Comp

Simple 4-bit/1 Hz virtual computer for learning purposes

For quick start see HOW TO RUN.

Memory

instructions - 4 bits
  |  +-- addresses - 4 bits
  v  v
----***-  <- 0  ----
--*-**-*  <- 1  ---*
---***--  <- 2  --*-
---*****  <- 3  --**
----***-  <- 4  -*--
---***-*  <- 5  -*-*
----**--  <- 6  -**-
---****-  <- 7  -***
-***---*  <- 8  *---
-*--****  <- 9  *--*
--------  <- 10 *-*-
--------  <- 11 *-**
--------  <- 12 **--
-------*  <- 13 **-*
-------*  <- 14 ***-
<OUTPUT>  <- 15 ****

• Execution starts at the first address (0).
• Execution stops when program reaches last address (15).
• Whatever gets written to the last address is sent to the printer.
• When reading from the last address (15), we get a random byte value.
• CPU has one 8 bit register.

Instruction set

• READ ---- - Copies the value at the specified address into register.
• WRITE ---* - Copies value of the register to the specified address.
• ADD --*- - Adds value at the specified address to the value of the register, and writes result to the register. If the result is bigger than the maximum possible value (255 = ********) then 255 gets written.
• SUBTRACT --** - Subtracts value at the speicfied address from the value of the register, and writes result to the register. If the result is smaller than 0 then 0 gets written.
• JUMP -*-- - Changes the value of the program counter to the specified address, meaning that in the next cycle execution will continue at that address.
• IF MAX -*-* - Jumps to the specified address if register has value 255 = ********.
• IF MIN -**- - Jumps to the specified address if register has value 0 = --------.
• SHIFT R -*** - Moves every bit of the register one spot to the right. This way rightmost bit gets lost, and a leftmost becomes '-'. This is the only instruction that doesn't use the address part, making the last four bits irrelevant.
• Any instruction that is not defined above is interpreted as READ ---- instruction.

How to run on…

Windows

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

UNIX

$ git clone https://github.com/gto76/comp-cpp.git
$ cd comp-cpp
$ ./run

Other versions

• Mark II, model with separate address space for code and data, and with more instructions. Programs can be saved and loaded and it can run without the interface (instead of a printer, it then uses stdout). Also input can be piped in.