x86 BIOS calls used by grub and Linux

The other day I was checking up to see which BIOS calls are used by grub and Linux.  I naively believed grub used just a few to get some disk geometry information and to do some reads as well as a little bit of console/keyboard I/O.  In fact Grub uses quite a few BIOS calls, namely:

BIOS "int 0x13 Function 0x41, bx=0x55aa" to check if LBA mode is
supported
BIOS "int 0x13 Function 0x42" read sectors from disk into memory (LBA
mode)
BIOS "int 0x13 Function 0x08" determine HDD geometry
BIOS "int 0x13 Function 0x02" read sectors from disk into memory
(non-LBA mode)
BIOS "int 0x13 Function 0x00" reset floppy drive (if not booted from
HDD)
BIOS "int 0x13 Function 0x4B01" check bootable CD-ROM emulation
status
BIOS "int 0x13 Function 0x4800" get driver paramaters

BIOS "int 0x15 Function 0xe801" get memory size (Phoenix BIOS v4.0)
BIOS "int 0x15 Function 0xe820" get memory map
BIOS "int 0x15 Function 0xc0" get ROM configuration
BIOS "int 0x15 Function 0x2400" disable A20 gate

BIOS "int 0x10 Function 0x4f00" get SVGA information (VBE)
BIOS "int 0x10 Function 0x4f01" get SVGA mode
BIOS "int 0x10 Function 0x4f02" set SVGA mode

BIOS "int 0x10 Function 0x01" set cursor shape
BIOS "int 0x10 Function 0x02" set cursor position
BIOS "int 0x10 Function 0x03" get cursor position and size
BIOS "int 0x10 Function 0x09" write character and attribute at cursor
BIOS "int 0x10 Function 0x0e" write character in teletype mode

BIOS "int 0x16 Function 0x00" keyboard - get keystroke
BIOS "int 0x16 Function 0x01" keyboard - check for keystroke

BIOS "int 0x1a Function 0x02" get real-time clock

BIOS Advanced Power Management (APM) calls: Grub will ignore APM if it does not detect it - it's not manditory to have APM for grub to work. So these are non required.

APM "int 0x15 Function 0x5300" check for APM
APM "int 0x15 Function 0x5301" connection real-mode interface
APM "int 0x15 Function 0x5304" disconnect interface
APM "int 0x15 Function 0x5307" set power state
APM "int 0x15 Function 0x530E" driver version


As for the Linux kernel, it also uses quite a range of BIOS calls, for example figuring out specific system settings and doing console work.

Below is a list of calls that I could find in the x86 arch specific code, I'm not sure if it's the fully definitive list, but I believe I found the majority of the BIOS calls.

BIOS "int 0x10 Function 0x00" set video mode
BIOS "int 0x10 Function 0x02" set cursor position
BIOS "int 0x10 Function 0x03" get cursor position and size
BIOS "int 0x10 Function 0x0e" write character in teletype mode
BIOS "int 0x10 Function 0x0f" get current video mode
BIOS "int 0x10 Function 0x0100" set cursor scan lines
BIOS "int 0x10 Function 0x1111" set 9x14 font
BIOS "int 0x10 Function 0x1112" set 8x8 font
BIOS "int 0x10 Function 0x1200" bl=0x10, check EGA/VGA
BIOS "int 0x10 Function 0x1200" bl=0x20, video, alternative function
select
BIOS "int 0x10 Function 0x1201" bl=0x34, turn of cursor emulation
BIOS "int 0x10 Function 0x1a00" get display combination code
BIOS "int 0x10 Function 0x4f00" get SVGA information (VBE)
BIOS "int 0x10 Function 0x4f01" get SVGA mode
BIOS "int 0x10 Function 0x4f02" set SVGA mode
BIOS "int 0x10 Function 0x4f08" get/sec DAC palette control
BIOS "int 0x10 Function 0x4f0a" get protected mode interface
BIOS "int 0x10 Function 0x4f15" check DDC capabilities

BIOS "int 0x15 Function 0x2401" enable A20 gate
APM "int 0x15 Function 0x5300" check for APM
APM "int 0x15 Function 0x5303" connect interface
APM "int 0x15 Function 0x5304" disconnect interface
BIOS "int 0x15 Function 0xe980" get Intel speedset information
BIOS "int 0x15 Function 0xec00" set BIOS CPU mode
BIOS "int 0x15 Function 0xc0" query MCA
BIOS "int 0x15 Function 0xe801" get memory size (Phoenix BIOS v4.0)
BIOS "int 0x15 Function 0xe820" get memory map
BIOS "int 0x15 Function 0x88" get extended memory size

BIOS "int 0x16 Function 0x00" keyboard - get keystroke
BIOS "int 0x16 Function 0x01" keyboard - check for keystroke
BIOS "int 0x16 Function 0x0305" set keyboard rate

BIOS "int 0x1a Function 0x02" get real time clock
BIOS "int 0x1a Function 0x02" get real time clock

Needless to say, some of these are not used for example, the use of some depend on how the console is configured.

Anyhow, it's quite surprising how much we still rely on core BIOS functionality to get a system up and running.  Kudos to Ralph Brown's interrupt list to help me figure out the BIOS int and function code mappings.

GCC hacks in the Linux Kernel

M. Tim Jones has written a concise and useful article on some useful special optimisation hints and capabilities found in version 4.3.2 of the GNU C Compiler.

The article describes how to use these methods and includes working examples found in Linux kernel. It will take just 5-10 minutes to read and is time worth spent to get up to speed with these useful GCC hacks

inteltool - dumps Intel CPU/chipset configuration settings

A little used tool from the coreboot project is inteltool. This tool is useful for dumping the configuration space of Intel CPUs and for examining the Northbridge and Southbridge settings.

The debian package can be found in universe, and can be installed using:

sudo apt-get install inteltool

..and has to be run using sudo.

At the simplest level, it can be run to get CPU and Northbridge/Southbridge version information:

$ sudo inteltool
Intel CPU: Family 6, Model f
Intel Northbridge: 8086:2a00 (PM965)
Intel Southbridge: 8086:2815 (ICH8-M)

However, one can drill down a bit, for example to get a dump of the GPIO settings, use the -g flag, e.g.:

$ sudo inteltool -g
Intel CPU: Family 6, Model f
Intel Northbridge: 8086:2a00 (PM965)
Intel Southbridge: 8086:2815 (ICH8-M)

============= GPIOS =============

GPIOBASE = 0x1180 (IO)

gpiobase+0x0000: 0x99541d02 (GPIO_USE_SEL)
gpiobase+0x0004: 0xe0fa7fc2 (GP_IO_SEL)
gpiobase+0x0008: 0x00000000 (RESERVED)
gpiobase+0x000c: 0xe1aa5fc7 (GP_LVL)
gpiobase+0x0010: 0x00000000 (GPIO_USE_SEL Override (LOW))
gpiobase+0x0014: 0x00000000 (RESERVED)
gpiobase+0x0018: 0x00000000 (GPO_BLINK)
gpiobase+0x001c: 0x00000000 (GP_SER_BLINK)
gpiobase+0x0020: 0x00080000 (GP_SB_CMDSTS)
gpiobase+0x0024: 0x00000000 (GP_SB_DATA)
gpiobase+0x0028: 0x00000000 (RESERVED)
gpiobase+0x002c: 0x00001900 (GPI_INV)
gpiobase+0x0030: 0x00000146 (GPIO_USE_SEL2)
gpiobase+0x0034: 0x00540f70 (GP_IO_SEL2)
gpiobase+0x0038: 0x00540f74 (GP_LVL2)
gpiobase+0x003c: 0x00000000 (GPIO_USE_SEL Override (HIGH))

There are other options to dump out the RCBA, Power Management, Memory Controller, EPBAR, DMIBAR and PCIEXBAR registers as well as the CPU MSRs.

It's an excellent utility for digging into the configuration of any Intel PC.

inotail - tail clone using inotify

I regularly use the tail command to look at the end of log files - generally I use the -f flag to follow (monitor and display) new entries that are appended to the log.

Tail will poll every second to see if the file size is changed and will act if data has been appended or the file has been truncated, and the poll interval is adjustable.  However, polling a file is plain ugly, it causes extra wakeup events and if one wants very fast updates then polling is not a good solution.

Hence, a better tool to use is inotail when following a file. Inotail uses inotify to only wake up inotail when a file has been modified, deleted or moved - hence no polling is required.  This also means inotail will output the changes to a file almost immediately - unlike tail which by default may wait almost a second before detecting a change.

To install inotail use:

sudo apt-get install inotail

..and use it the same was as tail.

UPDATE:  Pete Graner has also informed me that the tailf command is also an inotify based tail -f like replacement too. Just goes to show there are many ways of doing the same thing...