Simple 4-bit virtual computer
Branch: master
Clone or download
Latest commit 7eb67e9 Apr 25, 2017
Type Name Latest commit message Commit time
Failed to load latest commit information.
doc Documentation Oct 28, 2015
examples Renamed cat-vs-mouse to cat-and-mouse Oct 4, 2015
src Sub changed to subtract Oct 4, 2015
tools Cleanup Oct 3, 2015
.gitignore Documentation Oct 28, 2015
Dockerfile Add support for docker Oct 4, 2015
LICENSE Initial commit May 25, 2015 Update Apr 25, 2017
makefile Finalizing Oct 3, 2015
run Can also quit during execution with q and Q keys Oct 4, 2015

Comp Mark II

Updated version of Comp – Simple 4-bit/1 Hz virtual computer for learning purposes


For quick start see HOW TO RUN


  • Processor has one 8 bit register.
  • Ram is separated in two address spaces; one for instructions, called CODE, and another for data, called DATA.
  • All instructions are 8 bits long.
  • Execution starts at the first address (0) of the CODE ram.
  • Execution stops when program reaches the last address (15) of the CODE ram.
  • Most of instructions consist of instruction code and address:
instruction code - 4 bits
  |  +-- address - 4 bits
  v  v
----***-  ->  READ 14  ->  Copy value stored at address 14 of DATA ram to register.
  • All addresses specified by instructions refer to DATA ram, except for addresses of Execution Control instructions (JUMP, IF MAX, IF MIN, ...). They refer to CODE part of ram.
  • Some instructions (JRI~<>&V) do not specify address. They operate on register (SHIFT L/R, NOT, ...) or between register and predefined DATA address (AND, OR, ...).
  • Whatever gets written to the last address is sent to the printer, or to stdout if program is running in a non-interactive mode.
  • When reading from the last address (15), we get a random byte value, or a single word from stdin, if program is running in non-interactive mode.
  • In this word every * is interpreted as true and all other characters as false. If word starts with a digit, it is then read as a number and converted appropriately.
  • Program will start in a non-interactive mode when any input is piped in, or any option is given. For instance echo | ./comp <file>
  • Programs can be saved with s key and loaded by passing their name as a parameter.
  • If more than one filename is passed, the computers will be chained together (stdin > comp-1 > comp-2 > ... > stdout).
  • If folder is passed, all files in that folder that have suffix .cm2 will be loaded in an alphabetical order.

Instruction set

Most of the instructions, together with the highlighted word that they use/modify, are pretty self-explainatory. Exception are instructions that start with -***, and are a part of JRI~<>&VX instruction cluster. They are:

  • J – JUMP_REG,
  • R – READ_REG,
  • I – INIT,
  • ~ – NOT,
  • < – SHIFT_L,
  • > – SHIFT_R,
  • & – AND,
  • V – OR,
  • X – XOR.

(Use Shift–Left/Right Arrow to easily shift between them.)
Detailed descriptions of all instructions can be found HERE.


  • Space – Flip bit
  • Tab – Switch address space
  • Enter – Start/pause execution
  • Esc – Cancel execution
  • Delete, Backspace – Delete word or move following words up if empty
  • Insert, ] – Insert empty word and move following words down
  • s – Save ram to textfile named punchcard-<num>.cm2. To load it, start program with ./comp <file>
  • q – Save and quit

Detailed descriptions of all keys can be found HERE.


  • --non-interactive, -n – Runs program without an interface. Output is sent to stdout instead of a printer. This option is not necessary if any other option is present, or if input is piped in.
  • --char-output, -c – Converts numbers to characters using ASCII standard when printing to stdout.
  • --filter, -f – Convert characters to numbers when reading from stdin, and numbers to characters when printing to stdout.
  • --game, -g – Same as filter, but reads characters directly from keyboard.
  • parse – Converts program to c++ code (other options may be specified).
  • compile – Compiles program to executable file, by converting it to c++ code and then running g++ compiler (other options from above may be specified). Only difference between compiled program and one run on the Comp Mark II is in execution speed.

How to run on…


  • Install Tiny Core Linux on VirtualBox using this instructions.
  • Run the UNIX commands.


Make and g++ need to be installed first. On Ubuntu and Debian you can get them by running sudo apt-get install build-essential, on OS X they get installed automatically after running make.

$ git clone
$ cd comp-m2
$ ./run


$ docker run -it --rm mvitaly/comp-m2 <options>


Fibonacci Sequence

$ ./comp --non-interactive examples/fibonacci.cm2
-------*   1
-------*   1
------*-   2
------**   3
-----*-*   5
----*---   8


$ echo "3 4" | ./comp examples/multiply.cm2
----**--  12

Hello World

$ ./comp --char-output examples/hello-world.cm2
Hello world

To Upper Case

$ echo "Hello world" | ./comp --filter examples/to-upper-case.cm2

Cat and Mouse

Two player game

$ ./comp compile --game examples/cat-and-mouse/
Compiled as cat-and-mouse
$ ./cat-and-mouse
#                #
#  C             #
#                #
#                #
#                #
#                #
#                #
#                #
#                #
#                #
#                #
#                #
#                #
#                #
#             m  #
#                #

Further Development

Check out the blueprint of Mark III model.