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README.md

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-# pill\_6502
+# pill\_6502: emulated 8-bit 6502 CPU and 6850 ACIA for STM32F103 blue pill
 
+[6502](https://en.wikipedia.org/wiki/MOS_Technology_6502) 8-bit CPU and
+[6850](http://www.cpcwiki.eu/imgs/3/3f/MC6850.pdf) ACIA emulator for the
+[STM32F103](https://www.reddit.com/r/stm32f103/) [blue pill](http://wiki.stm32duino.com/index.php?title=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.
+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.
+
+![screenshot](https://user-images.githubusercontent.com/26856618/34910886-d1869f5e-f872-11e7-8dd9-e034348619dd.png)
+
+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 and instruction count |
+| ^G | Show help |
+
+Intended to be somewhat compatible with [Grant Searle's Simple6502](http://searle.hostei.com/grant/6502/Simple6502.html),
+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 |