xrxs is an experimental game server using the Plan 9 protocol 9p.

The client is a specialized uxn ROM that can load other ROMs over the network via the 9p service.


This is the working structure of the 9p filesystem:

  • /ctl: Read/write control file for inputing system commands. Reading the file shows the status of the last input command: 1 for success, 0 for failure; logout is a special case, and the status code will be -1 if it was succesful. the following are valid command syntax:

    • login PW: Authenticate with xrxs – password is hashed against realm password hash.
    • logout: Gracefully remove yourself from the users table.
    • load CART: Load a cartridge.
    • chunk TYPE N: Load data of type TYPE and chunk number N.
    • create REALM: Create a new realm (start a new game) – must have a cartridge loaded
    • protect PW: Protect the curent realm with a password if you are the master.
    • transfer USER: Transfer ownership of the realm to another user.
    • delete REALM: Delete the realm off the server if you are the master and it is empty.
    • enter REALM: Join an existing realm.
    • leave: Leave the current realm.
    • unload: Unload the cartridge.
  • /users: Read-only; Self and others in the realm are readable from here, one per line. It contains only yourself before joining a realm. Your username on your machine is used as your username in xrxs – if your name is taken, you will get an error on attaching.

  • /carts: Available game/app cartridges for this server, read only; Carts are listed per line upon reading the file. It is backed by files on the server in a directory structure like carts/CART_NAME/{CART_NAME.rom, data/, realms/}.

  • /slot: After loading the cartridge, its ROM is read from here; Read-only.

  • /data/: Any supporting data that comes with the cartridge will be found here; They are in three parts: sprite, audio, and text. While uxn has the ability to seek through a file up to 4GB in size, it can be beneficial to separate game assets into discrete files. The chunk command should be used to page different files into the service when needed. The files on the server should be like TYPEN where TYPE is one of sprite, audio, and text, and N is any sequence of characters (canonically a nonnegative integer). When first loading the cartridge, N == 0. Issuing the command chunk TYPE XXX will attempt to load data from file carts/CART_NAME/data/TYPEXXX into the correct data file. If TYPE is not one of sprite, audio, or text, or the file TYPEXXX doesn’t exist in the data directory, the chunk command does nothing.

  • /realms: Open/saved realms, read-only. Realms and their associated universe are backed by real files on the server so that they can be preserved across service instantiations, in a directory structure like: carts/CART_NAME/realms/REALM_NAME/{realm, universe}. Realms can either be solo, open, or protected; Open or protected realms can have limited member numbers. Depending on the cartridge, these settings can be user-managed or managed by the cartridge itself. Realms are listed per line upon reading the file like: REALM_NAME 1 4 1. First would obviously be the name of the realm. The first number is number of members, second is member limit, third is 1 if protected, 0 if not. 0 1 1 represents a protected solo realm that is empty (saved game with password). 0 1 0 represents an unprotected solo realm that is empty (saved game with no password).

  • /universe: Write here to update serverside state for this cart/realm; Read from here to get the complete current state. This is backed by a key-value-pair list on the server.

  • /scope: Write here to tell the server the names of the Atoms (key/value pair of a Universe) you’re interested in (one per line), and read from here to retrieve their values (one per line). In many cases this will be preferrable to fetching the entire Universe.

  • /random: Read-only, get a random number from 0 to 99.

  • /grandom: Read-only, get a random number from 0 to 99 – These are doled out on a per-realm basis, and the number stays the same until everyone in the realm has had a chance to read it. If you’ve already read it this round or aren’t in a realm, it will be empty.

  • /version: Read-only, outputs the version of the xrxs server.

realm format

Each realm directory on the server should have the following files:

  • realm: Basic data for the realm, file should contain only the maximum number of members, the master’s name, and the password hash, if any (otherwise 0), separated by spaces.
  • universe: The actual game state for the realm as key value pairs, one per line, like KEY = VALUE; limit 15 characters for keys, 63 for values.

The realm should be synchronized to disc when realm membership, limit, or password change. Fenagling some periodic autosave should be possible…


config.h in the source contains the following configuration macros:

  • MAX_USERS: the maximum number of simultaneous users able to attach to the xrxs service
  • DATA_DIR: the path to the root of the cartridge and realm storage; can be absolute or relative to the xrxs executable, but must have the trailing /



The xrxs server is built/tested in a Linux environment with plan9port and the C standard library as the only dependencies. With minimal modifications it will probably run just as well on Plan9, *BSD, WSL, and MacOS.

Running ./build.sh from the server directory should build the xrxs executable.

You can run a local server (for testing, split-screen games, or single-player games) with:

./xrxs -m /path/to/mountpoint

or expose a service on the network (uses 9pserve to support multiple users and gracefully handle disconnects) with:

./xrxs-srv.sh start

Add the -d option to the above command to enable 9p debugging output. The default port is 5460 but can be changed by setting the XRXS_PORT environment variable.

Similarly, you can stop the service with:

./xrxs-srv.sh stop

The executable itself supports the following options, one of which is required (no options prints the help text):

  • -m MOUNTPOINT: mount the 9p filesystem locally at MOUNTPOINT
  • -s SOCKET: serve the 9p filesystem over a socket named SOCKET
  • -v: print the version information
  • -h | --help: show the help text


There are two versions of the client ROM. One is the normal xrxs client, and one is a standalone client for use as a generic uxn bootloader. The latter reads a ROM list out of a file called index, and appends the .rom extension to your selection and loads a file so named. Note that while the 9p filesystem will be located under the parent directory of ./n, the standalone bootloader assumes a flat filesystem.

You can run ./build.sh with no options to build the normal xrxs client, or with any of the following options:

  • -r: build the normal client and run it for local testing (don’t mount the remote 9p filesystem)
  • -l: build the standalone bootloader
  • -lr: build the standalone bootloader and run it

When the client ROM has been built, run ./uxn-xrxs.sh to mount the 9p service and run the client. As with the server, the default port is 5460 but can be overridden with the XRXS_PORT environment variable. The default server address is but can be similarly changed by setting the XRXS_ADDR environment variable.

The scripts assume you have uxnasm and uxnemu in your PATH. 9pfuse is used to mount the service, but other implementations could possibly be used.