By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Page Table of Contents
Even our language treats 'color' differently—here in Oz we write 'colour' and in the USA they write 'color'. Likewise, '2600 units in different countries don't quite speak the same language when it comes to color.
We have already seen why there are 3 variants of '2600 units—these variations (PAL, NTSC, SECAM) exist because of the differences in TV standards in various countries. Specifically, the color information is encoded in different ways into the analogue TV signal for each system, and the '2600 hardware is responsible for inserting that color information in the data sent to the TV.
Not only do these different '2600 systems write the color information in different ways, they also write totally different colors! What is one color on a NTSC system is probably NOT the same color on PAL, and almost certainly not the same color on SECAM!
Here are some wonderful color charts that show the colors used by each of the systems. . .
Below are adapted versions of those color charts. . .
NTSC (128 unique colors)
PAL (104 unique colors)
SECAM (8 unique colors)
Colors are represented on the '2600 by numbers. How else could it be? The color to number correspondence is essentially an arbitrary association—so, for example on a NTSC machine the value $1A is yellowish, on PAL the same color is gray, and on SECAM it is aqua (!). If the same color values were used on a game converted between a NTSC and PAL system, then everything would look very weird indeed! To read the color charts on the page linked to above, form a 2-digit hex number from the hue and the lum values (ie: hue 2, lum 5 -> $25 value -> brown(ish) on NTSC, and as it happens, a very similar brown(ish) on PAL.
[Instead of getting your color values from static charts, you can do it the easy way and use the interactive palettes on the TIA Color Charts and Tools page. It includes an NTSC/PAL color conversion tool and Atari 2600 color compatibility tools that can help you quickly find colors that go great together (possibly saving you a lot of time and energy).]
We've already played with colors in our first kernel! In the picture section (the 192 scanlines) we had the following code. . .
; 192 scanlines of picture... ldx #0 REPEAT 192; scanlines inx stx COLUBK sta WSYNC REPEND
We should know by now what that 'sta WSYNC' does—and now it's time to understand the rest of it. Remember the picture that the kernel shows? A very pretty rainbow effect, with color stripes across the screen. It's the TIA producing those colors, but it's our kernel telling the TIA what color to show on each line. And it's done with the 'stx COLUBK' line.
Remember how the TIA maps to memory in locations 0 - $7F, and that WSYNC is a label representing the memory location of the TIA register (which happens, of course, to be called WSYNC). In similar fashion, COLUBK is a label which corresponds to the TIA register of the same name. This particular register allows us to set the color of the background that the TIA sends to the TV!
A quick peek at the symbol table shows. . .
COLUBK 0009 (R )
In fact, the very best place to look is in the Stella Programmer's guide—for here you will be able to see the exact location and usage of this TIA register. This is a pretty simple one, though—all we do is write a number representing the color we want (selected from the color charts linked to, above) and the TIA will display this color as the background.
Remember that it also depends on what system we're running on! If we're doing a PAL kernel, then we will see a different color than if we're doing a NTSC or SECAM kernel. The bizarre consequence of this is that if we change the number of scanlines our kernel generates, the COLORS of everything also change. That's because (if we are running on an emulator or plug a ROM into a console) we are essentially switching between NTSC/PAL/SECAM systems, and as noted these systems send different color information to the TV! It's weird, but the bottom line is that when you choose colors, you choose them for the particular TV standard you are writing your ROM to run on. If you change to a different TV system, then you will also need to rework all the colors of all the objects in your game.
Let's go back to our kernel and have a bit of a look at what it's doing to achieve that rainbow effect. There's remarkably little code in there for such a pretty effect.
As we've learned, the 6502 has just three 'registers'. These are named A, X, and Y—and allow us to shift bytes to and from memory—and perform some simple modifications to these bytes. In particular, the X and Y registers are known as 'index registers', and these have very little capability (they can be loaded, saved, incremented and decremented). The accumulator (A) is our workhorse register, and it is this register used to do just about all the grunt-work like addition, subtraction, and bit manipulation.
Our simple kernel, though, uses the X register to step a color value from 0 (at the start), writing the color value to the TIA background color register (COLUBK), incrementing X by one each scanline. First (outside the repeat) we have 'ldx #0'. This instruction moves the numeric value 0 into the X register. ld is an abbreviation for 'load', and we have lda, ldx, ldy. st is the similar abbreviation for store, and we have stx sty sta. Inside our repeat structure, we have 'stx COLUBK'. As noted, this will copy the current contents of the x register into the memory location 9 (which is, of course, the TIA register COLUBK). The TIA will then *immediately* use the value we wrote as the background color sent to the TV. Next we have an instruction 'inx'. This increments the current value of the X register by one. Likewise, we have an 'iny' instruction, which increments the y register. But, alas, we don't have an 'ina' instruction to increment the accumulator (!). We are also able to decrement (by 1) the x and y registers with 'dex' and 'dey'.
The operation of our kernel should be pretty obvious, now. The X register is initialized with 0, and every scanline it is written to the background color register, and incremented. So the background color shows, scanline by scanline, the color range that the '2600 is capable of. In actual fact, you could throw another 'inx' in there and see what happens. Or even change the 'inx' to 'dex'—what do you think will happen? As an aside, it was actually possible to blow up one early home computer by playing around with registers like this (I kid you not!)—but you can't possibly damage your '2600 (or emulator!) doing this. Have fun, experiment.
Since we're only doing 192 lines, the X register will increment from 0 to 192 by the time we get to the end of our block of code. But what if we'd put two 'inx' lines in? We'd have incremented the X register by 192 x 2 = 384 times. What would its value be? 384? No—because the X register is only an 8-bit register, and you would need 9 bits to hold 384 (binary %110000000). When any register overflows—or is incremented or decremented past its maximum capability, it simply 'wraps around'. For example, if our register had %11111111 in it (255, the maximum 8-bit number) and it was incremented, then it would simply become %00000000 (which is the low 8-bits of %100000000). Likewise, decrementing from 0 would leave %11111111 in the register. This may seem a bit confusing right now, but when we get used to binary arithmetic, it will seem quite natural. Hang in there, I'll avoid throwing the need to know this sort of stuff at you for a while.
Now you've had a little introduction to the COLUBK register, I'd just like to touch briefly on the difference apparent between the WSYNC register and the COLUBK register. The former (WSYNC) was a strobe—you could simply 'touch' it (by writing any value) and it would instantly halt the 6502. Didn't matter what value you wrote, the effect was the same. The latter register (COLUBK) was used to send an actual VALUE to the TIA (in this case, the value for the color for the background)—and the value written was very much important. In fact, this value is stored internally by the TIA and it keeps using the value it has internally as the background color until it changes.
If you think about the consequences of this, then, the TIA has at least one internal memory location which is in an unknown state (at least by us) when the machine first powers on. We'd probably see black—which happens to be value 0 on all machines), but you never know. I believe it is wise to initialize the TIA registers to known-states when your kernel first starts—so there are no surprises on weird machines or emulators. We have done nothing, so far, to initialize the TIA—or the 6502, for that matter—and I think we'll probably have a brief look at system startup code in a session real-soon-now.
Until then, have a play with the picture-drawing section, and see what happens when you write different values to the COLUBK register. You might even like to change it several times in succession and see what happens. Here's something to try (with a bit of head-scratching, you should be able to figure all this out by now). . .
; 192 scanlines of picture... ldx #0 ldy #0 REPEAT 192; scanlines nop nop nop nop nop nop nop nop nop nop inx stx COLUBK nop nop nop dey sty COLUBK sta WSYNC REPEND
One caution: as the above code is wrapped inside a repeat structure which creates 192 copies of the enclosed code, we're actually running short of ROM space! With the above code installed, there's only 10 bytes free in our entire ROM! Clearly, using REPEAT in this sort of situation is wasteful, and the code should be written as a loop. We covered looping for scanline draw early on—but because both X and Y registers are in use at the moment, it's a bit more tricky.
So for now, we'll just have to accept that we can't add any more code—but at least you can see what effect adding/removing cycles can have on the existing code.
Here's a screenshot:
Here's the .bin file to use with an emulator:
Other Assembly Language Tutorials
Session 11: Colorful Colors
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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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.