Plug and Play BIOS Specification

Plug and Play BIOS Specification 1.0A Page 22
INT 13h is called, the Option ROM may make the assumption that the base I/O address is the one that
goes with that entry point.
3.6 Error Recovery: Returning to the Boot flow
In the current boot model for standard PC compatible systems, once the system BIOS turns control over to
the Initial Program Load (IPL) device's boot sector, there is no way for the boot sector to return control to
the system BIOS in the event that an OS loader is not present on the disk, or the IPL fails for some other
reason.
In the Plug and Play Boot model, an attempt is made to correct this. If at any time after control has been
turned over to the IPL device's boot sector either the boot sector or some other portion of the OS loader
determines that the IPL device is incapable of supporting the boot process, control may be returned to the
system BIOS (so that the system BIOS can attempt to boot off of a different IPL device) by issuing either
an INT 19h or an INT 18h. The BIOS will intercept this INT vector and attempt to continue the bootstrap
process.