Core War is a 1984 programming game created by D. G. Jones and A. K. Dewdney in which two or more battle programs (called "warriors") compete for control of a virtual computer. These battle programs are written in an abstract assembly language called Redcode.

This is what is looks like in action:

Let’s focus on the high-level game dynamics:

• The game board, the memory of our virtual computer. It’s represented in a 64 X 64 grid of bytes.
• The players, small programs represented in different colors. The white parts have blank memory.
• Cursors, the moving parts with inverted color. They read from the game board.

The cursors have a couple of features. They can jump on the game board, store and write values, and clone themselves. The cursors belong to no-one, they just read from the game board.

When the game starts, all players have one cursor at the beginning of their program. The game advances as the cursors read the commands stored in their programs. If they end up on a faulty command or a blank memory, it moves to the next byte.

Below, we see how the pink player starts by cloning their cursors. It then starts attacking the blue player.

Let’s get into a little bit more depth.

Every byte you see, the pairs of numbers or letters, are represented in hexadecimal. Each hexadecimal has the value of a digit, from 0 - 255.

There are 16 operations in the game. The operations decide if the cursor should jump to a different address, clone itself and so on. The first 16 hexadecimal are coding bytes, they all store a different operation. The remaining digits, 17 - 255, are regular numbers.

The coding byte decides which operation to execute and how many bytes to read. In the image above, the pink player goes to the hexadecimal 0c. It's 12 in decimal, telling the cursor to clone itself. The two bytes after decide where the cloned cursor starts.

There are three main components to determine who wins:

• Game rounds, every game round is measured in cycles. It determines how much each cursor can read from the game board.
• Lives, if a cursor reads a player life from the game board, they are given a life for that round. These are visualized in the colorful progress bars.
• Cycle to die, for every game round, the number of bytes a cursor can read from the game board is reduced.

You win if your program is the last one to receive a life.

There are more nuances to the game that I don’t cover. The best way to learn about them is to run it with the below code.

Executing Core War

Open your terminal and make it full-screen, copy paste the below, and hit enter.

git clone https://github.com/maximilien-alt/CPE_corewar_2019.git ; cd CPE_corewar_2019 ; make

The usage for the Core War game:

Usage:
 	Example: ./corewar filename1.cor filename2.cor filename3.cor filename4.cor
 	Default Player Numbers: 1, 2, 3, 4...
 	Dump: ./corewar -dump 300 [Players](prints the memory after 300 cycles)
 	Game on! 👾 🤖 🔫

If you want to create and then compile players, use the asm:

./asm [filename.s]

Technical Implementation Overview

The Core War consists of players that are written in assembly, a compiler to turn them into binary, and the virtual computer to run the programs on.

The program is written using read, open, write, malloc, free and exit from stdlib.h.

This is an example of a simple player, a normal player has around 150 lines of assembly code:

    .name "zork"
    .comment "I'M ALIIIIVE"
    
    l2:          sti r1,%:live,%1
                 and r1,%0,r1
    
    live:        live %1
                 zjmp %:live

This complies to binary which is later represented in hexadecimal. The above program looks like this when it’s compiled:

    0000000 00 ea 83 f3 7a 6f 72 6b 00 00 00 00 00 00 00 00
    0000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    *
    0000080 00 00 00 00 00 00 00 00 00 00 00 17 49 27 4d 20
    0000090 41 4c 49 49 49 49 56 45 00 00 00 00 00 00 00 00
    00000a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    *
    0000890 0b 68 01 00 0f 00 01 06 64 01 00 00 00 00 01 01
    00008a0 00 00 00 01 09 ff fb

The first part includes an identification code and the name, followed by the comment, and ending with the program which will be stored in the memory of the virtual machine.