How to Draw A Playfield

by Nick Bensema (adapted by Duane Alan Hahn)

Table of Contents

Original File

Download the original asm file from:

Nick Bensema's Atari 2600 Programming Page

Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.


Since the CPU must hold the TIA's hand through all graphical operations, the flow ought to go like this:



What I will do is create an outline, and explain everything I can. This program will display "HELLO" and scroll it down the screen.


In writing this program, I will take the opportunity to show you how a few simple modifications can completely change a program's appearance or behavior. I will invite you to comment out a few lines of code, and alter others, so that you can observe the results.


I will be using DASM for now. Conversion to A65 should be trivial.

	processor 6502
	include vcs.h

	org $F000
Temp       = $80
PlayfieldY = $90







Let's Clear Things Up

The 2600 powers up in a completely random state, except for the PC which is set to the location at $FFC. Therefore the first thing we must do is to set everything up inside the 6502.

	SEI  ; Disable interrupts, if there are any.
	CLD  ; Clear BCD math bit.

You may feel the need to use the stack, in which case:

	LDX  #$FF

	TXS  ; Set stack to beginning.









Set Up Joysticks and Such

At this point in the code we would set up things like the data direction registers for the joysticks and such.

	JSR  GameInit






Program Flow

Here is a representation of our program flow.

	JSR  VerticalBlank ;Execute the vertical blank.
	JSR  CheckSwitches ;Check console switches.
	JSR  GameCalc      ;Do calculations during Vblank
	JSR  DrawScreen    ;Draw the screen
	JSR  OverScan      ;Do more calculations during overscan
	JMP  MainLoop      ;Continue forever.







It is important to maintain a stable screen, and this routine does some important and mysterious things. Actually, the only mysterious part is VSYNC. All VBLANK does is blank the TIA's output so that no graphics are drawn otherwise the screen scans normally. It is VSYNC which tells the TV to pack its bags and move to the other corner of the screen.


Fortunately, my program sets VBLANK at the beginning of the overscan period, which usually precedes this subroutine, so it is not changed here.

VerticalBlank  ;*********************** VERTICAL BLANK HANDLER
	LDX  #0
	LDA  #2
	STA  VSYNC ;Begin vertical sync.
	STA  WSYNC ; First line of VSYNC
	STA  WSYNC ; Second line of VSYNC.

But before we finish off the third line of VSYNC, why don't we use this time to set the timer? This will save us a few cycles which would be more useful in the overscan area.


To insure that we begin to draw the screen at the proper time, we must set the timer to go off just slightly before the end of the vertical blank space, so that we can WSYNC up to the ACTUAL end of the vertical blank space. Of course, the scanline we're going to omit is the same scanline we were about to waste VSYNCing, so it all evens out.









Game Switch Switcheroo

Checking the game switches is relatively simple.


It just so happens that I'm not going to check any game switches here. I'm just going to set up the colors, without even checking the B&W switch! HA!

CheckSwitches ;*************************** CONSOLE SWITCH HANDLER
       LDA #0
       STA COLUBK  ; Background will be black.






Game Calculations

Minimal game calculations, just to get the ball rolling.

GameCalc ;******************************* GAME CALCULATION ROUTINES
	INC PlayfieldY   ;Inch up the playfield






Screen Drawing Routines

This is the scariest thing I've done all month.

DrawScreen ;**************************** SCREEN DRAWING ROUTINES
	BNE DrawScreen ; Whew!
	STA VBLANK  ;End the VBLANK period with a zero.

Now we can do what we need to do. What sort of playfield do we want to show? A doubled playfield will work better than anything if we either want a side scroller (which involves some tricky bit shifting, usually) or an asymmetrical playfield (which we're not doing yet). A mirrored playfield is usually best for vertical scrollers. With some creativity, you can use both in your game.










GameInit could conceivably be called when the Select key is pressed, or some other event.

	LDA #0
	STA PlayfieldY






Graphics Data

Graphics are placed so that the extra cycle in the PFData, X indexes is NEVER taken, by making sure it never has to index across a page boundary. This way our cycle count holds true.

	org $FF00 ; *********************** GRAPHICS DATA

PFData0  ;H       4 5 6 7
       .byte $00,$f0,$00,$A0,$A0,$E0,$A0,$A0
PFData1  ;EL      7 6 5 4 3 2 1 0
       .byte $00,$FF,$00,$77,$44,$64,$44,$74
PFData2  ;LO      0 1 2 3 4 5 6 7
       .byte $00,$FF,$00,$EE,$A2,$A2,$A2,$E2
PFLColor ; Left side of screen
       .byte $00,$FF,$00,$22,$26,$2A,$2C,$2E
PFRColor ; Right side of screen
       .byte $00,$1F,$00,$6E,$6C,$6A,$66,$62
	org $FFFC
	.word Start
	.word Start






Reversed Bit Order

This is the tricky part of drawing a playfield: actually drawing it. Well, the display routine and all that binary math was a bit tricky, too, but still, listen up.


Playfield data isn't stored the way most bitmaps are, even one-dimensional bitmaps. We will use the left side of the screen only, knowing that the right side is either repeated or reflected from it.


In PF0 and PF2, the most significant bit (bit 7) is on the RIGHT side. In PF1, the most significant bit is on the LEFT side. This means that relative to PF0 and PF2, PF1 has a reversed bit order. It's just really weird.



This is important to remember when doing calculations on bytes intended for the PF registers. Defender gives a good example of this.


It will become necessary to write a program that makes this easier, because it is easy to become confused when dealing with this system.




Other Assembly Language Tutorials

Be sure to check out the other assembly language tutorials and the general programming pages on this web site.

Atari 2600 BASIC

If assembly language is too hard for you, try batari Basic. It's a BASIC-like language for creating Atari 2600 games. It's the faster, easier way to make Atari 2600 games.

Try batari Basic

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