Cross-posting from r/ti994a since the implementation might be of interest beyond the TI crowd. Just open-sourced Swift 99/a, a native macOS emulator for the Texas Instruments TI-99/4A (1981 home computer built around the 16-bit TMS9900).
Github: https://github.com/swryder/Swift-TI-99 (BSD-2)
What's interesting about it:
It's written from scratch in pure Swift — SwiftUI for the chrome, AppKit where it has to be, Metal for the visualizer, and zero third-party dependencies. Most emulators in this space are C/C++/Rust ports of Classic99 / Win994a / js99er. This one isn't a port — it's a fresh take, which means the bugs are mine and the architecture is whatever felt clean in Swift (chip-per-file, protocol-oriented peripherals on a common bus, no cross-cutting framework).
What's emulated:
- TMS9900 CPU — full instruction set, workspace-pointer architecture, context switching, status-register lookup tables (65K-entry word table, 256-entry byte table) for the hot path
- TMS9918A VDP — Graphics I/II, Text 40-col, Multicolor, Bitmap. 32 hardware sprites with collision detection. NTSC timing.
- SN76489 PSG — three tone channels + noise, fractional clock accumulator for pitch accuracy across sample rates
- TMS5220 speech synthesizer
- TMS9901 I/O — full CRU bit-addressable interface
- Disk DSR — V9T9 image format, full PAB protocol (open, close, read, **write**, delete, status, directory listing), FDR parsing
- Cartridges — raw `.bin` / `.rom` plus the LZW-compressed `.ticart`container format. Banking: standard 8KB, GROM-only, 378 / inverted 379, MBX (fixed-low / banked-high / 1KB RAM)
Fun things:
- **Live memory map visualizer.**A 256×256 GPU-rendered heatmap of the entire 64KB CPU address space, redrawn at 60Hz. Blue for reads, red for writes, purple for both, with white-hot decay tracking recent activity. Wall-clock-based exponential decay so the fade looks the same at 15fps and 60fps. Honestly the most fun part of the project — you can watch a BASIC program execute and see exactly which pages off memory it's hammering. Key for such a weird architecture machine
- "Monitor Mode." A chromeless window with a TI-99/4 monitor bezel asset, transparent background, draggable from anywhere. Sits on the desktop like a CRT. Pure macOS-native (NSWindow + SwiftUI overlay).
- Clipboard paste as keyboard events. Paste from macOS into the TI's keyboard buffer. Surprisingly useful for typing in BASIC programs.
Performance approach:
- Two execution modes: real 3 MHz (DispatchSourceTimer-paced) and uncapped (tight async loop). On Apple Silicon the uncapped mode hits >100× real speed.
- Per-byte peripheral routing tables instead of switch-on-address — avoids branch mispredictions in the memory access hot path
- Read shadow mirror so most reads bypass the peripheral indirection entirely
What's not done:
PEB peripherals beyond disk + speech, save states, cassette (CS1/CS2), a couple of obscure cart banking schemes. Issues and PRs welcome.
Background:
The project was the working artifact behind a LinkedIn article I wrote about how complex projects scale in the AI era — full disclosure if you're curious about the AI-assisted angle: https://www.linkedin.com/pulse/complexity-conserved-how-do-big-things-ai-era-scott-ryder-zyybc/
Cartridges and disk images are BYO.
BSD2 Open Source - Have Fun
