Plug and Play BIOS Specification

Plug and Play BIOS Specification 1.0A Page 15
Step 4 Enable Input and Output Devices
Select and enable the Input and Output Device. Compatibility devices in the system that are not
configurable always have precedence. For example, a standard VGA adapter would become the
primary output device. If configurable Input and Output Devices exists, then enable these devices
at this time. If Plug and Play Input and Output Devices are being selected, then initialize the
option ROM, if it exists, using the Plug and Play option ROM initialization procedure (See
section 3).
Step 5 Perform ISA ROM scan
The ISA ROM scan should be performed from C0000h to EFFFFh on every 2K boundary. Plug
and Play Option ROMs are disabled at this time (except input and output boot devices) and will
not be included in the ROM scan.
Step 6 Configure the IPL device
If a Plug and Play device has been selected as the IPL device, then use the Plug and Play Option
ROM procedure to initialize the device. If the IPL device is known to the system BIOS, then
ensure that interrupt 19h is still controlled by the system BIOS. If not, recapture interrupt 19h
and save the vector.
Step 7 Enable Plug and Play ISA and other Configurable Devices
If a static resource allocation method is used, then enable the Plug and Play ISA cards with
conflict free resource assignments. Initialize the option ROMs and pass along the defined
parameters. All other configurable devices should be enabled, if possible, at this time.
If a dynamic resource allocation method is used, then enable the bootable Plug and Play ISA
cards with conflict free resource assignments and initialize the option ROMs.
Step 8 Initiate the Interrupt 19H IPL sequence
Start the bootstrap loader. If the operating system fails to load and a previous ISA option ROM
had control of the interrupt 19h vector, then restore the interrupt 19h vector to the ISA option
ROM and re-execute the Interrupt 19h bootstrap loader.
Step 9 Operating system takes over resource management
If the loaded operating system is Plug and Play compliant, then it will take over management of
the system resources. It will use the runtime services of the system BIOS to determine the
current allocation of these resources. It is assumed that any unconfigured Plug and Play devices
will be configured by the appropriate system software or the Plug and Play operating system.
3.0 Option ROM Support_________________________
This section outlines the Plug and Play Option ROM requirements. This Option ROM support is directed
specifically towards boot devices; however, the Static Resource Information Vector permits non-Plug
and Play devices which have option ROMs to take advantage of the Plug and Play Option ROM expansion
header to assist a Plug and Play environment whether or not it is a boot device. A boot device is defined
as any device which must be initialized prior to loading the Operating System. Strictly speaking, the only
required boot device is the Initial Program Load (IPL) device upon which the operating system is stored.
However, the definition of boot devices is extended to include a primary Input Device and a primary
Output device. In some situations these I/O devices may be required for communication with the user.
All new Plug and Play devices that support Option ROMs should support the Plug and Play Option ROM
Header. In addition, all non-Plug and Play devices may be "upgraded" to support the Plug and Play
Option ROM header as well. While these static ISA devices will still not have software configurable
resources, the Plug and Play Option ROM Header will greatly assist a Plug and Play System BIOS in
identification and selection of the primary boot devices.
It is important to note that the Option ROM support outlined here is defined specifically for computing
platforms based on the Intel X86 family of microprocessors and may not apply to systems based on other
types of microprocessors.