LDX #64 
  200 STX 2040 
  210 INX 
  220 STX 2041 
  230 INX 
  240 STX 2042 
  250 INX 
  260 STX 2043 
  270 INX 
  280 STX 2044 
  290 INX 
  300 STX 2045 
  310 INX 
  320 STX 2046 
  330 INX 
  340 STX 2047 
  350 LDY # 70 
  360 STY 53249 
  370 STY 53251 
  380 STY 53253 
  390 STY 53255 
  400 STY 53257 
  410 STY 53259 
  420 STY 53261 
  430 STY 53263 
  440 LDX #30 
  450 STX 53248 
  460 LDX #60 
  470 STX 53250 
  480 LDX #90 
  490 STX 53252 
  500 LDX #120 
  510 STX 53254 
  520 LDX #150 
  530 STX 53256 
  540 LDX #180 
  550 STX 52358 
  560 LDX #210 
  570 STX 53260 
  580 LDX #240 
  590 STX 53262 
  610 DOWN LDX #100 
  620 LOOP1 LDY #100 
  630 LOOP DEY 
  640 BNE LOOP 
  650 DEX 
  660 BNE LOOP1 
  670 INC 53249 
  680 INC 53251 
  690 INC 53253 
  700 INC 53255 
  710 INC 53257 
  720 INC 53259 
  730 INC 53261 
  740 INC 53263 
  750 LDA 53249 
  760 CMP #200 
  770 BNE DOWN 
  780 UP LDX #100 
  790 LOOP3 LDY #100 
  800 LOOP2 DEY 
  810 BNE LOOP2 
  820 DEX 
  830 BNE LOOP3 
  840 DEC 53249 
  850 DEC 53251 
  860 DEC 53253 
  870 DEC 53255 
  880 DEC 53257 
  890 DEC 53259 
  900 DEC 53261 
  910 DEC 53263 
  920 LDA 53249 
  930 CMP #70 
  940 BNE UP 
  950 JMP DOWN

So how does it work ?

10 & 20 clears the screen

30,40,50 give the border & background the colour black

60,70,80 turn on all 8 sprites and enable multi coloured sprites

90, 100 sets sprite multicolour register 0 to RED

110,120 sets sprite multicolour register 1 to CYAN

130 - 180 sets each sprite colour to PURPLE using a Compare & Branch Loop

190 - 340 sets each sprite data pointer. The sprites here would be in bank 0 and the value 64 gives them a location in memory of 1000 Hex or 4096 decimal. This is where you would load your sprite data. Of course you would change the colours above to suit your sprites. The formula to calculate where your sprite data is, is shown here:

  LOCATION = (BANK * 16384) + (SPRITE POINTER VALUE * 64)

So for our case we have:

  LOCATION = ( 0 * 16384) + (64 * 64)

Hence

  LOCATION = 4096 Decimal (1000 Hex)

So the first sprite uses 64 as its value, and the next 65 etc until all sprites have there data pointer location.

350 - 430 These set the Y position on the screen for each sprite to 70

440 - 590 These set the X position on the screen for each sprite, starting at 30 for sprite 0, to 240 for sprite 7

600 (Oops I made a mistake and missed this line, no matter)

610 - 770 These lines of code move the sprites down the screen. The code uses a time delay loop to slow the process down, an example of this loop is shown below:

  LDX #100 
  LOOP1 LDY #100 
  LOOP DEY 
  BNE LOOP 
  DEX 
  BNE LOOP1

This is really simple to follow, Firstly it sets the X and Y registers to 100, then -1 from Y until Y = 0, then -1 from the X register and see if X = 0, if not then branch back to line LDY #100 and start again. The effect is a delay. Without this delay the sprite movement would be too quick to see. You could change these values to suit yourself. Of course the maximum value you can have in each loop is 255.

The way the sprite is moved is simply by incrementing the Y position for each sprite by +1. (You increment to move down as that is the way the screen is setup, i.e. the top left is position (0,0)

The code then checks the position of sprite 0, if this position is 200 then the program falls through the branch to move the sprite up the screen. If the position is 200 then the program branches back to continue to move the sprite down the screen.

780 - 950 Exactly the same as moving down except we move the sprites up by incrementing each sprite Y position and we check the position for 70. If not 70 move the sprite up some more, if position = 70 then this time the program jumps back to line labelled DOWN to again move the sprite down again.

And so the process would go on & on moving the sprite up & down the screen.

Well i hope you could follow that. It is really easy, honest. It looks long but most of the code is for actually setting up the sprite colours and pointers and initial screen positions.

This example doesn't move the sprites in the X direction, but you do that the same. The hardest part with that is getting the sprites in and out of the MSB i.e. past 255 in the X direction. Yes you heard right, the screen in the X direction is split into two visible sections. These are 24 to 255 & then in the next section 0 to 91. We will do that in another section.

In the next lesson we will produce more efficient coding of the same thing, but this time I'll show you how to make those sprites bounce instead of just moving up & down.