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(Not so) OISC

The (Not so) OISC, or NOISC, is a computer that operates primarily using the subleq instruction. However, unlike a typical OISC, this computer has five instructions. Therefore, it could also be called a FISC (Five Instruction Set Computer).

Specifications

  • Signed 8-bit data and 16-bit address width
  • 5 instructions (subleq, set, in, out, hlt)

Each instruction has a variable length, making this computer similar to a CISC architecture. But the only instruction that performs actual computation is subleq, while the other instructions are used to write/read values to/from specific memory addresses or perform operations that would be difficult with just subleq.

Instructions of the NOISC Computer

  1. subleq a, b, c
    perform b = b - a, and if b <= 0, branch to c. a and b, c are addressees. c can either be a label or a 16-bit address, but it is recommended to use labels. 7 Bytes long.

  2. set addr, value
    Sets the value at memory address addr to value. 4 bytes long.

  3. in addr
    Reads input from the user and stores it at memory address addr. 3 bytes long.

  4. out addr
    Prints the value stored at memory address addr. 3 bytes long.

  5. hlt
    Halts the program. 1 byte long.

How to use

This code has only been tested in a Linux environment with GCC. However, since the codes are simple, it is likely to work on other compilers or operating systems as well.

On Linux

If you are using Debian/Ubuntu,

sudo apt update && sudo apt upgrade
sudo apt install python3 gcc git
git clone https://github.com/Aurorasphere/not-so-oisc/
cd not-so-oisc
make

If everything is done correctly, the computer executable will be located in the bin directory.

Assembler

  1. Write some assembly code and save.
  2. type python src/assembler.py [YOUR CODE'S NAME].s [OUT FILE'S NAME].out in your terminal.

Computer

  1. type bin/oiscomputer [OUT FILE'S NAME].out in your terminal.

Example Program

Here is an example program that compares two numbers and prints the result:

// Compare A and B, 
// If A > B, Print 0.
// If A <= B, Print 1.

in      0x00F0                  // Read A, Address of A is 0x00F0
in      0x00F1                  // Read B, Address of B is 0x00F1
set     0x00FF 0x00             // Set Bool = 0, Address of Bool value is 0x00FF
subleq  0x00F1 0x00F0 RETURN    // If A - B <=0, go to RETURN 
out     0x00FF                  // If not, Print 0 
hlt                             // Stop the Program

RETURN:                         
    set 0x00FF 0x01             // Set Bool = 1
    out 0x00FF                  // Print 1
    hlt                         // Stop the program