By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Page Table of Contents
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 . . .
. . . and the other (TIMETOCHANGE) which is used in the code as a number for comparison
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.
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. . .
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. . .
See you then, then!
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
Session 13: Playfield Basics
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
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).
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.
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.
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.
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.
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.
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).
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.
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.
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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Watch these two YouTube videos for more information:
Take a look at my page called The H Word and Beyond. You might also want to look at my page called Zinc and Quercetin. My sister and I started taking those two supplements near the end of 2020 in the hopes that they would scare away the flu and other viruses.
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