About this thing
A working MOS 6502 microcomputer simulator running entirely in your browser
— CPU, RAM, ROM, video chip, and a real 6522 VIA peripheral, all wired
to a memory-mapped bus. The same code runs as a terminal app and as this WebAssembly build.
Long-term goal: a teaching tool for how 6502-era machines actually worked — chip selects,
memory-mapped I/O, polling vs. interrupts, and writing assembly that transfers to real silicon.
How it's built
- Go One simulator core, two front-ends.
- TUI Native build runs in a real terminal via tcell.
- WASM The browser build compiles the same code with
GOOS=js and renders the TUI to a canvas.
- Graphics-mode pixels overlay the cell grid using a sentinel-rune trick — so windows, shadows, and z-order all just work over the bitmap.
The FoxPro UI
Windows, menus, status bar, drop shadows, drag-to-move — this is
foxpro-go, a from-scratch homage to FoxPro for DOS's TUI
(and dBASE before it). The simulator embeds it as the host shell.
Try: F10 opens the menu bar. Drag any window by its
title. Right-click on the desktop to drag the whole layout.
What's in the machine
- CPU — two backends: a fast interpreter and a transistor-level netlist sim built on data from Visual6502.
- VIC — 40×13 char/color planes plus a 160×100 4-bpp graphics plane with hardware blits, fills, lines, and circles.
- VIA #1 — 65C22 chip with Timer 1 free-running on its own 1 MHz oscillator, IFR/IER, polling-style pacing.
- RAM/ROM — 8 KB each, reset vector at
$FFFC.
Memory map
| Range | Device |
|---|
| $0000-$1FFF | RAM (8 KB) |
| $2000-$9FFF | open / expansion |
| $A000-$A3FF | VIC color plane |
| $A400-$A7FF | VIC char plane |
| $A800-$ABFF | VIC controller |
| $B000-$B0FF | 6522 VIA #1 |
| $B100-$BFFF | peripheral slots (15×) |
| $C000-$DFFF | VIC graphics plane |
| $E000-$FFFF | ROM |
Try this
- Open Demo menu — bouncing balls, scrollers, a graphics quad-rotator.
- Open VIA #1 window: watch the timer count down even when you stop the CPU.
- Open Memory: hit
v to cycle Hex / Disasm / Labels.
- Type a base address in Memory to navigate — e.g.
$B000 to inspect the VIA.
Hardware-faithful intent
Each peripheral lives on its own chip-select region, decoded by high
address bits exactly the way a 74HC138 would on a real board.
The VIA's registers mirror every 16 bytes through its
256-byte CS block — same as a real 65C22 with only 4 register-select pins.
The intent: demos written here transfer to real silicon unmodified.
What's on screen
- CPU — live A/X/Y/PC, status flags, half-cycle counter, address & data buses, R/W direction, IRQ/NMI lines. Reset button on the right.
- Clock — Run, Stop, Step (one instruction), Tick (one half-cycle), and a speed selector (1 Hz → Max). The pacemaker for the whole machine.
- VIC — the video chip's framebuffer (text or graphics mode), with a column of buttons for every controller command (Clear, Shifts, Rotates, Fills). Drag-to-scroll the memory strip below.
- VIA #1 — the 6522 timer/peripheral chip. Counter ticks down on its own crystal (independent of the CPU clock), IFR/IER bit dots flicker as the timer fires. Watch it stay alive while you single-step.
- Memory — hex view, with trace tinting (yellow = changed write, green = read). Press
v to cycle Hex / Disasm / Labels.
Transistor data, node IDs, and the die color palette are derived from
Visual6502 by Greg James,
Brian Silverman, and Barry Silverman
(CC BY-NC-SA 3.0).
Carl Edwards ·
thecarledwards.com