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Atari 2600 Programming for Newbies

Session 13: Playfield Basics

By Andrew Davie (adapted by Duane Alan Hahn)

Table of Contents

Original Session

In the last few sessions, we started to explore the capabilities of the TIA. We learned that the TIA has "registers" which are mapped to fixed memory addresses, and that the 6502 can control the TIA by writing and/or reading these addresses. In particular, we learned that writing to the WSYNC register halts the 6502 until the TIA starts the next scanline, and that the COLUBK register is used to set the color of the background. We also learned that the TIA keeps an internal copy of the value written to COLUBK.

 

Today we're going to have a look at playfield graphics, and for the first time learn how to use RAM. The playfield is quite a complex beast, so we may be spending the next few sessions exploring its capabilities.

 

The '2600 was originally designed to be more or less a sophisticated programmable PONG-style machine, able to display 2-player gamesbut still pretty much PONG in style. These typically took place on a screen containing not much more than walls, two "players"usually just straight lines and a ball. Despite this, the design of the system was versatile enough that clever programmers have produced a wide variety of games.

 

The playfield is that part of the display which usually shows "walls" or "backgrounds" (not to be confused with THE background color). These walls are usually only a single color (for any given scanline), though games typically change the color over multiple scanlines to give some very nice effects.

 

The playfield is also sometimes used to display very large (square, blocky looking) scores and words.

 

Just like with COLUBK, the TIA has internal memory where it stores exactly 20 bits of playfield data, corresponding to just 20 pixels of playfield. Each one of these pixels can be on (displayed) or off (not displayed).

 

 

 

 

 

Horizontal Resolution

The horizontal resolution of the playfield is a very-low 40 pixels, divided into two halvesboth of which display the same 20 bits held in the TIA internal memory. Each half of the playfield may have its own color (we'll cover this later), but all pixels either half are the same color. Each playfield pixel is exactly 4 color-clocks wide (160 color clocks / 40 pixels = 4 color clocks per pixel).

 

The TIA manages to draw a 40 pixel playfield from only 20 bits of playfield data by duplicating the playfield (the right side of the playfield displays the same data as the left side). It is possible to mirror the right side, and it is also possible to create an "asymmetrical playfield"where the right and left sides of the playfield are NOT symmetrical. I'll leave you to figure out how to do that for nowwe'll cover it in a future session. For now, we're just going to learn how to play with those 20 bits of TIA memory, and see what we can do with them.

 

 

 

 

 

Sample Code

Let's get right into it. Here's some sample code which introduces a few new TIA registers, and also (for the first time for us) uses a RAM location to store some temporary information (a variable!). There are three TIA playfield registers (two holding 8 bits of playfield data, and one holding the remaining 4 bits)PF0, PF1, PF2. Today we're going to focus on just one of these TIA playfield registers, PF1, because it is the simplest to understand.

 

; '2600 for Newbies
; Session 13 - Playfield





                processor 6502

                include "vcs.h"

                include "macro.h"


;------------------------------------------------------------------------------



PATTERN         = $80                  ; storage location (1st byte in RAM)

TIMETOCHANGE    = 20                   ; speed of "animation" - change as desired
;------------------------------------------------------------------------------



                SEG

                ORG $F000



Reset



   ; Clear RAM and all TIA registers



                ldx #0 

                lda #0 

Clear           sta 0,x 

                inx 

                bne Clear



       ;------------------------------------------------

       ; Once-only initialization...



                lda #0

                sta PATTERN            ; The binary PF 'pattern'



                lda #$45

                sta COLUPF             ; set the playfield color



                ldy #0                 ; "speed" counter



       ;------------------------------------------------



StartOfFrame



   ; Start of new frame

   ; Start of vertical blank processing



                lda #0

                sta VBLANK



                lda #2

                sta VSYNC



                sta WSYNC

                sta WSYNC

                sta WSYNC               ; 3 scanlines of VSYNC signal



                lda #0

                sta VSYNC           





       ;------------------------------------------------

       ; 37 scanlines of vertical blank...

            

                ldx #0

VerticalBlank   sta WSYNC

                inx

                cpx #37

                bne VerticalBlank



       ;------------------------------------------------

       ; Handle a change in the pattern once every 20 frames

       ; and write the pattern to the PF1 register



                iny                    ; increment speed count by one

                cpy #TIMETOCHANGE      ; has it reached our "change point"?

                bne notyet             ; no, so branch past



                ldy #0                 ; reset speed count



                inc PATTERN            ; switch to next pattern

notyet





                lda PATTERN            ; use our saved pattern

                sta PF1                ; as the playfield shape



       ;------------------------------------------------

       ; Do 192 scanlines of color-changing (our picture)



                ldx #0                 ; this counts our scanline number



Picture         stx COLUBK             ; change background color (rainbow effect)

                sta WSYNC              ; wait till end of scanline



                inx

                cpx #192

                bne Picture



       ;------------------------------------------------



   

 

                lda #%01000010

                sta VBLANK          ; end of screen - enter blanking



   ; 30 scanlines of overscan...



                ldx #0

Overscan        sta WSYNC

                inx

                cpx #30

                bne Overscan











                jmp StartOfFrame




;------------------------------------------------------------------------------



            ORG $FFFA



InterruptVectors



            .word Reset          ; NMI

            .word Reset          ; RESET

            .word Reset          ; IRQ



      END

Here's a screenshot:

Here's the .bin file to use with an emulator:

kernel_13.bin

 

What you will see is our rainbow-colored background, as beforebut over the top of it we see a strange-pattern of vertical stripe(s). And the pattern changes. These vertical stripes are our first introduction to playfield graphics.

 

 

 


       ; Handle a change in the pattern once every 20 frames

       ; and write the pattern to the PF1 register



                iny                    ; increment speed count by one

                cpy #TIMETOCHANGE      ; has it reached our "change point"?

                bne notyet             ; no, so branch past



                ldy #0                 ; reset speed count



                inc PATTERN            ; switch to next pattern

notyet





                lda PATTERN            ; use our saved pattern

                sta PF1                ; as the playfield shape

This is the first time we've seen an instruction like "inc PATTERN"the others we have already covered. "inc" is an incrementand it simply adds 1 to the contents of any memory (mostly RAM) location. We initialized PATTERN (which lives at $80, remember!) to 0. So after 20 frames, we will find that the value gets incremented to 1. 20 frames after that, it is incremented to 2.

 

 

 

 

Binary Number System

Now let's go back to our binary number system for a few minutes. Here's the binary representation of the numbers 0 to 10. . .

 

 

Have a real close look at the pattern there, then run the binary again and look at the pattern of the stripe. I'm telling you, they're identical! That is because, of course, we are writing these values to the PF1 register and where there is a set bit (value of 1) that corresponds directly to a pixel being displayed on the screen.

 

See how the PF1 write is outside the 192-line picture loop. We only ever write the PF1 once per frame (though we could write it every scanline if we wished). This demonstrates that the TIA has kept the value we write to its register(s) and uses that same value again and again until it is changed by us.

The diagram above shows the operation of the PF1 register, and which of the 20 TIA playfield bits it modifies. You can also see the color-register to color correspondence.

 

 

 

 

Play with It

The rest of the code is identical to our earlier tutorialsso to get our playfield graphics working, all we've had to do is write a color to the playfield color register (COLUPF), and then write actual pixel data to the playfield register(s) PF0, PF1 and PF2. We've only touched PF1 this timefeel free to have a play and see what happens when you write the others.

 

You might also like to play with writing values INSIDE the picture (192-line) loop, and see what happens when you play around with the registers 'on-the-fly'. In fact, since the TIA retains and redraws the same thing again and again, to achieve different 'shapes' on the screen, this is exactly what we have to dowrite different values to PF0, PF1, PF2 not only every scanline, but also change the shapes in the middle of a scanline!

 

Today's session is meant to be an introduction to playfield graphicsdon't worry too much about the missing information, or understanding exactly what's happening. Try and have a play with the code, do the exercisesand next session we should have a more comprehensive treatment of the whole shebang.

 

 

 

 

 

Summary

Subjects we will tackle next time include. . .

 

See you then, then!

 

 

 

 

 

Exercises

  1. Modify the kernel so that instead of showing a rainbow-color for the background, it is the playfield which has the rainbow effect
  2. What happens when you use PF0 or PF2 instead of PF1? It can get pretty bizarrewe'll explain what's going on in the next session.
  3. Can you change the kernel so it only shows *ONE* copy of the playfield you write (that is, on the left side you see the pattern, and on the right side it's blank). Hint: You'll need to modify PF1 mid-scanline.

 

We'll have a look at these exercises next session. Don't worry if you can't understand or implement themthey're pretty tricky.

 

 

 

Other Assembly Language Tutorials

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

 

 

< Previous Session

 

 

Next Session >

 

 

 

 

Session Links

Session 1: Start Here

Session 2: Television Display Basics

Sessions 3 & 6: The TIA and the 6502

Session 4: The TIA

Session 5: Memory Architecture

Session 7: The TV and our Kernel

Session 8: Our First Kernel

Session 9: 6502 and DASM - Assembling the Basics

Session 10: Orgasm

Session 11: Colorful Colors

Session 12: Initialization

Session 13: Playfield Basics

Session 14: Playfield Weirdness

Session 15: Playfield Continued

Session 16: Letting the Assembler do the Work

Sessions 17 & 18: Asymmetrical Playfields (Parts 1 & 2)

Session 19: Addressing Modes

Session 20: Asymmetrical Playfields (Part 3)

Session 21: Sprites

Session 22: Sprites, Horizontal Positioning (Part 1)

Session 23: Moving Sprites Vertically

Session 24: Some Nice Code

Session 25: Advanced Timeslicing

 

 

 

 

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 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 with Examples

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

 

 

6502.org

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

 

 

Guide to 6502 Assembly Language Programming by Andrew Jacobs

Below are direct links to the most important pages.

 

 

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.

 

 

MiniDig

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.

 

 

Stella

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.

 

 

JAVATARI

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.

 

 

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Disclaimer

View this page and any external web sites at your own risk. I am not responsible for any possible spiritual, emotional, physical, financial or any other damage to you, your friends, family, ancestors, or descendants in the past, present, or future, living or dead, in this dimension or any other.

 

Use any example programs at your own risk. I am not responsible if they blow up your computer or melt your Atari 2600. Use assembly language at your own risk. I am not responsible if assembly language makes you cry or gives you brain damage.

 

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