Inside the Atari 800XL
Some info on its chips
Although I now have an Atari 800XL, it is not the one I had as a kid. This one is a later model because it has the Rev. C BASIC built-in. It is not modified in any way. If you zoom in on the picture below a bit, you’ll notice the Option key is somewhat worn out. This is pretty common because you have to hold down Option to disable the built-in BASIC, which is usually needed when running disk-based machine language games.
It really should not be necessary to do that since an ML game can disable BASIC itself, but it seems most didn’t know that was a thing at the time.
Otherwise, the computer is in good shape and the coloring is fine. It’s perhaps not quite as white as it would have been originally, but it’s not bad. The keyboard works great.
I took everything apart a while ago to clean it, so here is a quick look at its insides:
Going from left to right, you can see:
The RAM chips, 64K in total. There are 8 chips, so
each chip contains 8K64K, but only 1 bit. To get a byte, one bit is read from each chip.
The GTIA chip or George’s Television Interface Adapter controls how the Atari talks to the TV. The 800XL can display 256 colors, 16 base colors each with 16 different luminescence levels.
ANTIC, one of the most famous Atari chips, was responsible for sending display information to the GTIA.
The 6502 CPU running at 1.8Mhz. You can tell by all the lines running to it from the parallel bus port at the top.
The Pot(entiometer) Keyboard IC or POKEY chip handles controllers, IO and sound. That is why you hear those beeping noises when loading files from cassette or disk. This chip was also used by many arcade games of the time.
The OS ROMs are next, which are 16K.
And lastly is the BASIC ROM (8K), which is Rev. C as mentioned above.
Edit: The chip in the bottom right is the PIA (Peripheral Interface Adapter), which handles the joystick ports.
This is a pretty straightforward design, overall.
Apparently my 800XL is one of the many (most?) that do not have the chroma line hooked up for monitor output, at least based on my testing. I need to do more research to see what it would take to fix that. I’m not big on soldering, but if this is easy enough perhaps I’ll give it a try. If anyone has information on this they’d like to point me towards, I’d appreciate it.
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