VCS Architecture on Programming the Atari VCS in Assembly

The Processor (CPU)

Mon, 01 Jan 0001 00:00:00 +0000

The Processor (CPU) — a 6507#

The CPU runs your program. On most computers that makes it the star; on the VCS it’s deliberately the least special of the four chips — a small, ordinary processor whose whole job is to operate the other three at exactly the right moments.

A 6502 with its wings clipped#

The 6507 is a cost-reduced version of the well-known 6502: the same instruction set, the same registers, the same programming model — but in a cheaper package with two consequential omissions:

The Video & Sound Chip (TIA)

Mon, 01 Jan 0001 00:00:00 +0000

The TIA: Programming the Television#

Every console has a graphics chip. What makes the TIA — the Television Interface Adapter — and therefore the VCS different is what it doesn’t have: a frame buffer. That single absence is the source of both the machine’s notorious difficulty and its peculiar charm, and it’s why the right mental model is not “drawing graphics” but programming the television itself.

What the TIA handles#

For one chip, it does a lot — and all of it in real time, in step with the beam:

Memory, I/O & Timer (RIOT)

Mon, 01 Jan 0001 00:00:00 +0000

The Memory, I/O & Timer chip (RIOT) — a 6532#

The RIOT (the 6532) — sometimes called the PIA — is the unglamorous but essential housekeeping chip. While the TIA gets the glory, the RIOT quietly provides the three things every program needs but the TIA doesn’t supply. Its name is an inventory of those jobs: RAM, I/O, and Timer.

RAM — your 128 bytes#

The RIOT holds the VCS’s entire read/write memory: 128 bytes, at addresses $80–$FF. Every variable your game keeps — score, positions, velocities, game state — lives here, and nowhere else. Internalize how little that is: a single screen’s worth of modern data structures would not come close to fitting. Tight memory discipline isn’t an advanced topic on the VCS; it’s the baseline.

The Program ROM

Mon, 01 Jan 0001 00:00:00 +0000

The Program ROM — your cartridge#

Three of the four chips are inside the console. The ROM is the one you supply — it’s the cartridge — and it holds your program and all of its constant data: code, sprite bitmaps, lookup tables, music.

Where it lives#

The CPU maps the cartridge into the top of the address space, the $F000–$FFFF window, with the all-important reset vectors at $FFFC. When the console powers on, the 6507 reads that vector and jumps to it — so the very top of your ROM is where execution begins. The mechanics of that startup are in Anatomy of a Minimal ROM.