re atari processors ifarqhar laurel ocs mq edu au ian farquhar writes in article apr das harvard edu squish endor uucp shishin yamada writes the atari used a cpu just like their entire bit line xe xe computers as well as the game machine wrong it has a for those who would like to know the was a without irq or nmi and with only address lines giving k memory space it had one custome chip not an asic in the sense that word is now understood the tia sometimes known as the although most other manufacturers list that as the cia complex interface adapter tia stands for television interface adapter and it handles sound paddles and the minimal video hardware the possessed didn t know about it being called the thats the cia found in the commodore i don t believe this chip existed when the was around the tia was mapped into the bottom bytes of page and shadowed in the bottom bytes of page 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 there was also a standard ram i o timer riot plus a voltage regulator and if memory serves a timer that s all the ram was mapped into the top bytes of page and shadowed in page the 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 unfortunately it also wants its stack to be in page the stack pointer is bits and the high byte of the address is hardwired to the iot section was mapped into page somewhere bit i o registers can t remember if each bit could be independantly set to i or o like the and and bit timer with a programmable prescaler i think this was some power of 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 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 likes to have rom right at the top of memory so the had to use roms with active high chip selects not exactly normal so some cartriges had a in them to convert this to the more usual active low chip select john west
