BIOS

Resources

The BIOS (Basic Input/Output System) was the standard firmware of IBM-PC-compatible computers until it was phased out in 2020. On a cold start, the BIOS performs system initialization, then loads the boot sector of the boot device specified in CMOS memory, and hands off execution to it. In most cases this boot sector is a Master Boot Record.

When it was originally released in 1981, the ROM BIOS was an entirely proprietary IBM product. Eventually, companies reverse engineered and documented the inner workings of the BIOS leading to a de facto standard. At first, this compatibility was incomplete but it grew and evolved over time until essentially 100% software compatibility existed between BIOS vendors.

Early BIOS implementations were written to ROM or EEPROM. This meant that upgrading the BIOS to address a required physically replacing the ROM chip. This is contrast to very late BIOS implementations and modern UEFI implementations where the firmware code is stored on flash memory. This change from read only memory to flash memory has security implications: malicious code can sometimes attach itself to the flash storage of firmware to evade detection or complicate removal.

Configuration

In order to configure the system, users need to enter the BIOS setup program by pressing a specific key at startup, usually within 2 seconds of hearing the start-up beep. This key press is not initially standardized and required users to check documentation in order to gain access to the system's firmware settings. By the mid 1990's a de facto standard began to emerge around a few possible keyboard combinations, with F2 being the most common.

Esc used by Coreboot.
Del
F1
F2 used by Phoenix BIOS.

By the late 1980's some BIOS manufacturers printed a notice on cold start indicating what button combination to press in order to enter the firmware configuration screen. Unfortunately, this practice was not uniform and early systems could have quite obscure keyboard input combintation requirements. This often lead to system administrators performing cold boots and simply guessing as to the correct keyboard input combination until their attempts were sucessful or all known combinations had been tried.

In the setup program's menu, users can configure several features:

Enable or disable built-in hardware devices.
Configure hardware features.
Set date and time.
Select boot drives and set a preference order.
Specify a boot password.

To operate the setup program with a USB keyboard, users may have to enable a function called USB Legacy, or something similar.

Set boot drive and order

Users can use the BIOS setup menu to change the boot drive and to set a preference order. They should set the drive where they installed the bootloader as their first preference, to speed up boot time. It takes time to check optical and floppy disk drives for a valid boot loader, so if those are placed first, the system will not boot as quickly as it might.

For USB boot drives users must enable a function called USB Legacy, or something similar.

Newer BIOS versions let users press a key during boot to open a menu, where they can select the boot drive. This is handy, if they want to boot from a LiveCD. Possible keys are:

F11
F12

Users can find the right key in their motherboard or computer manual, but it may also be shown at boot time, right after after power on. To show the boot option menu, users must press the right key about two seconds after power on, even if there is no screen displayed.

BIOS Memory Map

Once the BIOS has been initialized, RAM below 1MiB looks like this:

Start	Stop	Size	Description
`0x00000000`	`0x000003ff`	1KiB	Interrupt Vector Table (IVT) in Protected Mode.
`0x00000400`	`0x000004ff`	256B	BIOS Data Area (BDA).
`0x00000500`	`0x00007bff`	29.75KiB	Conventional memory available to programs.
`0x00007c00`	`0x00007dff`	512B	Reserved for Boot Sector.
`0x00007e00`	`0x0007ffff`	480.5KiB	Conventional memory available to programs.
`0x00080000`	`0x0009ffff`	128KiB	Extended BIOS Data Area (EBDA).
`0x000a0000`	`0x000bffff`	128KiB	Video memory.
`0x000c0000`	`0x000c7fff`	32KiB	Video card BIOS.
`0x000c8000`	`0x000effff`	160KiB	BIOS Extensions.
`0x000f0000`	`0x000fffff`	64KiB	Motherboard BIOS

BIOS as a UEFI Application for Backwards Compatibility

When a EFI or UEFI system is set to BIOS compatibility mode, then an EFI application called Compatibility Support Module (CSM) is loaded. The CSM operates as a shim that intercepts BIOS system calls and translates them into their UEFI equivalents, effectively acting as a BIOS emulation.

Booting from a Master Boot Record (MBR) partition means that the firmware must give the system a BIOS-like environment as a starting point, so that BIOS bootsectors and BIOS bootloaders continue to work. When an EFI/UEFI system (with Secure Boot disabled) is booted with an MBR partitioned media, the firmware will automatically load the CSM. It is not available when Secure Boot is enabled, which is the standard setting on computers with Microsoft Windows 10 and newer.

In UEFI after 2020 the Compatibility Support Module was discontinued, and with it all BIOS compatibility. In native (U)EFI mode, the firmware will directly load a compatible EFI bootloader from the EFI System Partition (ESP), which requires a GUID Partition Table (GPT). In theory, none of this prevents the creation of an open source UEFI application that can be installed to the UEFI partition and act as a BIOS emulator. In practice, it's not clear what barriers OEM's might erect to prevent this.

BIOS and Coreboot

Coreboot provides just enough firmware to initialize a system from a cold start and pass execution to a designated payload. While Coreboot itself provides any of several payloads, a UEFI payload is perhaps the most common. Several Coreboot distributions reject UEFI and prefer a SeaBIOS payload by default, but offer alternatives such as booting directly to GRUB or a Linux kernel. From the perspective these Coreboot forks, such as Libreboot, there is a simple reason for preferring a BIOS payload: it provides a much smaller attack surface relative to a UEFI implementation.

Boot Sector

It is a common misunderstanding that a BIOS will analyze the partition layout of the Master Boot Record and boot the active partition, although there are BIOS implementations that will check for an active partition and refuse to boot, if in fact no partition is set active.^[1] Some BIOSes also support special service partitions, and will boot a very specific system from it when the function is requested. Examples include the BIOS Setup for certain Compaq models^[2] (which is unavailable if that specific partition is missing), or the Hidden Protected Area (HPA) of ThinkPads (sometimes also called Predesktop Area or Rescue and Recovery).

Normally, a system firmware fully compatible with the original IBM PC's ROM BIOS will not do any of that. Instead it will simply verify if the signature of the boot sector, the last two bytes at offset 0x01FE (or 510 decimal), are 0xAA55, load the sector into memory and execute it in 16 bit x86 Real Mode. If the BIOS is unable to find a valid boot sector on any drive, that is in the list of bootable devices, it will display a simple text message, such as NO BOOTABLE DEVICE.

External Resources

How to enter BIOS on old computers
Coreboot — open source firmware that can deploy BIOS and UEFI (among others) as second stage payloads.
GLaBIOS — A modern, scratch-built, open-source ROM BIOS for PC, XT, 8088 PCs.

References

[1] ttps://www.rodsbooks.com/gdisk/bios.html

[2] ttps://www.seriss.com/people/erco/compaq-deskpro-2000/

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