For comment: draft BIOS use document for the kernel

Alan Cox (
Fri, 22 Jun 2001 17:20:33 +0100 (BST)

Linux 2.4 BIOS usage reference

Boot Sequence

Linux is normally loaded either directly as a bootable floppy image or from
hard disk via a boot loader called lilo. The kernel image is transferred
into low memory and a parameter block above it.

When booting from floppy disk the BIOS disk parameter tables are replaced
by a new table set up to allow a maximum sector count of 36 (the track size
for a 2.88Mb ED floppy)

int 0x13, AH=0x02 is issued to to probe and find the disk geometry.
int 0x13, AH=0x00 is used to reset the floppy controller.
int 0x13, AH=0x02 is then issued repeatedly to load tracks of data. The
boot loader ensures no issued requests cross the track boundaries

int 0x10 service 3 is used during the boot loading sequence to obtain the
cursor position. int 0x10 service 13 is used to display loading messages
as the loading procedure continues. int 0x10 AH=0xE is used to display a
progress bar of '=' characters during the bootstrap

Control is then transferred to the loaded image whether by the floppy boot
loader or other services

Kernel Setup

The initial kernel setup executes in 16bit mode. While in 16bit mode the
kernel calls and caches data from several 16bit calls whose data is not
available in 32bit mode

It then uses int 0x10 AH=0x0E in order to print initial progress banners so
that immediate feedback on the boot status is available. The 0x07 character
is issued as well as printable characters and is expected to generate a

Memory detection is done as follows, attempting to handle the various
methods that have been available over time

Memory Sizing

Firstly a call is made to BIOS INT 15 AX=0xE820 in order to read the
E820 map. A maximum of 32 blocks are supported by current kernels. The
'SMAP' signature is required and tested. In addition the SMAP signature
is restored each call, although not required by the specification in order
to handle some know BIOS bugs.

If the E820 call fails then the INT 15 AX=0xE801 service is called and the
results are sanity checked. In particular the code zeroes the CX/DX return
values in order to detect BIOS implementations that do not set them
usable memory data. It also handles older BIOSes that return AX/BX but not
AX/BX data.

When service E801 is used the kernel assumes that usable memory extends from
4K to the bottom of the EBDA, and from 1Mbyte to the top of the E801 area.

If neither service is available then INT 0x15 AH=0x88 is invoked in order to
get the memory size, up to 64Mb by the original IBM PC BIOS service.


Having sized memory the kernel moves on to set up peripherals. The BIOS
INT 0x16, AH=0x03 service is invoked in order to set the keyboard repeat
rate and the video BIOS is the called to set up video modes.

The kernel tries to identify the video in terms of its generic features.
Initially it invokes INT 0x10 AH=0x12 to test for the presence of EGA/VGA
as oppose to CGA/MGA/HGA hardware.

INT 0x10 AH=0x03 is used to obtain the cursor position, and INT 0x10,
AH=0x0F is used to obtain the video page and the mode. If EGA or VGA
are present the normal BIOS locations of 0x485 and 0x484 are used to obtain
the font size and the screen height.

VESA BIOS video services are used to obtain the amount of video memory
(INT 0x10 AX=0x4F00) and then to obtain the VESA 2.0 protected mode interface
data if available (INT 0x10, AX=0x4F0A). Users are able to select graphical
video modes (INT 0x10 AX=0x4F02), or if not available the pre VESA mode
setup. The presence of the VESA BIOS is checked by the VESA get mode
information call (INT 0x10 AX=0x4F01)

Special modes will also invoke INT 0x10 AH=0x1200 (Alternate print screen),
INT 0x10 AH=0x11 (to set 8x8 font), INT 0x10 AH=0x1201 (to turn off cursor
emulation) and INT 0x10 AH=0x01 (to set up the cursor).

Having completed video set up the hard disk data for hda and hdb is copied
from the low memory BIOS area into the kernel tables. INT 0x13 AH-0x15 is
used to check if a second disk is present.

INT 0x15, AH=0xC0 is invoked in order to check for MCA bus machines. If an
MCA systab is available the first block of the table is also saved into
the kernel's own data areas.

INT 0x11 is used to check for a PS/2 mouse. If this function reports that
a PS/2 pointing device is present the kernel will also verify directly with
the PS/2 controller itself that the mouse is attached.

Power Management

Linux supports APM power management. It will issue APM BIOS service calls in
order to set up power management, and if present will then issue calls to
the 32bit APM services after boot up.

During boot the kernel issues INT 0x15 AX=0x0530 in order to do an APM BIOS
installation check. It requires that a 32bit capable APM BIOS is present.
Assuming a valid 32bit capable APM BIOS is reported the kernel will then
issue an APM disconnect (INT 0x15 AX=0x5304) followed by a 32bit connect
(INT 0x15 AX=0x5303).

The kernel then issues an APM installation check again (INT 0x15 AH=0x5300)
in order to check if the BIOS feature flags have changed now 32bit mode
has been selected. Finally it checks the signature and saves the parameters.

At this point use of 16bit BIOS services cease and the kernel begins talking
directly to the hardware. It enters 32bit mode and transfers execution to
the 32bit kernel proper.

32bit Bootstrap

The 32bit bootstrap runs mostly independently of BIOS services. It does
however scan for and use certain tables if they are present.

PCI BIOS is used if the user requests it or PCI configuration type 1 and
type 2 are not available on the system. At that point the kernel searches
for the BIOS32 signature and then for the PCI signatures "PCI " and "$PCI"

The kernel invokes the PCI_BIOS_PRESENT function initially, in order to
test the availability of PCI services in the firmware. Assuming this is
calls are issued as the PCI service are configured, along with

In the majority of systems the kernel will directly invoke type 1 or type 2
configuration. In this situation the kernel will search for a $PIR PCI
IRQ routing table in the BIOS area (0xF0000->0xFFFFF) with a revision of
0x100 (1.0).

Multiprocessor Machines

The kernel will also search for an Intel MP 1.1 or MP 1.4 table. If this is
found it is used to obtain the multiprocessor data required to boot the
other processors as well as the APIC information, including IRQ routing
tables. Some older Linux boot loaders would overwrite the EBDA if it was
more than 4K, so the SMP tables are best placed elsewhere for Linux
compatibility. One extension the Linux kernel makes to the official rules
for parsing this table, is that in the presence of PCI/ISA machines it will
probe for and use the EISA ELCR configuration register if it appears to be

If multiprocessor capability is detected then APM BIOS service usage is
disabled except for poweroff. The APM specification and the behaviour of
existing APM BIOS implementations under SMP conditions are at best described
as 'variable'.


If the user selects a VESA BIOS console the VESA 32bit BIOS calls for
screen panning are used in order to scroll the VESA linear frame buffer and
to do colour manipulation.

Providing the 32 bit VESA BIOS calls are available the kernel calls
function 0x4F07 (pan display) in order to implement scrolling. When using an
8bit console depth the palette reload (0x4F09) function is used to reload
colour tables. In the absence of the 32bit interface the kernel does
software scrolling and assumes the colour registers are VGA compatible

The memory region segment option is not supported. If this is found then
the kernel acts as if 32bit VESA extensions are not available.


BIOS plug and play 32bit services are used if present. The functions used
are 0x00 (GET_NUM_SYS_DEV_NODES) , 0x01 (GET_SYS_DEV_NODE), 0x02
(SET_SYS_DEV_NODE). Docking station and ESCD services are not currently
utilised at all. Linux currently makes fairly minimal use of the PnPBIOS
services, simply using them to find certain motherboard device

32bit Power Management

When 32bit APM services are available the kernel will use APM facilities to
do power management, instead of issuing 'hlt' instructions when idle.

After the initial boot up the APM kernel thread issues APM function
VERSION (0x530E) specifying a maximum version of 1.2. If this fails it
assumes APM 1.0 services. ENGAGE_PM (0x530F) is then issued if the power
management is currently disengaged.

The APM task then loops handling pending APM requests once a second.
GET_EVENT (0x530B) is invoked to process pending events. When the kernel
wishes to change power state it will issue SET_STATE (0x5307) in order to
transfer state.

The idle transition loops will invoke IDLE (0x5305) and BUSY (0x5306) as
appropriate to allow the BIOS to execute its power management policy.

GET_STATUS(0x530A) is issued when user processes request battery status in
order to implement battery monitoring applications.

SET_STATE is also issued when the display timer expires in the OS. In this
situation the kernel tries to blank the primary display device (0x100) or if
this fails to blank all video devices (0x1ff)

Power Management and BIOS Bugs

The APM BIOS is a complex subsystem and has historically had many bugs,
particularly in older laptop systems. To handle this the APM BIOS supports a
variety of options to work around problems

Linux will ignore small segment limits provided by the BIOS and always
set the segment limit to 64K. This is necessary as some BIOSes get the
limit values wrong.

Linux will call BIOS functions with a selector of 0x40 pointing to the real
mode address base of 0x40:0. Many BIOS functions rely on this selector being
present even though the specification does not permit the assumption.

Options control whether BIOS functions are invoked with interrupts enabled
or disabled. A correctly functioning APM BIOS should not care but some do.

Options also control whether the power off is issued 32bit or 16bit, with
the kernel transitioning to 16bit before issuing the power off request.

Finally the battery status querying can be disabled to work around a small
number of BIOSes which crash when this function is used from 32bit space.
These options can be keyed from the DMI table scanner, so that, if we are
made aware of BIOSes requiring options set specific ways we can
automatically set the options correctly for that BIOS without user

Power Management Assumptions

The Linux 2,4 kernel power management code will restore certain devices
itself but it does rely on the BIOS to correctly save and restore the
o MTRR registers
o AGP configuration
o Hard disk parameters - including waking the drive properly
on a resume

The SMM BIOS code needs to be aware that Linux is not windows. In particular
we have seen problems where the save/restore code for the video BIOS is not
capable of handling any mode not used by windows.

The Linux kernel makes use of certain advanced features, and while these
should not intrude into the SMM mode some of them are worth mentioning

Processor State
Linux makes use of machine check exceptions, 36bit
extensions, MTRR registers, and 4Mbyte pages

Interrupt Controllers
Linux will use the APIC when available, including on newer
single processor machines. SMM code must be aware that the
IRQ routing may have been configured for this

In some circumstances Linux will assign PCI bus resources
and potentially renumber and relocate devices. It tries
where possible to keep the existing BIOS setup

Testing Strategy

Because Linux makes little use of the BIOS services it is relatively easy to
run a test sequence to get basic validation of APM functionality under

The recommended procedure is as follows

1. Boot Linux on the system
Verify the system boots
[Does basic verification of PCI services, boot up 16bit calls]

Login to the system

1.1 Type 'free'
Verify the amount of free memory appears correct
[Verifies memory reporting is working correctly]

1.2 Type 'cat /proc/apm'
Verify that the machine does not crash
[Verifies GET_STATUS 32bit call]

1.3 Type 'apm -s'
The machine should standby

1.4 Wake it and type 'apm -S'
The machine should suspend

1.5 Verify the BIOS hot-keys for suspend etc work

1.6 Verify the BIOS suspend to disk works if applicable

1.7 Resume the machine and type 'poweroff'

The system should now shutdown

2. Boot Linux on the system
Start the graphical user interface

Repeat steps 1.3 to 1.6 from a terminal window in the GUI

This verifies that the APM suspend/restore correctly handles
the GUI save/restore

Add the line "vga=0x0311" to the /etc/lilo.conf
Rerun lilo

Repeat step 1.7

3. Boot the system
This time the VGA option will try to use VESA BIOS services
to set a 640x480 16bit mode.

3.1 Login to the system

3.2 Type 'ls -lR /'

This will cause the screen pan/scrolling BIOS calls to be tested
After this has scrolled for a bit hit ^C

3.3 Remove the "vga=0x0311" option from /etc/lilo.conf
Rerun lilo

This ensures the setup is correct for any future test run

4. Additional Laptop Test

4.1 Boot Linux on the system

4.2 Insert a PCMCIA card, ensure the kernel detects it

4.3 Remove the PCMCIA card, ensure the kernel detects the change

4.4 Insert a cardbus card, ensure the kernel detects it

4.5 Verify the cardbus device is usable

4.6 Remove the cardbus device, ensure the kernel detects it

Compatibility With Older (2.2) Linux

Linux 2.2 makes fairly similar BIOS calls to the 2.4 kernel. It does not
support PCI plug and play setup and should generally be configured in the
BIOS as a non plug and play operating system. On machines with large amounts
of memory the earlier 2.2 kernels frequently do not see all of it as they
lack the E820 memory sizing code. 2.2 users should probably be advised to
upgrade to Linux 2.2.19 or higher on such machines.

Future Paths

Intel are currently working on ACPI support for Linux. While much of this is
functional it is not yet stable enough that vendors enable it. Linux does
not require APM services to do minimal power management, nor does it require
PnPBIOS services to function happily. It does however need to know about
interrupt routing. For minimal Linux compatibility a 'legacy free' BIOS
should probably provide the $PIR table, even if it does not provide non ACPI
versions of other services.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at