Writing custom OSes

Starting with OpenComputers 1.3 computers need an “Operating System” to function - OpenOS is no longer built-in. As of 1.4(.2?) you even need an EEPROM in your computers, which acts as the BIOS of the machine. While this makes it slightly more annoying to set up computers, it opens up a lot more possibilities for people that want to write their own OSes; which is what this article is all about!

Booting

The only thing built-in now is the “machine” booting the BIOS. This “machine” wrapper is a non-editable Lua script responsible for setting up the sandbox and super-low-level interaction with the “host” side of things (that being Java/Scala). When a computer is powered on, that script is loaded and run. After estabilishing the sandboxed environment it looks for an EEPROM and tries to run the data on it as a Lua script. If that fails the computer crashes. Otherwise that BIOS is run. For the provided one (Lua BIOS, crafted using EEPROM + book) it will perform the actions previously done by the machine itself:

  1. It looks for a file system with address set via computer.setBootAddress (read via computer.getBootAddress).
  2. On that file system it tries to load and execute a Lua script from a file named init.lua.
  3. If it succeeds, that's it. The user script is now in control. If it fails, it iterates over all present file systems, performing step 2 again until it succeeds.
  4. If no working init.lua script is found, the computer crashes with a no bootable medium found error.

But since you can program the EEPROM however you like you could also boot from a network device, for example.

What's Available?

There are a few libraries documented on the wiki that are in fact part of OpenOS, so when you're implementing your own operation system, those won't be at your disposal. Most notably that includes the io, package and filesystem libraries. For a definite reference for what's available in an init script, check the kernel. The following list isn't guaranteed to be complete and/or up-to-date!

snippet.lua
_G
_VERSION
 
assert
error
getmetatable
ipairs
load
next
pairs
pcall
rawequal
rawget
rawlen
rawset
select
setmetatable
tonumber
tostring
type
xpcall
 
bit32.arshift
bit32.band
bit32.bnot
bit32.bor
bit32.btest
bit32.bxor
bit32.extract
bit32.lrotate
bit32.lshift
bit32.replace
bit32.rrotate
bit32.rshift
 
coroutine.create
coroutine.resume
coroutine.running
coroutine.status
coroutine.wrap
coroutine.yield
 
debug.getinfo
debug.traceback
 
math.abs
math.acos
math.asin
math.atan
math.atan2
math.ceil
math.cos
math.cosh
math.deg
math.exp
math.floor
math.fmod
math.frexp
math.huge
math.ldexp
math.log
math.max
math.min
math.modf
math.pi
math.pow
math.rad
math.random
math.randomseed
math.sin
math.sinh
math.sqrt
math.tan
math.tanh
 
os.clock
os.date
os.difftime
os.time
 
string.byte
string.char
string.dump
string.find
string.format
string.gmatch
string.gsub
string.len
string.lower
string.match
string.rep
string.reverse
string.sub
string.upper
 
table.concat
table.insert
table.pack
table.remove
table.sort
table.unpack
 
checkArg
 
component.doc
component.fields
component.invoke
component.list
component.methods
component.proxy
component.slot
component.type
 
computer.address
computer.addUser
computer.beep
computer.energy
computer.freeMemory
computer.getArchitectures
computer.getArchitecture
computer.getBootAddress
computer.maxEnergy
computer.pullSignal
computer.pushSignal
computer.removeUser
computer.setArchitecture
computer.setBootAddress
computer.shutdown
computer.tmpAddress
computer.totalMemory
computer.uptime
computer.users
 
unicode.char
unicode.charWidth
unicode.isWide
unicode.len
unicode.lower
unicode.reverse
unicode.sub
unicode.upper
unicode.wlen
unicode.wtrunc

Accessing components in BIOS

Unlike in OpenOS you don't have the primary system for components in BIOS. That means you cannot just write component.redstone.setOutput, you have to manually create your proxy for the component first. For several reasons the proxy system is provided via the machine itself, so you can still use that. To get a proxy, use the component.proxy method. It expects the address of the component to wrap. You can either write that down manually, or get the list of components using component.list and go from there.

For example, to get the first best redstone component, you can do the following:

snippet.lua
local r = component.proxy(component.list("redstone")())

This works because component.list returns a Lua iterator: a function that can be called repeatedly to get the next element in whatever it iterates over, returning nil once there are no more elements.

A few more things

When an init script returns, the computer powers down, so you'll want to have a “main loop” in there, regardless of how you structure your OS otherwise. Signals must be processed using computer.pullSignal, you're free to use any model to provide this to programs running in your OS you please - this is what OpenOS' event.pull does, for example.

You'll also want to take care of setting up attached components during your OS' boot routine, such as binding GPUs to screens.

Aside from that, go crazy, be creative!