mcos

mcos is the operating system of Modular Computers. It is written in Lua, it lives on a disk you craft, and it is modeled on Linux: a kernel that owns every resource, programs that reach it only through system calls, a shell, files, pipes, sockets and services.

If you know Linux, most of it will look familiar; where mcos differs from Linux, the pages and the programs’ pages say so. If you don’t, it is a good way to learn how a real operating system works, because the parts are small enough to read.

The pieces

• Boot: from the power button to a shell prompt.
• The filesystem: disks, mounts, /dev, /tmp and links.
• The shell: sh, its syntax and its line editing.
• /dev: devices as files.
• Writing programs: blocking instead of yielding, with complete examples.
• Networking: UDP and TCP sockets over the network cards.
• Services: /etc/rc, /etc/rc.d and rc.conf.
• Remote shell: rsh, rshd and rcp.
• The sys library: the system calls a Lua program uses.
• The devices library: the bus’s devices as tables of their methods, and waiting for one to come.
• Programs: every command, one page each.

How it is built

• The kernel runs on its own coroutine and owns open files, pipes, terminals, mounts and processes. A program is a process that runs its own code and asks the kernel for anything else through sys, the table of system calls. A system call stops the process where it is (this is what the CPU’s trap does: see Persistence) and the kernel serves it on its own stack. A process is also stopped when input arrives, and when its time slice is spent, so a busy program never keeps the others, or the kernel, from running.
• Every program is Lua. Every process starts in /lib/ld.lua, which plays the part of Linux’s ld.so and crt1: it gives the program its standard library (io, os, print, require) over the process’s system calls, calls it, and ends the process with what it returns. /bin/lua is just Lua’s interpreter, for typing Lua at a prompt; it is not needed to run a program.
• Commands are programs in /bin. The shell contains only shell behavior (cd, export, redirections, pipelines); line editing is a library, /lib/readline.lua.
• Interfaces are Linux-shaped. Devices speak tables; mcos turns them into text, as Linux does. A redstone card’s file is a line a face, not Lua syntax.
• No lazy loading, no old compatibility. A program loads the libraries it asks for, and nothing is kept for the sake of an older way.

What it needs

A tier 1 memory stick is enough to run mcos to its shell. Every disk tier comes with mcos: its files are made when the disk is first installed. A disk crafted alone is blank. The Disk Station can format a disk, clone it, and install or update mcos on it: see the Disk Station tutorial.

Getting started

Your first computer builds one and gets it to a prompt. After that, your first program writes a Lua program, and Redstone controls the world with it.