


For those who would like to know, the 6507 was a 6502 without IRQ or NMI,
and with only 13 address lines (giving 8K memory space).


Didn't know about it being called the 6526 - thats the CIA found in the
Commodore 64 (I don't believe this chip existed when the 2600 was around).
The TIA was mapped into the bottom 128 bytes of page 0, and shadowed in the
bottom 128 bytes of page 1. To get an image on the screen, you had to wait
until the raster line your picture started at, load bitmaps for this line
into the 'sprite' registers, tell them what x positions to take, how many
duplicates you wanted, etc, then do it all again for the next line. This
took all of the processor's time during the visible portion of the
display.


The RAM was mapped into the top 128 bytes of page 0, and shadowed in page
1. The 6502 has a zero-page addressing mode on most instructions that is
both shorter and faster than the normal ones, so it is important on a
machine like this to have the scratchpad RAM in page 0. Unfortunately, it
also wants its stack to be in page 1 (the stack pointer is 8 bits, and the
high byte of the address is hardwired to 1).

The IOT section was mapped into page 2 somewhere. 2 8 bit I/O registers
(can't remember if each bit could be independantly set to I or O like the
6522 and 6526), and 1(?) 8(?) bit timer with a programmable prescaler (I
think this was some power of 2). There were no interrupts in the system, so
many games would set up the timer at some known time, go away and do
something else for a while, then sit and wait for the timer to run down.

There was no logic in the box to do address decoding - the RIOT had at
least 2 chip select pins (probably one active high, one active low), and I
imagine the TIA was much the same. Various address lines were fed directly
to chip selects. The 6507 likes to have ROM right at the top of memory, so
the 2600 had to use ROMs with active high chip selects - not exactly
normal, so some cartriges had a 7404 in them to convert this to the more
usual active low chip select.