How to Draw A Playfield

by Nick Bensema (adapted by Duane Alan Hahn, a.k.a. Random Terrain)

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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:


Clear memory and registers

Set up variables


Do the vertical blank

Do game calculations

Draw screen

Do more calculations during overscan

Wait for next vertical blank

End Loop


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.





Clear Memory and TIA Registers

Now the memory and TIA registers must be cleared. You may feel the need to write a subroutine that only clears out a certain section of memory, but for now a simple loop will suffice.


Since X is already loaded to 0xFF, our task becomes simply to count everything off.

	LDA #0
B1      STA 0,X

The above routine does not clear location 0, which is VSYNC. We will take care of that later.






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.





Cycle Count

Atari says we have to have 37 scanlines of VBLANK time. Since each scanline uses 76 cycles, that makes 37*76=2888 cycles. We must also subtract the five cycles it will take to set the timer, and the three cycles it will take to STA WSYNC to the next line. Plus the checking loop is only accurate to six cycles, making a total of fourteen cycles we have to waste. 2888-14=2876.


We almost always use TIM64T for this, since the math just won't work out with the other intervals. 2880/64=44. something. It doesn't matter what that something is, we have to round DOWN.

	LDA  #44





Clear Collision Latches

And now's as good a time as any to clear the collision latches.

	LDA #0 





End VSYNC Period

Now we can end the VSYNC period.

	STA  WSYNC ; Third line of VSYNC.
	STA  VSYNC ; (0)

At this point in time the screen is scanning normally, but the TIA's output is suppressed. It will begin again once 0 is written back into VBLANK.







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.






The "score" bit is useful for drawing scores with playfield graphics as Combat and other early games do. It can also create a neat effect if you know how to be creative with it. One useful application of score mode would be always having player 1 on the left side, and player 0 on the right side. Each player would be surrounded in the opposite color, and the ball graphic could be used to stand out against either one. On my 2600jr, color from the right side bleeds about one pixel into the left side, so don't think it's perfect. It's really far from perfect because PC Atari does not implement it at all both sides appear as Player 0's color. A26 does, though.


To accommodate this, my routine puts color values into COLUP0 for the left side, and COLUP1 for the right side. Change the LDA operand to 0 or 1 to use the normal system. The code in the scanning loop accounts for both systems.

	LDA  #2





Initialize Display Variables

There aren't any display variables!





Counting Scanlines with Y Register

I'm going to use the Y register to count scanlines this time. Realize that I am forfeiting the use of the Y register for this purpose, but DEC Zero Page takes five cycles as opposed to DEY's two, and LDA Zero Page takes three cycles as opposed to TYA's two.


I'm using all 191 remaining scanlines after the WSYNC. If you want less, or more, change the LDY line.


This is a decremental loop, that is, Y starts at 191 and continues down to 0, as do all functions of Y such as memory locations, which is why the graphics at the end of this file are stored "bottom-up". In a way, one could say that's how the screen is drawn. To turn this into an incremental loop, change the number to 65 (255-191) and change the DEY at the end of the scanning loop to INY.

	LDY #191





A Clean Routine

Okay, now THIS is scary. I decided to put the bulk of my comments BEFORE the code, rather than inside it, so that you can look at the code all at once.


This routine came out surprisingly clean. There are no branches, and most updates are made even before PF0 becomes visible at cycle 23, even though PF1 and PF2 don't become visible until, by my estimate, cycles 29 and 40, respectively. We could use this time to get player shape and colors from temp variables and sneak them in, but that's another file. I put less-than signs in there to demonstrate that each register is changed before the "deadline cycle" at which point the changes would not completely appear on the current scanline.


The playfield will only be moved up every 4 scanlines, so it doesn't look squished. I could have updated it every 2 scanlines, and that would have saved two cycles. I could have saved another two cycles by having it change EVERY scanline. Comment out one or both of the ASL's to see what this would look like. I realize that it updates the PF registers whether it needs it or not, but it would be pointless to branch around these updates. Better to know you're wasting cycles and have them counted than to get unlucky and have your code spill into the next scanline every time too many things get updated.





Moving Playfield

This is going to be a moving playfield. For a stationary playfield, comment out the SEC and SBC lines. That's probably what most of you all are going to want, anyway. And for a really good moving playfield, like in River Raid or Vanguard, you'll need slightly more interesting code than I'm prepared to provide.


I also could have made the playfield graphic 16 bytes high, or 32, or 64, by changing only my data and the AND line. AND can serve as a modulus for any power of two (2^n) up to 128, by ANDing a byte with that number minus one ( (2^n)-1 ). 8 bytes * every 4 scanlines == 32, which is a power of two, which is why this works. Try commenting out the AND line and see how the program interprets it. Remember that PlayfieldY goes up to 255.


But you won't need to worry about that if you're drawing a stationary playfield where the graphics data is so large, it doesn't need to repeat. In that case, you don't need the AND line and you don't need to make sure your graphics are 2^n bytes tall. Comment out the AND, SEC and SBC lines, and add a third LSR to the block of two. It indexes a bit too far at the top of the screen, which explains the garbage. You can fix that problem either by adding more data to the end of each array, or by decreasing the resolution by adding a fourth or fifth LSR.


And who's to say you'll need all three playfield registers? Perhaps you have a rather narrow playfield, or one that's always clear in the middle. Either choice will save you five cycles per scanline.


As you can see, it can be trimmed down quite a bit, and I still have a lot of cycles left over. The maximum, if you recall, is 73 if you plan to use STA WSYNC, and I pity the fool who doesn't.

ScanLoop ; ============================== SCANNING LOOP
; Result of the following math is:
;  X = ( (Y-PlayfieldY) /4 ) mod 7  
	SBC PlayfieldY
	LSR   ;Divide by 4
	AND #7  ;modulo 8
	LDA PFData0,X           ;Load ahead of time.
; WSYNC is placed BEFORE all of this action takes place.
	STA WSYNC                
	STA PF0                 ;[0] +3 = *3*   < 23
	LDA PFLColor,X          ;[3] +4
	;In a real game, I wouldn't be this redundant.
	STA COLUP0              ;[7] +3 = *10*  < 23
	STA COLUPF              ;[10]+3 = *13*  < 23
	LDA PFData1,X           ;[13]+4
	STA PF1                 ;[17]+3 = *20*  < 29
	LDA PFRColor,X          ;[20]+4
	STA COLUP1              ;[24]+3 = *27*  < 49
	LDA PFData2,X           ;[27]+4
	STA PF2                 ;[31]+3 = *34*  < 40
	BNE ScanLoop





Prevent Bleeding

Clear all registers here to prevent any possible bleeding.

	LDA #2
	STA WSYNC  ;Finish this scanline.
	STA VBLANK ; Make TIA output invisible,
	; Now we need to worry about it bleeding when we turn
	; the TIA output back on.
	; Y is still zero.





Overscan Calculations

For the Overscan routine, one might take the time to process such things as collisions. I, however, would rather waste a bunch of scanlines, since I haven't drawn any players yet.

OverScan   ;********************* OVERSCAN CALCULATIONS
	LDX #30
	 BNE KillLines







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.


4 5 6 7 7 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7


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.


Useful Links

Easy 6502 by Nick Morgan

How to get started writing 6502 assembly language. Includes a JavaScript 6502 assembler and simulator.



Atari Roots by Mark Andrews (Online Book)

This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).



Machine Language For Beginners by Richard Mansfield (Online Book)

This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.

The Six Instruction Groups

The 6502 Instruction Set broken down into 6 groups.

6502 Instruction Set

Nice, simple instruction set in little boxes (not made out of ticky-tacky).



The Second Book Of Machine Language by Richard Mansfield (Online Book)

This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.

6502 Instruction Set

An easy-to-read page from The Second Book Of Machine Language.



6502 Instruction Set with Examples

A useful page from Assembly Language Programming for the Atari Computers.

Continually strives to remain the largest and most complete source for 6502-related information in the world.

NMOS 6502 Opcodes

By John Pickens. Updated by Bruce Clark.



Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.


Goes over each of the internal registers and their use.

Instruction Set

Gives a summary of whole instruction set.

Addressing Modes

Describes each of the 6502 memory addressing modes.

Instruction Reference

Describes the complete instruction set in detail.



Stella Programmer's Guide

HTMLified version.



Nick Bensema's Guide to Cycle Counting on the Atari 2600

Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.



How to Draw A Playfield by Nick Bensema

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.



Cart Sizes and Bankswitching Methods by Kevin Horton

The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.



Atari 2600 Specifications

Atari 2600 programming specs (HTML version).



Atari 2600 Programming Page (AtariAge)

Links to useful information, tools, source code, and documentation.




Atari 2600 programming site based on Garon's "The Dig," which is now dead.



TIA Color Charts and Tools

Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.



The Atari 2600 Music and Sound Page

Adapted information and charts related to Atari 2600 music and sound.



Game Standards and Procedures

A guide and a check list for finished carts.




A multi-platform Atari 2600 VCS emulator. It has a built-in debugger to help you with your works in progress or you can use it to study classic games. Stella finally got Atari 2600 quality sound in December of 2018. Until version 6.0, the game sounds in Stella were mostly OK, but not great. Now it's almost impossible to tell the difference between the sound effects in Stella and a real Atari 2600.




A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.



batari Basic Commands

If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.



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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Negative Ions = Good

Negative ions are good for us. You might want to avoid positive ion generators and ozone generators. A plain old air cleaner is better than nothing, but one that produces negative ions makes the air in a room fresher and easier for me to breathe. It also helps to brighten my mood.



Litterbugs = Bad

Never litter. Toss it in the trash or take it home. Do not throw it on the ground. Also remember that good people clean up after themselves at home, out in public, at a campsite and so on. Leave it better than you found it.



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The Climate Agenda



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Charlie Robinson had some good advice about waking up normies (see the link to the video below). He said instead of verbally unloading or being nasty or acting like a bully, ask the person a question. Being nice and asking a question will help the person actually think about the subject.


Interesting videos:

Charlie Robinson Talks About the Best Way to Wake Up Normies

Georgia Guidestones Explained

The Men Who Own Everything


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