Atari 2600 Programming for Newbies
Session 13: Playfield Basics
By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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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 games—but 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).
The horizontal resolution of the playfield is a very-low 40 pixels, divided into two halves—both 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 now—we'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.
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:
What you will see is our rainbow-colored background, as before—but 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.
Have a good look at what this demo does; although it is only writing to a single playfield register (PF1) which can only hold 8 bits (pixels) of playfield data, you always see the same stripe(s) on the left side of the screen, as on the right. This is a result, as noted earlier, of the TIA displaying its playfield data twice on any scanline—the first 20 bits on the left side, then repeated for the right side.
Let's walk through the code and have a look at some of the new bits…
PATTERN = $80 ; storage location (1st byte in RAM) TIMETOCHANGE = 20 ; speed of "animation" - change as desired
At the beginning of our code we have a couple of equates. Equates are labels with values assigned to them. We have covered this sort of label value assignation when we looked at how DASM resolved symbols when assembling our source code. In this case, we have one symbol (PATTERN) which in the code is used as a storage location …
sta PATTERN
… and the other (TIMETOCHANGE) which is used in the code as a number for comparison
cpy #TIMETOCHANGE
Remember how we noted that the assembler simply replaced any symbol it found with the actual value of that symbol. Thus the above two sections of code are exactly identical to writing "sta $80" and "cpy #20". But from our point of view, it's much better to read (and understand) when we use symbols instead of values.
So, at the beginning of our source code (by convention, though you can pretty much define symbols anywhere), we include a section giving values to symbols which are used throughout the code. We have a convenient section we can go back to and "adjust" things later on.
Here's our very first usage of RAM…
lda #0
sta PATTERN ; The binary PF 'pattern'
Remember, DASM replaces that symbol with its value. And we've defined the value already as $80. So that "sta" is actually a "sta $80", and if we have a look at our memory map, we see that our RAM is located at addresses $80 - $FF. So this code will load the accumulator with the value 0 (that's what that crosshatch means—load a value, not a load from memory) and then store the accumulator to memory location $80. We use PATTERN to hold the "shape" of the graphics we want to see. It's just a byte, consisting of 8 bits. But as we have seen, the playfield is 20 bits each being on or off, representing a pixel. By writing to PF1 we are actually modifying just 8 of the TIA playfield bits. We could also write to PF0 and PF2—but let's get our understanding of the basic playfield operation correct, first.
lda #$45
sta COLUPF ; set the playfield color
When we modified the color of the background, we wrote to COLUBK. As we know, the TIA has its own internal 'state', and we can modify its state by writing to its registers. Just like COLUBK, COLUPF is a color register. It is used by the TIA for the color of playfield pixels (which are visible—ie: their corresponding bit in the PF0, PF1, PF2 registers is set).
If you want to know what color $45 is, look it up in the color charts presented earlier. I just chose a random value, which looks reddish to me :)
ldy #0 ; "speed" counter
We should be familiar with the X, Y and A registers by now. This is loading the value 0 into the y register. Since Y was previously unused in our kernel, for this example I am using it as a sort of speed throttle. It is incremented by one every frame, and every time it gets to 20 (or more precisely, the value of TIMETOCHANGE) then we change the pattern that is being placed into the PF1 register. We change the speed at which the pattern changes by changing the value of the TIMETOCHANGE equate at the top of the file.
Speed Throttle and Pattern Change
That speed throttle and pattern change is handled in this section…
; 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 increment—and 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.
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…
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
00001010
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.
The rest of the code is identical to our earlier tutorials—so 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 time—feel 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 do—write 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 graphics—don't worry too much about the missing information, or understanding exactly what's happening. Try and have a play with the code, do the exercises—and next session we should have a more comprehensive treatment of the whole shebang.
Subjects we will tackle next time include…
- The other playfield registers (PF0, PF2)
- The super-weird TIA pixel -> screen pixel mapping
- Mirrored playfields
- Two color playfields
- Asymmetrical playfield
See you then, then!
- Modify the kernel so that instead of showing a rainbow-color for the background, it is the playfield which has the rainbow effect
- What happens when you use PF0 or PF2 instead of PF1? It can get pretty bizarre—we'll explain what's going on in the next session.
- 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 them—they'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.
Amazon: Atari 2600 Programming (#ad)
Amazon: 6502 Assembly Language Programming (#ad)
Atari 2600 Programming for Newbies (#ad)
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Session 2: Television Display Basics
Sessions 3 & 6: The TIA and the 6502
Session 5: Memory Architecture
Session 7: The TV and our Kernel
Session 9: 6502 and DASM - Assembling the Basics
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 20: Asymmetrical Playfields (Part 3)
Session 22: Sprites, Horizontal Positioning (Part 1)
Session 22: Sprites, Horizontal Positioning (Part 2)
Session 23: Moving Sprites Vertically
Session 25: Advanced Timeslicing
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 6502 Instruction Set broken down into 6 groups.
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.
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.
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.
Gives a summary of whole instruction set.
Describes each of the 6502 memory addressing modes.
Describes the complete instruction set in detail.
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 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.
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.
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.
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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Thought Experiment: What Happens After the Jab?
The Truth About Polio and Vaccines
What Is Causing the Mysterious Self-Assembling Non-Organic Clots and Sudden Deaths?
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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.
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.
Climate Change Cash Grab = Bad
Seems like more people than ever finally care about water, land, and air pollution, but the climate change cash grab scam is designed to put more of your money into the bank accounts of greedy politicians. Those power-hungry schemers try to trick us with bad data and lies about overpopulation while pretending to be caring do-gooders. Trying to eliminate pollution is a good thing, but the carbon footprint of the average law-abiding human right now is actually making the planet greener instead of killing it.
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Interesting videos:
Charlie Robinson Talks About the Best Way to Wake Up Normies
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