Option ROM

The term "Option ROM" denotes an additional ROM code that augments a PC's system BIOS, and extends the BIOS by device-specific functions. For example, a SCSI adapter's option ROM enables the PC BIOS to boot from a SCSI disk that is attached to the SCSI adapter.

A network adapter's option ROM extends the system BIOS by network boot capabilities, so that the PC can boot over the network by means of the network adapter's boot code. If a network adapter does not come with a boot code, it can be post-fitted with a boot code like the TCP/IP BOOT-PROM or PXE PROM code.

This method can be used to provide diskless computers with a network-booted operating system, and so turn them into "Thin Clients".

Also, it is used for unattended installation purposes, where a "helper operating system" is booted over the network, and serves as a platform for the operating system installer.

More, this method can be used in emergency boot scenarios, where an emergency operating system is booted over the network in order to perform maintenance activities, such as an offline virus scan, a firmware update, etc.