* BIOS interrupt call

A special BIOS routine — software interrupt 0x15, subfunction 0x85 — was added to signal the OS when SysRq was pushed or released.

If all else fails, it should issue an INT 18h BIOS interrupt call ( followed by an INT 19h just in case INT 18h would return ) in order to give back control to the BIOS, which would then attempt to boot off other devices, attempt a remote boot via network or invoke ROM BASIC.

A protected mode OS can also be written for the 80286, but DOS application compatibility was more difficult than expected, not only because most DOS applications accessed the hardware directly, bypassing BIOS routines intended to ensure compatibility, but also that most BIOS requests were made by interrupt vectors that were marked as " reserved " by Intel.

VBE is made available through the video card's BIOS, which installs during boot up some interrupt vectors that point to itself.

* BIOS interrupt call

Over time, DOS-virus cloaking methods became more sophisticated, with advanced techniques including the hooking of low-level disk INT 13H BIOS interrupt calls to hide unauthorized modifications to files.

Nearly all PC BIOS ROMs, even modern UEFI based ROMs, include code which is backwards compatible with the original AT BIOS interrupt calls.

Many MS-DOS programs and device drivers interacted with DOS in a low-level way ; for example, by patching low-level BIOS interrupts such as int 13h, the low-level disk I / O interrupt.

A southbridge chipset handles all of a computer's I / O functions, such as USB, audio, serial, the system BIOS, the ISA bus, the interrupt controller and the IDE channels.

A BIOS interrupt handler would then translate the program's request to match the hardware that was actually present.

For example, to print a character to the screen using BIOS interrupt 0x10, the following x86 assembly language instructions would be executed:

A list of common BIOS interrupt classes can be found below.

Adding six bits for sectors and eight bits for heads results in the 24 bits supported by BIOS interrupt 13h.

On the PC, the BIOS ( and thus also DOS ) traditionally maps the master 8259 interrupt requests ( IRQ0-IRQ7 ) to interrupt vector offset 8 ( INT08-INT0F ) and the slave 8259 ( in PC / AT and later ) interrupt requests ( IRQ8-IRQ15 ) to interrupt vector offset 112 ( INT70-INT77 ).

# REDIRECT BIOS interrupt call

Before it had hooked the interrupt there may have been no disks on the system, but by intercepting the interrupt and altering the values returned, the SCSI BIOS can make all the disks on the SCSI bus visible to the system.

Communication occurs both ways ; power management events are sent from the BIOS to the APM driver, and the APM driver sends information and requests to the BIOS via function calls.

Power management happens in two ways ; through the above mentioned function calls from the APM driver to the BIOS requesting power state changes, and automatically based on device activity.

A program that wrote directly to video memory could achieve output rates 5 to 20 times faster than making standard calls to the BIOS and MS-DOS.

These patterns were impossible to detect using standard DOS or BIOS calls, so direct access to the disk controller hardware was necessary for the protection to work.

This is opposed to the " traditional " int 10h BIOS calls, which are limited to resolutions of 640 × 480 pixels with 16 color ( 4-bit ) depth or less.

This mode-switching technique is also the one used by DPMI ( under real, not emulated, DOS ) and DOS extenders like DOS / 4GW to allow protected mode programs to run under DOS ; the DPMI system or DOS extender switches to real mode to invoke DOS or BIOS calls, then switches back to return to the application program which runs in protected mode.

Several shortcomings such as the inability to access the BIOS or DOS calls due to inability to switch back to real mode without resetting the processor prevented widespread usage.

The book was a popular and comprehensive guide to low-level programming on the original PC platform ( covering BIOS and MS-DOS system calls in great detail ).

While DJGPP runs in 32-bit protected mode, its stub and library heavily rely upon many 16-bit DOS and BIOS calls.

On IBM Personal Computers and compatibles, the Basic Input Output System ( BIOS ) and DOS system calls provide a way to write text on the screen, and the ANSI. SYS driver could process standard ANSI escape sequences.

ANSI. SYS was extremely slow as it mapped to the BIOS calls.

NTVDM emulates BIOS calls and tables as well as the Windows 3. 1 kernel and 16-bit API stubs.

A payload called SeaBIOS can be used to provide BIOS calls and thus allow coreboot to load operating systems that require those services, however most modern operating systems access hardware in another manner and only use BIOS calls during early initialization and as a fallback mechanism.

Once it has done this, any subsequent calls to INT 13H will be " caught " by the SCSI option ROM ( or " SCSI BIOS "), allowing it to insert details about any disks that may exist on the SCSI bus.

Instead of adding code to the kernel, as in the old scheme, or using kernel facilities as in the kernel-based scheme, it traps all calls to a file and identifies if it should be redirected to the kernel's equivalent function or if it has to be handled by the specific filesystem driver, and the filesystem driver " directly " access the disk contents using low-level BIOS functions.

There are 12 power events ( such as standby, suspend and resume requests, and low battery notifications ), plus OEM-defined events, that can be sent from the APM BIOS to the operating system.

The BIOS provides a small library of basic input / output functions used to operate and control the peripherals ( such as the keyboard, text display functions and so forth ), and these software library functions are callable by external software.

There are a few alternatives for " Legacy BIOS " in the x86 world: Extensible Firmware Interface, Open Firmware ( used on the OLPC XO-1 ), and coreboot.

These items are only available by obtaining a patched V64 BIOS.

In many desktop computers, for example, the bootstrapping process begins with the CPU executing software contained in ROM ( for example, the BIOS of an IBM PC ) at a predefined address ( some CPUs, including the Intel x86 series are designed to execute this software after reset without outside help ).

16-bit BIOS interfaces are required by certain x86 operating systems, such as DOS and Windows 3. 1 / 95 / 98 ( and all when not booted via UEFI ).

This makes it a suitable replacement for older read-only memory ( ROM ) chips, which are used to store program code that rarely needs to be updated, such as a computer's BIOS or the firmware of set-top boxes.

The most notable exceptions are probably the Dreamcast game console which includes a minimal BIOS, like the PlayStation, but can load the Windows CE operating system from the game disk allowing easily porting of games from the PC world, and the Xbox game console, which is little more than a disguised Intel-based PC running a secret, modified version of Microsoft Windows in the background.

PCI video cards and other cards that contain a BIOS are problematic, although video cards conforming to VESA Standards may be used for secondary monitors.

There are also optimizations to prevent boot slowdown due to BIOS POST operations.

** MSX-Audio BIOS ( 32 KB ) ( optional, no machines are known with this BIOS built in )

Many motherboards are sold, and advertised, with extensive facilities for overclocking implemented in hardware and controlled by BIOS settings.

Motherboards purchased separately to build a custom machine are more likely to allow the user to edit the multiplier and FSB settings in the PC's BIOS.

Jumper blocks and jumpers are also often used on motherboards to clear the CMOS information, resetting the BIOS configuration settings.

A few months later they found that some laptops are sold with a legitimate rootkit, known as CompuTrace or LoJack for Laptops, preinstalled in the BIOS.

At system start, only the BIOS drivers are available for LILO to access hard disks.

When MSX BASIC is invoked, the ROM code for BIOS and the BASIC interpreter itself are visible on the lower 32K of the Z80 addressing space.

Function Keys are also heavily used in the BIOS interface.

Some of the more common nomenclatures used are: SoC ( System on Chip ), BIOS ( Basic Input Output System ), CPU ( Central Processing Unit ), and etc.

For this reason a separate filesystem, often of only a few megabytes, is often created at the start of the disk to store the kernel image and a few auxiliary files needed at boot time, ensuring that they are readable by the BIOS.

To further overcome this limit, INT 13H Extensions were introduced with BIOS Enhanced Disk Drive Services specification, which removed practical limits on disk size for operating systems which are aware of this new interface, such as DOS 7. 0 component in Windows 95.