The 2600 was a 4-bit system.
No, it's 8-bit.
As I recall, The 2600 used a 6507, which is an eight bit processor (a member of the 650x family) with an 8-bit data bus and a 13-bit address bus. It used 8-bit memory and 8-bit opcodes (the same as in the 6502).
I don't know why people erroneously say it was 4-bit. The only other difference of note is that its stack area was 128 bytes instead of the 256 normally seen in 650x's, but that takes 7-bits to address, not 4.
I'd really have to strain my brain to remember the TIA specs, but I'm fairly confident it was 8-bit as well.
-J
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