pill_6502: emulated 8-bit 6502 CPU and 6850 ACIA for STM32F103 blue pill

6502 8-bit CPU and 6850 ACIA emulator for the STM32F103 blue pill microcontroller boards and similar. Turn your modern micro into a retrocomputing processor.

To build, run make, to install, flash src/pill_6502.bin to the blue pill. Prebuilt binaries are available from CircleCI. Connect the board to your computer using USB, this will reset the CPU and a virtual serial port should appear (e.g., /dev/cu.usbmodem6502), connect using GNU Screen, and you should see the output from the emulated ACIA communication interface chip. Type to send to the ACIA.

Special commands built-in, not passed to the ACIA and 6502:

Keystroke	Action
^V	Show pill_6502 version
^P	Pause/resume 6502 processor execution
^R	Reset the 6502 processor (also reset on USB reset)
^E	Local echo toggle, off by default
^T	Show clock ticks, instruction count, and frequency since last ^T
^G	Show help

Intended to be somewhat compatible with Grant Searle's Simple6502, and includes the Microsoft OSI BASIC ROM by default (C) Microsoft 1977. Other ROMs can be loaded by replacing ROM.HEX and rebuilding. Memory map:

Start	End	Usage
0x0000	0x3fff	16KB internal RAM
0x4000	0x9fff	(unused)
0xa000	0xbfff	6850 ACIA serial comm interface, emulated connecting to USB CDC-ACM virtual serial port
0xc000	0xffff	16KB internal ROM

Thanks to Mike Chambers for the fake6502 processor emulation core.

For implementation notes see blog post pill_6502: 8-bit 6502 CPU and 6850 ACIA emulation on the STM32 blue pill to run Microsoft BASIC from 1977.