Every bus cycle
The 6502 models each cycle, dummy reads and writes included, plus the undocumented opcodes. Every access waits for a cycle ANTIC leaves free, so cycle stealing, WSYNC and interrupt timing fall out of the bus.
Named after the chip
The 800 XL's CPU is a custom 6502C that Atari called "Sally". This emulator is written in C and Objective-C with nothing beyond the system frameworks: Cocoa, Metal, Core Audio and GameController.
Mid-line changes land
GTIA draws each line in segments up to the beam before any register write, so colors and positions changed in the middle of a line show up exactly where they should.
Even frames
On a 60 or 120 Hz display, Atari frames lock to refreshes, so each one is on screen for the same time. Pixels are drawn at whole multiples, so nothing ripples while scrolling.
Event-driven sound
POKEY's channels are integrated between divider events for each audio sample. The sample rate is nudged to keep the buffer level steady instead of drifting into dropouts.
Its own little OS
A small built-in OS, written in 6502 assembly and assembled at startup, handles reset, interrupts and the vertical blank. No Atari ROMs needed to run cartridge games.
At home on the Mac
Drop a cartridge on the window or double-click it in Finder. Game controllers work as joysticks, and the last cartridge comes back at the next launch.
Controls
The 800 XL's metal keys sit on Command and the digits, in the order they appear on the machine. Its plastic keys are on plain Mac keys.
| Mac | Atari |
|---|---|
| Arrow keys | Joystick |
| Option | Fire |
| Cmd-1 Cmd-2 Cmd-3 | START, SELECT, OPTION (held) |
| Cmd-4 | HELP (held) |
| Cmd-5 Cmd-6 | RESET, power cycle |
| The key left of 1 | Inverse |
| fn-Delete | BREAK |
| Other keys | The Atari keyboard, by character |
| Game controller | Joystick (A, B or R2 fire) |
Cmd-P pauses, Cmd-0 fits the window to the picture, Ctrl-Cmd-F enters full screen.
Something to play
QUADROP is a falling blocks game for the 800 XL on a 16K cartridge: seven colors in a character mode, pieces that glide a scanline at a time, and Korobeiniki on POKEY. Download the .car file and drop it on Sally's window.
Build from source
You need macOS 14 and the Xcode command line tools.
git clone https://github.com/ecmel/sally.git
cd sally
make # build/Sally.app
make run ROM=game.rom # or drop a cartridge onto the window
make test # CPU tests
make install # copy to /ApplicationsCartridges are plain 8K or 16K images (.rom, .bin) or .car files of those types, or of the bank-switched MegaCart and SIC!. The machine has 128K of RAM, banked as on the 130XE. Sally does not ship with any games.
Not done yet
Disk drives and SIO (no .atr or .xex), bank-switched cartridges other than MegaCart and SIC!, paddles, a way to load the BASIC ROM, and the finer GTIA quirks. Only NTSC is emulated.