ARCS is the name for the firmware used by SGI machines to boot an IRIX kernel and store important NVRAM variables. It's also commonly called in common parlance the PROM by users, though this generally refers to the physical chip storing ARCS.
ARCS is a fully graphical firmware that has basic understanding of TCP/IP (though it can't ping/trace and has no understanding of routes. It only knows TFTP and RSH commands enough to boot a kernel or application like fx), it can boot CD, Tape and Network installations of IRIX.
The Command Monitor is the area most users will have the most interest in learning as it contains useful commands to set common startup variables for IRIX and the system in general, preparing the system for remote network installs, and displaying installed hardware (hinv).
This section will cover useful commands and their functions:
hinv - Hardware inventory. Displays installed and detected hardware, usually disks, network controllers, CPU, memory etc. Similar to the IRIX command, but far more limited.
setenv - changes environment variables to change the behavior of the system.
printenv - prints all active env variables
fill - sets portions of the NVRAM memory to new values - notable for use in fixing the SGI Indy's MAC address failures.
netaddr - the default IP of the system.
console - sets graphical (g) or serial console (d). Serial will always work, but d will explicitly ignore the graphics adapter.
AutoLoad - will the system automatically load and boot its disk? y / n parameters.
hostname - default hostname, used for network booting.
eaddr - MAC address of the host.
SystemPartition - The partition that contains the startup code for the SGI.
OSLoadPartition - The partition that contains the operating system.
OSLoader - sets the shell or loader used. Usually sash (standalone shell).
OSLoadFilename - the filename of the kernel used. /unix for IRIX.