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Compaq Computer Corporation
Phoenix Technologies Ltd.
Intel Corporation
BIOS Boot Specification
Version 1.01
January 11, 1996
This specification has been made available to the public. You are hereby granted the right to use,
implement, reproduce, and distribute this specification with the foregoing rights at no charge. This
specification is, and shall remain, the property of Compaq Computer Corporation ("Compaq"),
Phoenix Technologies Ltd ("Phoenix"), and Intel Corporation ("Intel").
NEITHER COMPAQ, PHOENIX NOR INTEL MAKE ANY REPRESENTATION OR
WARRANTY REGARDING THIS SPECIFICATION OR ANY PRODUCT OR ITEM
DEVELOPED BASED ON THIS SPECIFICATION. USE OF THIS SPECIFICATION FOR
ANY PURPOSE IS AT THE RISK OF THE PERSON OR ENTITY USING IT. COMPAQ,
PHOENIX AND INTEL DISCLAIM ALL EXPRESS AND IMPLIED WARRANTIES,
INCLUDING BUT NOT LIMITED TO THE IMPLIED WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND FREEDOM FROM
INFRINGEMENT. WITHOUT LIMITING THE GENERALITY OF THE FOREGOING,
NEITHER COMPAQ, PHOENIX NOR INTEL MAKE ANY WARRANTY OF ANY KIND
THAT ANY ITEM DEVELOPED BASED ON THIS SPECIFICATION, OR ANY PORTION
OF IT, WILL NOT INFRINGE ANY COPYRIGHT, PATENT, TRADE SECRET OR OTHER
INTELLECTUAL PROPERTY RIGHT OF ANY PERSON OR ENTITY IN ANY COUNTRY.
Table of Contents
1.0 INTRODUCTION 5
1.1 REVISION HISTORY 5
1.2 RELATED DOCUMENTS 5
1.3 PURPOSE 5
1.4 TERMS 6
2.0 OVERVIEW 9
2.1 DESCRIPTION 9
3.0 IPL DEVICES 10
3.1 REQUIREMENTS FOR IPL DEVICES 10
3.1.1 IPL T ABLE 10
3.1.2 PRODUCT NAME STRING 11
3.2 BAID S 11
3.3 DEVICES WITH PNP EXPANSION HEADERS 11
3.4 LEGACY IPL DEVICES 12
3.5 IDENTIFYING IPL DEVICES 13
3.5.1 BAID S 14
3.5.2 PNP EXPANSION HEADER 14
3.5.3 PCI D EVICES 14
3.5.4 IDENTICAL IPL DEVICES 15
4.0 IPL PRIORITY 16
4.1 MAINTAINING THE IPL PRIORITY 16
4.2 IPL P RIORITY PSEUDOCODE 18
5.0 BCV PRIORITY 19
5.1 INTRODUCTION 19
5.2 INT 13 H DEVICE CONTROLLERS 19
5.2.1 ATA D RIVE SUPPORT IN THE BIOS 19
5.2.2 PNP CARDS WITH BCVS 20
5.2.3 LEGACY CARDS WITH OPTION ROMS 20
5.2.4 H ARD DRIVE BAID 20
5.2.5 CONTROLLER INSTALLATION GUIDELINES 20
5.2.6 N OTES ON INT 13H DEVICES 21
5.3 INSTALLATION ORDERING 21
5.4 POST P SEUDOCODE 23
2
6.0 POST SEQUENCE 24
6.1 POWER-ON INITIALIZATION 24
6.1.1 INITIALIZING BAIDS 24
6.1.2 PNP BOOT DEVICES 24
6.2 PNP OPTION ROM I NITIALIZATION 24
6.3 CHECK IPL PRIORITY AND BCV PRIORITY 25
6.4 INT 13 H DEVICE CONTROLLER INSTALLATION 25
6.4.1 BOOT CONNECTION VECTORS 26
6.4.2 D ISCONNECT VECTOR 26
6.4.3 LEGACY ROM S CAN 27
6.4.4 O N-BOARD ATA S UPPORT 27
6.5 INT 19 H PROCESSING 27
6.5.1 BOOTING FROM BAIDS 29
6.5.2 BOOTING FROM BEVS 29
6.6 INT 19 H PSEUDOCODE 30
6.7 INT 18 H PSEUDOCODE 30
6.8 NOTES ON THE POST P ROCESS 31
APPENDIX A: DATA STRUCTURES 32
A.1 IPL T ABLE AND BCV TABLE ENTRY DATA STRUCTURE 32
A.2 PNP OPTION ROM H EADER 33
A.3 PNP EXPANSION HEADER 33
A.4 PCI D ATA STRUCTURE 33
APPENDIX B: RUN-TIME FUNCTIONS (OPTIONAL) 34
FUNCTION 60H - GET VERSION AND INSTALLATION CHECK 35
FUNCTION 61H - GET DEVICE COUNT 36
FUNCTION 62H - GET PRIORITY AND TABLE 37
FUNCTION 63H - SET PRIORITY 38
FUNCTION 64H - GET IPL DEVICE FROM LAST BOOT 39
FUNCTION 65H - GET BOOT FIRST 40
FUNCTION 66H - SET BOOT FIRST 41
APPENDIX C: BOOT MENU (OPTIONAL) 42
C.1 BOOT MENU POP-UP 42
C.2 BOOT MENU INT 19H PSEUDOCODE 42
C.3 BOOT FIRST RUN-TIME FUNCTIONS 42
APPENDIX D: RECOMMENDED BOOT SECTOR CHANGES (OPTIONAL) 43
D.1 USE DL FOR DRIVE NUMBER 43
3
D.2 INT 18 H ON BOOT FAILURE 43
APPENDIX E: PCI WITH MULTIPLE PNP HEADERS (OPTIONAL) 44
E.1 DESCRIPTION 44
E.2 REQUIREMENTS 44
E.3 OPTION ROM I NITIALIZATION 44
E.3.1 BEFORE OPTION ROM P LACEMENT 44
E.3.2 PLACING THE PCI OPTION ROM 44
E.3.3 CALLING THE PCI OPTION ROM 45
E.3.4 N O DEVICES PRESENT 45
E.3.5 D EVICES ARE PRESENT 45
E.4 ENUMERATING PNP EXPANSION HEADERS 46
E.5 CALLING THE BCVS 46
4
1.0 Introduction
1.1 Revision History
Version Changes
0.80 Original version.
0.81 Grammatical corrections.
1.00 Finalized for public release.
1.01 Added the BiosSelector parameter to the run-time functions.
Moved run-time functions 65h and 66h from Appendix C to Appendix B.
Cleaned up documentation on the run-time functions.
You may obtain the latest copy of the BIOS Boot Specification from the Phoenix
world wide web site at http://www.ptltd.com, or by contacting a representative from
one of the authoring companies.
Technical Editor:
Scott Townsend
Phoenix Technologies Ltd.
2575 McCabe Way
Irvine, CA 92714
Phone: (714) 440-8000
Fax: (714) 440-8300
Email: [email protected]
1.2 Related Documents
Title Version Author
Plug and Play BIOS Specification 1.0A Compaq/Phoenix/Intel
Hardware Design Guide for Microsoft Windows 95 1.0 Microsoft Corporation
Enhanced Disk Drive Specification 1.1 Phoenix
"El Torito" Bootable CD-ROM Format 1.0 Phoenix/IBM
Specification
PCI Local Bus Specification 2.1 PCI Special Interest Group
1.3 Purpose
The purpose of this specification is to describe a methodology by which the BIOS will
identify all IPL (Initial Program Load) devices in the system, prioritize them in the
order the user selects, and then sequentially go through each device and attempt to
boot. The BIOS must become more intelligent about booting because the Plug and
Play BIOS Specification places additional requirements on the BIOS during the boot
process, and there are now more devices that are bootable such as CD-ROM, network
remote boot, PCMCIA, etc. It is important that this specification define a boot scheme
that is generic and flexible enough to allow booting from virtually any existing IPL
device, and for the definition of future IPL devices as well.
5
1.4 Terms
To avoid confusion and to be consistent with the Plug and Play BIOS Specification,
definitions of some of the terminology used in this document are listed below.
ATA
An Advanced Technology Attachment drive, also known as an IDE drive, is a hard
drive with the interface built-in.
ASCIIZ
An ASCIIZ string is simply a string of ASCII characters terminated by a NULL (0
byte).
BAID
A BIOS Aware IPL Device is any device that can boot an O/S, but requires the BIOS
to have specific code to support it. Some examples are: the first floppy drive, the first
hard drive, ATAPI CD-ROM, PCMCIA, embedded network adapter, etc.
BCV
A Boot Connection Vector is a pointer that points to code inside the option ROM that
will perform device initialization, detect if a peripheral (such as a SCSI hard drive) is
attached, and optionally hook INT 13h. The BCV resides in a PnP option ROM
Expansion Header. An example of an option ROM with a BCV is a PnP ISA SCSI
controller.
BDA
The BIOS Data Area is a data storage area in RAM. The BDA is used by the BIOS to
manage the various peripherals and resources in the system. The BDA starts at
segment address 0040h.
BEV
A Bootstrap Entry Vector is a pointer that points to code inside an option ROM that
will directly load an O/S. The BEV resides in a PnP option ROM Expansion Header.
An example of an option ROM with a BEV is a PnP ISA ethernet controller.
BIOS
The Basic Input/Output System is the software embedded on a chip located on the
computer's main board. The BIOS executes POST to test and initialize the system
components and then loads (boots) the O/S. The BIOS also handles the low-level
input/output to the various peripheral devices connected to the computer.
6
Boot Device
A Boot Device is any device that must be initialized prior to loading the O/S. This
includes the primary input device (keyboard), the primary output device (display), and
the initial program load device (floppy drive, hard drive, etc.). An IPL device is one
form of a boot device.
CDR
Conflict Detection and Resolution is a method by which a PnP BIOS first detects the
resource requirements for PnP cards, and then allocates them in a conflict-free way.
CSN
A Card Select Number is a number that uniquely identifies a PnP ISA card and is used
to communicate exclusively to that card. The CSN is assigned by the PnP BIOS to
each PnP ISA card in the system.
DDIM
The Device Driver Initialization Model is a method of initializing an option ROM
whereby the option ROM is first copied to shadow RAM, then its initialization vector
is called with the shadow RAM write-enabled. When the option ROM completes
initialization it may dispose of code not needed at run-time by re-sizing the ROM
memory footprint. Finally, after the option ROM returns and the BIOS regains
control, the ROM is write-protected.
DV
A Disconnect Vector is a pointer that points to code inside the option ROM that will
perform clean-up after the Boot Connection Vector has already been called. The DV
resides in a PnP option ROM Expansion Header. An example of an option ROM with
a DV is a PnP ISA SCSI controller.
IPL Device
An Initial Program Load Device is any device in the system that can boot and load an
O/S. In standard AT machines, this is the floppy drive or hard drive.
Legacy Card
A Legacy Card is a standard ISA card that contains no PnP compatible configurability,
and no PnP Expansion Header.
NV
Non-Volatile memory is memory that is retained even when the power has been shut
off. The most common type of NV memory on a PC is the CMOS RAM that is used
to store system configuration information.
O/S
An Operating System is loaded from an IPL device when that device is selected for
booting.
7
PFA
A PCI Function Address is a unique number assigned to a PCI function on a PCI
device. The PFA consists of a function number, a device number, and a bus number.
PnP
Plug and Play is a term used to identify anything defined by the Plug and Play BIOS
specification or the Plug and Play ISA specification. The term will typically be used
to reference some device or behavior that is specific to PnP technology.
PnP Cards
PnP Cards consist of any cards that contain an option ROM with a PnP Expansion
Header.
POST
The Power-On Self Test is the part of the BIOS that takes control immediately after the
computer is turned on. POST initializes the computer hardware so that an O/S can be
loaded.
Setup
The system Setup program is the part of the BIOS that is executed after a user
specified is pressed during the BIOS initialization. Setup allows the user
to set up and configure the system as well as select the IPL Priority of the system.
8
2.0 Overview
2.1 Description
The BIOS Boot Specification defines a feature within the BIOS that creates and
maintains a list of all the IPL devices found in the system and stores this list in NV
memory. IPL devices come in three flavors: BAID, PnP Card, and Legacy. Only
BAIDs and PnP Cards are enumerated. Legacy devices are not supported for several
reasons. First, they tend to take control of the boot process altogether making them
rather unfriendly. Second, they provide no means for identifying themselves as an IPL
device. Finally, the BIOS cannot selectively boot from one of several Legacy IPL
devices in a system.
The BIOS Boot Specification provides one basic feature, the IPL Priority. The IPL
Priority is a user-specified priority of IPL devices that is arranged in Setup. This boot
order is similar to the common feature of boot A: then C: or vice versa, but supports
additional IPL devices. Also, the number of IPL devices in the system may vary from
one power-on to another. Each time the user turns on the system all IPL devices in the
system are enumerated.
Additionally, the BIOS Boot Specification defines the BCV Priority. The BCV
Priority is a user-specified priority list of INT 13h Device Controllers that is arranged
in Setup. This list specifies the order that the controllers will be called to install their
INT 13h drive support during POST.
If an IPL device fails to load an O/S, the BIOS regains control and attempts to boot
from the next available IPL device. This procedure will continue until all possible IPL
devices have been exhausted. Only then will the BIOS display a message that an O/S
cannot be found, wait for a key stroke, and then invoke INT 19h again. This method
ensures that the BIOS has intelligently made every attempt to boot.
The BIOS Boot Specification encompasses the boot process of both PnP and non-PnP
systems. The support for PnP Cards wherever mentioned is only pertinent to systems
which include PnP support in their BIOS. A standard AT compatible system (also
called a Legacy system) is much simpler than one with a PnP BIOS because it only
supports BAIDs. A Legacy system does not need to provide any dynamic IPL device
enumeration or configuration, nor does it support PnP Cards in their native mode.
This is because the number of IPL devices in such a system will never change.
9
3.0 IPL Devices
3.1 Requirements for IPL Devices
An IPL device can be virtually any device that has the ability to load and execute an
O/S. This includes floppy drives, hard drives, CD-ROM drives, PCMCIA
controllers/cards, PnP Cards, Legacy cards, and could, in the future, include virtually
any devices such as serial ports, parallel ports, etc. An IPL device falls into one of
three categories: BAID, Legacy IPL Device, or PnP Card. There are two varieties of
PnP Cards, BCV devices and BEV devices.
One of the key points of concern about an IPL device is that it be able to return back to
the BIOS with an error if the O/S load fails. The BIOS Boot Specification defines INT
18h as the recovery vector for failed boot attempts. For example, if a PnP ISA
network card was configured to boot from the network, but the network cable wasn't
attached, the card's option ROM should signal to the BIOS that it cannot boot by
invoking INT 18h.
If a Legacy card's option ROM code hooks INT 19h during its initialization call it
controls the boot process. If a Legacy card hooks INT 18h, the boot failure recovery
will not work. For this reason, it is recommended that all future Legacy boot devices
include the PnP Expansion Header and its associated support in their option ROMs so
that their priority in the booting process can be controlled.
3.1.1 IPL Table
Each BAID and BEV device must have corresponding entries in the IPL Table. An
IPL Table for a typical PC would have two BAID entries, one for diskette drive A: and
one for hard drive C:, and one for each BEV device found. A sample IPL Table is
shown in section 4.1.
The information stored in the IPL Table consists of identification information, pointers
to description strings, and pointers to handlers that will initiate an O/S load. The IPL
Table and BCV Table data structure is defined in detail in Appendix A.
10
3.1.2 Product Name String
The Plug and Play BIOS Specification defines fields in the PnP Expansion Header for
optional description strings. Vendors of PnP Cards must have meaningful strings in
the Product Name String and Manufacturer String fields of the PnP Expansion Header.
The Product Name String is particularly important since it will be displayed to the user
as an identifier for that device. Although the only current limitation to these strings is
that they be terminated by a NULL (0 byte), it is declared here that only the first 32
characters (bytes) of the Product Name String are significant. Characters beyond 32
will not be displayed. This is a reasonable length to provide the user with a unique
name for that device. Product Name Strings shorter than 32 will be displayed in their
entirety.
3.2 BAIDs
BAIDs include the first floppy drive, the first hard drive, and any other bootable
device that has specific code in the BIOS to support it. A BAID is able to launch a
bootstrap only because the BIOS has built-in support as part of the INT 19h service
routine. Some examples of this might be an ATAPI compatible CD-ROM drive or a
PCMCIA card/controller. BAIDs do not have option ROMs, nor can they function
without specific code embedded in the BIOS.
Each BAID will have an entry in the IPL Table. This entry will contain the following
information: the device type, a device sub-type, a pointer to a description string, and a
pointer to code that will attempt to boot from the device. The INT 19h handler will
read this table and be able to boot from any one of these devices without having to
know what type of device it is. The table consists of a contiguous array of data
structures of which each corresponds to a particular IPL device.
Note that only the first floppy drive and the first hard drive are considered IPL devices.
The main reason why a system cannot boot from other drives is that the boot sector
code specifically addresses only these drives (00h for floppy and 80h for hard drive)
when INT 13h is called to load the O/S. See Appendix D for recommended changes
to the O/S boot sector that could solve this problem.
11
3.3 Devices with PnP Expansion Headers
All IPL devices with option ROMs must contain a valid option ROM header that
resides between system memory addresses C0000h and EFFFFh on a 2k boundary and
begins with 55AAh. A Device's booting can only be controlled if it has a PnP
Expansion Header. The Expansion Header, whose address resides within the standard
option ROM header at offset +1Ah, contains important information used to configure
the device. It also contains pointers to code in the device's option ROM (BCV or
BEV) that the BIOS will call to boot from the device. See Appendix A for the
structure of the PnP Expansion Header. There are two ways an IPL device with a PnP
Expansion Header can be booted. It must contain a BCV or a BEV.
A BEV device, typically a network card, is booted by the BIOS making a far call
directly to its BEV. If the device fails to boot, it executes an INT 18h which returns
control back to the BIOS. BEV devices behave much like BAIDs in that they are
selectively bootable.
A BCV device, typically a SCSI controller, is not directly bootable. Rather, it merely
adds its drives to the system by hooking into the BIOS' INT 13h services and
appending drive numbers to any existing drives. Since only drive 80h is bootable, a
BCV would only be able to boot one of its drives if it installed before any other drives
in the system. For this reason, it is necessary to control the order that all drives are
installed into the system, including on-board ATA drives and those controlled by
Legacy devices such as older SCSI controllers. The only way to control a BCV
device's drives in the boot order is by allowing the user to specify the order of
initialization among ATA, BCV, and Legacy option ROMs. This will be discussed in
detail in section 5.0.
3.4 Legacy IPL Devices
This is a standard ISA card with a valid option ROM that resides in system memory
address space between C0000h and EFFFFh on a 2k boundary. An example of this
type of device is a Legacy SCSI hard drive controller with a bootable ROM installed.
Legacy IPL devices do not contain PnP Expansion Headers in their option ROMs.
Option ROMs are detected during the BIOS' scan and their initialization vectors are
called. During initialization, devices may hook INT 19h so that they will gain control
when the BIOS issues INT 19h to boot the system. Or, they may hook INT 18h so that
if all other IPL devices in the system fail to boot they will take over. Or, they may
simply hook INT 13h and only respond to calls that reference their drive number. In
any case, the first two methods provide no consistent way for the BIOS to regain
control if the device fails to boot.
12
Legacy IPL devices will be allowed to take control of the system (via hooking
interrupts) in both Legacy and PnP systems. The Plug and Play BIOS specification
recommends that Legacy devices that hook a bootstrap interrupt such as INT 19h, 18h,
or 13h have the interrupt re-captured by the BIOS. This is not done because grabbing
an interrupt vector back after a device has hooked it can produce unpredictable results.
Further, by allowing the card to take control, the behavior of these Legacy cards will
be the same on both PnP and Legacy machines.
Legacy IPL devices are not enumerated or handled in any special way. Their behavior
will be largely unknown by the BIOS. If a Legacy IPL device traps INT 19h it takes
control of the boot process. This is acceptable because it is consistent with what
Legacy systems do now. If a Legacy IPL device traps INT 13h, it becomes the local
hard drive from the BIOS' perspective since it would then respond to INT 13h
function calls executed in the INT 19h handler.
13
3.5 Identifying IPL Devices
The large variety of possible IPL devices dictates that a generic methodology be
implemented so that all existing and future devices can be made bootable with a
minimum of difficulty. All IPL devices in the system need to be enumerated during
POST, but before INT 19h. All IPL devices fall into one of three categories: BAID,
PnP Card, or Legacy.
IPL Device Type
First floppy drive BAID
First ATA Hard drive BAID
PCI ATA card w/ drive BAID
ATAPI CD-ROM drive BAID
PCMCIA controller w/ bootable card BAID
Ethernet controller w/ code embedded in BIOS BAID
PnP Token Ring card PnP (BEV)
PnP ethernet card PnP (BEV)
Non-PnP card w/ PnP Expansion Header PnP (BEV or
BCV)
PnP SCSI card w/ drive PnP (BCV)
3.5.1 BAIDs
All BAIDs are automatically identified because the BIOS has specific code to support
them. In addition, each BAID has a corresponding entry in the IPL Table.
3.5.2 PnP Expansion Header
A PnP Card is defined by having a PnP Expansion Header in its option ROM. Any
IPL device with an option ROM can contain the PnP Expansion Header, including PCI
and ISA devices. If a PnP Expansion Header exists, the device will be treated as a
PnP Card. If it is a BEV device it will be included in the IPL Priority. If it is a BCV
device the BCV will be called. The PnP Expansion Header method of identifying boot
devices with option ROMs is expected to become the future standard.
3.5.3 PCI Devices
From the standpoint of booting, most PCI IPL devices with option ROMs behave just
like Legacy IPL devices. Currently, most PCI SCSI controllers will typically hook
INT 13h and most PCI network cards typically hook either INT 19h or INT 18h in
order to be an IPL device. It is recommended that all future PCI devices with option
ROMs employ the PnP Expansion Header method for booting so that they may enjoy
the benefits of appearing on a boot menu and allowing users to select its boot order.
14
PCI devices without option ROMs are either not an IPL device, or they behave like a
BAID. For example, a PCI ATA controller will not have an option ROM, but will
become the system ATA controller and utilize the BIOS' INT 13h services to interface
to attached ATA drives.
The address space for option ROMs conforming to the Device Driver Initialization
Model (DDIM), such as PCI, shall be write-enabled both when their initialization
vector is called, and when their BCV is called.
3.5.4 Identical IPL Devices
In an implementation of the BIOS Boot Specification, the system BIOS may choose to
distinguish between identical IPL devices so that the user will be able to choose the
correct device, given a menu of selections. For instance, the system BIOS may
recognize that two identical PnP SCSI controllers are present in the system and
distinguish them by numbering them, by displaying their CSN, or some other method.
Currently, no specific method is recommended nor defined here. Recognizing and
dealing with identical devices is optional and not required for BIOS Boot Specification
compliance.
15
4.0 IPL Priority
Once all BAIDs and BEV devices in the system have been identified and enumerated,
the BIOS needs to decide in which order they will be selected for booting. This is
handled by the IPL Priority.
The IPL Priority consists of an array of ordinal values, one for each BAID and BEV
device found in the system at POST. An IPL Priority ordinal value is the index of a
BAID or BEV device entry in the IPL Table. The priority for IPL devices is
configurable by the user by editing the IPL Priority in Setup. The order the user
selects is then stored in NV memory so that it can be retrieved by the BIOS at INT 19h
time. The IPL Priority specifies not only the number of BAIDs and BEV devices in
the system, but their order of booting as well.
When the INT 19h handler gains control, the first IPL device in the IPL Priority is
used to attempt to boot. If that fails, the next device is tried, and so on until all devices
in the IPL Priority have been tried. At that point an error message will be displayed
since all attempts to boot have been exhausted.
4.1 Maintaining the IPL Priority
Since the ordinal values in the IPL Priority are simply the index of the IPL device in
the IPL Table, no information is stored as to the specific IPL device that the ordinal
value represents. Here we assume that the number of BAIDs in a system does not
change because the BIOS contains the code for them. The BIOS recognizes only a
change in the number of BEV devices, and not a change in the type of BEV devices.
If the user changes the BEV devices they have in their system without changing the
total number of BEV devices, the BIOS will not detect this and will keep the previous
IPL Priority. For example, if there is a PnP ISA ethernet card in a system and the user
removes it and installs a different PnP ISA ethernet card, the IPL Priority would not
change because the BIOS does not track individual BEV devices. It merely tracks the
number of them and their order in the IPL Priority. The IPL Table would contain an
entry for the new PnP ethernet card in the same location as where the old one was
before. Again, as long as the total number of BEV devices doesn't change, the BIOS
uses the last order that was stored in NV memory.
16
The way the IPL Priority is constructed is by taking the range of ordinal values for
BAIDs and BEV devices, and storing them in NV memory in the IPL Priority order.
The maximum number of IPL devices supported is implementation specific, and is
defined at BIOS build time. The number of BAIDs also does not change. The number
of BEV devices that were detected at the last boot is stored in NV memory so that a
change in this number can be recognized by the BIOS. If the number of BEVs found
during POST differs from the number found at last boot as indicated by the value
stored in NV memory, the BIOS should either reset the IPL Priority automatically to
reflect the changes, or prompt the user to enter Setup.
When the system boots for the first time, or if the NV memory is corrupted, a default
IPL Priority is created. The default IPL Priority consists of the BAIDs followed by the
BEV devices found in the system. The BEV devices are placed at the end of the IPL
Priority in the order that they are found. Optionally, the user can be prompted to enter
Setup.
The IPL Priority remains in effect until either the user changes it or the number of
BEV devices in the system changes. However, the IPL Priority could change in a
system with more than one BEV device if the order the BEV devices were found by
the PnP BIOS changed. This seems odd at first to have the IPL Priority change and
the user not be notified. But ultimately, if the user is changing boot devices they will
probably want to manually edit the IPL Priority anyway. This caveat saves a lot of
potential consumption of NV memory that would be used to store unique serial
numbers for each BEV device. Additionally, BIOS code space is saved by not
tracking individual BEV devices as opposed to simply enumerating the existing ones
without reference to what type of device they are.
IPL Table
0 Floppy A:
1 Hard Drive C:
2 CD-ROM
3 BEV #1
4 BEV #2
5
6
7
IPL Priority
0 1 2 3 4 5 6 7
3 4 1 2 0
When INT 19h is executed, the BIOS will process the IPL Priority shown above in the
following order: BEV #1, BEV #2, Hard Drive C:, CD-ROM, Floppy A:.
17
4.2 IPL Priority Pseudocode
The following pseudocode describes an example of managing the IPL Priority during
POST.
Created at BIOS build time:
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