Burning to EPROM & Real Hardware on Programming the Atari VCS in Assembly

Choosing the Chip

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

Choosing the Chip#

“EPROM” is the traditional name, but you have a few families to pick from, and the choice mostly comes down to how painful it is to reprogram when you find a bug — and you will find bugs.

The three families#

Family	Examples	Erase method	Reusable?	Notes
UV EPROM (windowed)	2716, 2732, 2764, 27256, 27512	UV light (~15–20 min in a UV eraser box)	Yes, many times	The classic. The quartz window over the die lets a UV eraser box wipe it clean for reuse. Just keep the window covered with a sticker in normal use, or ambient UV slowly corrupts it.
OTP EPROM (no window)	same part numbers, plastic package	—	No — one shot	Electrically identical die, but the plastic package has no window, so there’s no way to get UV to it. Programs once, then it’s permanent. Cheap for production; avoid while iterating.
EEPROM	28C16, 28C64, 28C256	Electrically, in the programmer	Yes, instantly	No UV lamp. Slightly pricier, sometimes slower write, very convenient for iteration.
Flash / “electrically-erasable EPROM”	W27C512, W27C020 (Winbond), SST39SF	Electrically, in the programmer	Yes, instantly	Cheap, reusable, no UV — the modern homebrew favorite. The Winbond W27C512 is a drop-in-ish 27512 replacement that erases in the programmer.

For iterating on a game, pick something electrically erasable (EEPROM or a Winbond/SST flash part). Reserve UV EPROMs for when you specifically want the vintage experience or already own a stack of them — the burn → test → “oops” → 20-minute-erase cycle gets old fast.

Preparing the ROM Image

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

Preparing the ROM Image#

The bytes you send to the programmer are not always the bytes DASM emitted. Two things have to be right: the reset vectors must land at the very top of the address the console fetches from, and the image must fill the chip in a way the cartridge wiring expects.

The reset vectors must be at the top#

When the console powers on, the 6507 reads its program-counter start address from the reset vector at $FFFC/$FFFD. On the cartridge that is the last two bytes of ROM space. Every ROM in this book ends with:

Wiring the Cartridge

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

Wiring the Cartridge#

Once the chip is programmed it has to be wired into something the console’s cartridge slot can read. This is where the project leaves software and becomes electronics — and where the mistakes stop being syntax errors and start being bus contention and floating address lines.

Strong recommendation: copy the connections from a proven, published cartridge schematic or reproduction PCB (AtariAge has well-documented designs) rather than improvising them. The pin-level details below are to help you understand and verify such a design, not to reverse-engineer one from scratch. This is the one part of the pipeline where “close enough” produces hardware that intermittently works, which is worse than hardware that never works.

Gotchas: Works in Stella, Dies on Hardware

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

Gotchas: Works in Stella, Dies on Hardware#

This is the page worth bookmarking. The bugs here share a signature: the game is perfect in the emulator and wrong on the console. That signature is diagnostic — it means the fault is in an assumption the emulator papers over, not in your game logic.

RAM is not zero on real hardware#

Stella conveniently clears the 128 bytes of RAM at startup. A real 6507 powers on with RAM full of random garbage. If any of your code reads a variable before writing it — a counter, a flag, a sprite position — it works in Stella (where it read 0) and does something random on hardware.