The PC's BIOS

NOTE: This page is still under construction; if a topic doesn't make sense, you may contact us for more information.

TO DO: We're planning on providing examples of different BIOS features to show how incredibly complex the code has become since the first PC was designed; another page will highlight a number of the routines at the assembly level of the BOCHS BIOS and a guide to using various tools to see what's inside the compressed BIOS code of some fairly recent motherboards.

A History of the PC's BIOS

It would take a group of accomplished authors working full-time to produce a textbook on the complete history of the PC BIOS in a reasonable amount of time. It will take us much longer to even produce a brief summary of how PC BIOS code has changed over the years. Though we have provided many details on special topics (such as Checksum Bytes and even some assembly listings), we are limited in the research we can conduct and the access we have to different types of computers. Nevertheless, we hope to provide some data on this subject you may not find anywhere else; we appreciate your feedback.

Though many of the details (and their order) have changed over the years, this is a list of the fundamental operations which the BIOS (Basic Input/Output System) code must complete before handing control over to an operating system. Due to the complexity of the modern BIOS, completing these items is now often referred to as the POST (Power On Self Test), since the BIOS has also contained code for many other tasks and features for quite some time:

As can be seen from the partial list above, a computer's BIOS is inextricably linked to its hardware. Therefore a history of the BIOS code must also include at least an historical summary of changes in PC hardware. Before computers with new processors, other internal devices (such as a real time clock), different types of memory, etc. ever became available to the public, they invariably required changes in the BIOS code. It's important to note the major differences (such as, bus width and addressable memory) in the Intel® processor family (see table below) for any historical study of the PC's BIOS.

Except for some early notebook CPUs (e.g.,

386SX), all Intel processors from 80386 and later, such as the

80486 (late 1989), were capable of addressing 4 GiB of Memory. But the maximum amount of memory an i486 through 6th-generation CPU based PC (i.e., Pentium™ Pro/-II/-III and Celeron™) could actually use was often determined by the Memory Control chips available to motherboard manufacturers. For example, a particular ASUS™ motherboard (CUSL-2C); circa 2000, designed for Socket 370 Pentium™ III or Celeron™ processors, only supported up to 512 MiB^[1] (even though CPUs which could access at least 4 GiB had been available for 15 years) due to the limitations of its MCH (Memory Controller Hub).^[2]

The advent of 64-bit processors meant computers might possibly access up to 16 EiB (16 Binary Exa Bytes) of memory at some point in the future. However, all 7th and 8th-generation 64-bit CPUs have been manufactured with far less than 64-bit address lines; 36-bit (for 64 GiB) and now 40-bit (for 1 TiB) are common. As noted above, a PC's chipset (such as its MCH) has often been the limiting factor regarding memory; and is still often true. For example, a GIGABYTE™ GA-G33M-DS2R from 2007, with an LGA775 socket for many different Intel 64-bit CPUs, only supported up to 8 GiB^[3] of system memory due to the limitations of its 82G33 Graphics and Memory Controller Hub (GMCH).^[4]

Of course, in order to generally make use of more than 4 GiB of RAM, a 64-bit OS is also needed. (Note: PCs with 32-bit OSs often show only about 3.2 to 3.3 GiB available when 4 GiB is installed, since addressable hardware; especially onboard video controllers, may use hundreds of MB for their own support.)

We will have more to say about how BIOS code was affected by changes in the amount of memory these processors could access.

Where is a PC's BIOS code stored? Various types of BIOS chips.

Over the course of the PC's history, BIOS code has been stored in many different types of non-volatile memory, the first of which was truly Read Only Memory (

ROM), because its programming had to be encoded in the chip at the time it was fabricated. Thus, BIOS chips on early IBM Personal Computers often had lines printed on them which not only indicated the manufacturer's IC type, but also an IBM part number for the code it contained. In the photo below, the logo for AMD™ (Advanced Micro Devices, Inc.) probably caught your attention first, being on four of the five ICs, but you'll also find the Motorola™ batwing logo (an 'M' inside a small circle) on U30. The first chip on the left (U33), with the lines "1501476 / AM9264DPC / 34146 / 8407VPM," contains the BIOS code for an IBM Model 5150, and its part number (1501476) identifies it as the 3rd version (October 27, 1982) of the code. These are all 24-pin, 8K-word x 8-bit ROMs, the BIOS/POST code being only 8 KiB long; U29 (5000019), U30 (5000021), U31 (5000022) and U32 (5000023) comprise the 32 KiB ROM BASIC code.

From a 2008 photo, courtesy of Rune Tapper, pc-museum.com. (This particular IBM 5150 had been "Made in Greenock, Scotland,"
some time after week 8 of 1984, since a number of chips soldered to the board had that date code; note the '8408' on U30 and U32.)

For testing or even small production runs,

PROMs (one time only, Programmable ROM) or

EPROMs (Erasable Programmable ROM) may have been used on some computers, since the manufacturer could then test and use revised code and not be left with any ROMs in their stock having either erroneous or obsolete data. Both PROMs and EPROMs usually have part numbers beginning with 27, but an EPROM has a small quartz window on top (often covered by a label), so its memory can be erased with a specific high intensity beam of ultra-violet light; allowing it be reused. Much later, motherboard manufacturers designed circuitry and utility programs for various types of

EEPROMs (Electrically Erasable Programmable ROM; part numbers usually begin with a 28 or 29) that allowed users to reprogram their own PC's BIOS. [This also meant the code could be corrupted if a power-loss occurred during the upgrade process! Of course, having a known-good battery-backup power supply could prevent that.]

If the BIOS chip became corrupted, it required purchasing a pre-programmed replacement chip; unless a user was so unlucky as to find the PC manufacturer had soldered the BIOS chip to the board! Even then, the BIOS chip generally still had 24 rather large pins, and an electronics hobbyist could disable (or cut away) the old chip and solder the new one to the existing legs (pins) without having to unsolder the old chip. Of course, if the cost of a programmed chip was too high, then a better board with more features would win out. The worst class of motherboards in relation to BIOS chips were those that stored the code in a chip soldered onto the board and also required the user to run an upgrade under Windows, without anything else running! So, forgetting to disconnect the Network cable or to use TASK MANAGER to kill all but the essential processes, meant you could easily turn such a motherboard into a useless piece of junk!

PCs with a better design philosophy, had a built-in bootstrap routine that would remain unaltered, thus allowing users to recover from failed BIOS upgrades. And more recent motherboards, may have a Dual BIOS system^[5], so users can re-boot from a secondary BIOS chip that was not in the process of being changed if a power-loss or some other event occurs. When the PC boots-up again, it will use the secondary BIOS and the user can go back to reprogramming the primary BIOS chip. Depending upon how the board manufacturer decided to implement their dual BIOS design, the second chip may be a safety chip that is 'read only' and can never be changed (GIGABYTE's design); which has the solid advantage of having some BIOS code that could always be relied upon, unless it had a hardware failure too. Or, a routine may exist to copy the successfully upgraded chip's contents to the other BIOS chip, after determining there is no problem with the updated code. The BIOS could then check that the copied code matches that of the upgraded chip, before allowing another upgrade (decidedly more difficult to program and might allow a hacker's code to be your new 'backup code').

Today, a PC's BIOS may reside in a small 8-pin,

Serial Flash (such as the Macronix™ MX25L8005; an 8 Mbit or 1024 KiB chip, with Boot Lock protection), its code being accessed by the motherboard's South Bridge (such as Intel's ICH9; I/O Controller Hub 9) using an

SPI bus. On other bords, you may still find a 32-pin PLCC package used for the BIOS. The Dell E521 PC used such a chip (SST 49LF040B 33-4C-NHE; a 4 Mbit chip to store up to 512 KiB of BIOS code). Pictured here is an early Gigabyte DualBIOS™ board using two PLCC chips.

Note: If you'd like to contribute to a listing of the different types of BIOS chips and methods used to access them, we'd appreciate hearing from you; perhaps including some digital photos of your motherboard (and/or a closeup of the BIOS chip, once we determine where it's located) You can email us here.

How to Identify your PC's Chipset

The best source of information, if it's available, would be from the engineers who designed your PC's motherboard. ASUS™ and GIGABYTE™ both provide manuals online for most of their products; other motherboard companies may as well. A number of chips on our GIGABYTE boards were listed in these manuals. However, most likely due to the cost of support, most PC manufacturers do not supply you with such details.

But utility programs also exist which will search PC buses for ID bytes that reveal the identity of a number of its ICs. One which we've used for years is called CPU-Z; available in both 32- and 64-bit OS versions of installable and portable downloads:

CPU-Z. The program's Mainboard TAB window displays whatever it knows about a PC's chipset and BIOS. Below you'll see examples for both a Gigabyte Motherboard and a Dell PC. On the left, most of the data is the same as we found in the manual. However, the data displayed here for the Dell PC (and other name-brand computers) is generally unavailable. Now we at least know it's using a chipset from NVIDIA, and some kind of Super I/O chip from SMSC ("Standard Microsystems Corporation") for its LPC (

Low Pin Count) I/O bus. We had to examine the board to find it was an SCH55140-NS chip; whereas the Gigabyte board's LPC I/O chip from ITE shows it was an IT8718. But neither board's actual BIOS chip is listed here. To see more detailed information about your PC, you could also download

PC-Wizard (from the same web site). The Hardware section provides information about the Mainboard, BIOS and Chipset; among many other items. And the data provided by its remaining sections is so detailed, it may overload your brain.

Once you have the Chipset data, you can search a chip manufacturer's site for details on how data in the chip might be accessed; that data can be used by independent utility programmers to show the actual contents of a BIOS chip, as well as by motherboard manufacturers for use in their own BIOS upgrade programs. Some Board manufacturers may even provide a Windows-based utility to make a copy of your BIOS code! [Note: We do not recommend performing a BIOS upgrade, unless you have been assured it will allow your PC to function in some new way you require it to; such as recognizing a larger disk drive, or fixing a problem. We are only pointing out it may be easy to copy the code from your PC's BIOS chip using a software tool; not to use such tools for overwriting BIOS code, without considering the possibility it may become corrupted.] The following shows one such utility running on a PC (we used the PrintScreen key to capture its image; NEVER do this when upgrading!) and added the yellow rectangle and caption:

Note: For anyone wishing to access the BIOS code on a modern PC - without such a utilty, you would first need to know how to use Intel's

Low Pin Count Specification and any possible variations in the LPC I/O Interface of the different

Super I/O chip manufacturers, and then figure out how to communicate with the actual BIOS chip itself; which could be an 8-pin or 32-pin flash chip of different manufacturers as well.

The Location of a PC's First Instruction

Virtually every PC, since the very first IBM® Personal Computer produced in 1981, to the latest Intel® or AMD® based PC, has had exactly the same Memory address hard-wired into its CPU as a reference for its first instruction!^[6] This means every PC's CPU will always begin executing machine code instructions from essentially the same location inside its BIOS chip, or for PCs that must first move (or decompress) their BIOS code to Memory, from an equivalent location in Memory. That address is:

F000:FFF0 (in Segment:Offset notation) or: FFFF0h (in Linear notation). You may also find it represented by FFFF:0000 [See Figure 1 below; as listed in IBM's original Technical Reference manual; P/N 6025008], or as just FFFF:0 (in Normalized Segment:Offset notation). [Read Section 3 of our page on Segment:Offset Addressing to understand why FFFF:0 is equivalent to the Segment:Offset pair F000:FFF0.]

IBM® named this address the "Power On Reset Vector " and it always contains a far jump instruction to the beginning of the BIOS chip's Power-on RESET code. Here's a display of what you'd find in that location and the next 12 bytes of an original IBM® PC's BIOS chip:

Offset: 0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F    ASCII chars. 
------ -----------------------------------------------  ---------------- 
0FFFF0 EA 5B E0 00 F0 30 34 2F 32 34 2F 38 31 FF FF EB  .[...04/24/81...

(The two FF bytes are used as padding and the EB at FFFFFh is the BIOS Checksum byte.)

The first five bytes (shown in green ) comprise the Power-On Reset Far Jump. These 5 bytes disassemble to:

As you can see, Segment F000: is embedded inside this instruction, thus the reason its location is often referenced as F000:FFF0. Although the location of this far jump instruction is essentially 'set in stone' for all PC BIOS, it's not a requirement that where it jumps to next always be the same; yet every PC BIOS we've ever examined always jumps to "F000:E05B".

Of the twelve IBM engineers assigned to create the IBM Personal Computer (model 5150), David J. Bradley^[7] developed the code for its BIOS. So he's the one who, among all its other details, decided where in Memory the BIOS would place and execute the code from the first sector of the IBM PC's first floppy diskette's Boot Record. The location he chose was 0x7C00 (or 0000:7C00 in Segment:Offset notation). Unlike the first 'jump address' mentioned above (to F000:E05B), later BIOS authors could not have chosen a different location in Memory for loading the initial bootstrap routines without having their code become incompatible with existing boot diskettes! So IBM (and all the PC-clone companies which followed) continued to use that same location in Memory for their hard disk drive's Master Boot Records (MBRs).

Release Date

The next eight bytes (shown in yellow ) were originally called the "RELEASE MARKER" by IBM; they always contain the release date of the BIOS code (which was 04/24/81 for those first PCs).

[ In Figure 1, there's an error in the machine code listing for the location of the Power On RESET code in the "JMP RESET" instruction. This is exactly how the original "IBM Personal Computer Technical Reference Manual," was printed. This should have been listed as: EA5BE000F0 rather than "EA5B0000F0"; if you check the address shown on page A-5, it's obvious "RESET" begins at the address we've specified.]

Anyone running a Microsoft/IBM OS from DOS through Windows™ 7 (32-bit), should be able to enter the following DEBUG commands (Windows users must first click on start —> Programs —> Accessories —> Command prompt to open a 'Command Prompt' window. Windows 7 users may be required to select 'Run As Administrator' next to the Command Prompt icon in order to access DEBUG), and obtain results similar to the following (DEBUG.EXE should already be in your path):

ENTER the commands shown in green at the DEBUG prompt ("-"). You can see we have the same Jump instruction as the original Personal Computer on this one, but this BIOS code was released on April 14, 2003 ("04/14/03"). (For more on the use of DEBUG, see our Guide to MS-DEBUG).

Here's a Windows™ 7 (32-bit) Command Prompt using DEBUG to show its PC's BIOS release date of "06/18/09":

On early PCs, RAM was an expensive and limited resource that was used as wisely as possible for the the execution of user's programs. Since the original PC could be purchased with only 16 KiB (16 x 1024 bytes = 16,384 bytes) of memory, we know for a fact it would have been impossible to fit the 32 KiB of ROM BASIC code into memory. But using half the available memory for even the 8 KiB of initial bootstrap code made no sense, since it could be accessed from its own ROM chip just as fast as any code in the dynamic RAM chips; both having a 250 nano second access time^[8]. Once RAM became much faster than ROM or EEPROM ICs and PCs commonly had many megabytes of memory, system engineers had motive enough to first copy bootstrap routines from the BIOS code into memory, then execute it there. This made it possible for BIOS programmers to explore a number of other advantages to having their code shadowed in RAM; which we'll point out in our next section. But first, we need to mention some methods for making copies of whatever BIOS code still remains in memory after a PC boots up:

How to Save BIOS code from the First 1 MiB of a PC's Memory

As we mentioned above, for first generation PCs, BIOS code always had to be read directly from its chip(s). Not only when the PC was booting-up, but every time a program used a system Interrupt or needed to access any other BIOS code stored in its ROM (Read Only Memory) chip(s). In those days, the address lines corresponding to the last segment of memory were hard-wired only to locations within the BIOS chip(s). Later on, PCs were made in which the BIOS code was first copied into RAM and then used from there instead of directly from the BIOS chip(s). If you have access to a PC with an Intel® 80286 (or possibly one with an i386) CPU, you might be able to use DEBUG to change various bytes of the BIOS code in memory, since early computers lacked the ability to write-protect portions of memory.

Motherboard and BIOS chip designs are now much more advanced and vastly different in how they function before even starting the POST than those early PCs, but it's still possible to examine some of the BIOS code that's copied to the last 64 KiB and other Segments of a computer's first Binary Megabyte of Memory. Although early PCs often revealed an exact duplicate of the code from their BIOS chip(s), what you'll find today, for example, on a PC running Windows XP, will only be a small portion of the full BIOS code that has been copied yet again into a Virtual Memory address space used by the NTVDM program to simulate the first Megabyte and beyond of a computer's physical memory. Nevertheless, these bytes are similar to (possibly even the same as) a small section of all the bytes archived in whatever hardware is actually used to store the PC's BIOS code.

To make a copy of this code on your own PC, simply download and run our BIOSDUMP Batch program which includes a DEBUG script file that uses MS-DEBUG to read all the bytes from F0000h to FFFFFh in memory and write them to the file BIOSDUMP.BIN. Being DEBUG purists, this script file was designed to make sure the first 256 bytes of DEBUG's memory space would not be altered. (Note: This download also includes VIDEODMP.BAT and a script file for dumping the C000: segment; the standard location for VGA video BIOS code.) The scripts cause DEBUG to perform a two-step process that first copies the code to a 65,280-byte file, followed by a 256-byte file. Then the two *.BIN files are combined using the DOS "COPY /b" command; lastly, the intermediate *.BIN files are deleted.
Instructions for using BIOSDUMP.ZIP: Simply extract all 4 files to a Windows or DOS folder of your choice, then double-click on one of the *.BAT files, or enter its name at a DOS prompt to run it. The Batch programs run the associated *.DSF Debug Script so you don't have to type in the DEBUG commands. (If run from a bootable floppy diskette, make sure a copy of DEBUG.EXE is in the same location or in the path.)

In order to quickly skim through the contents of any *.BIN file created by these programs, or to search for either a text string or hex byte pattern, we recommend using the free hex/disk editor, HxD.

BIOS Checksum Bytes

The last location in an early PC's BIOS chip was called its CHECKSUM byte. It was used to help check that none of the chip's bytes had become corrupted (changed values). For example, the Checksum Byte of the first IBM Personal Computer is "EB". This value was computed by first adding together every preceding byte of the PC's 8 KiB BIOS code, two bytes at a time, keeping only the last 8-bits (one byte) of the sum, and finally subtracting that value from 100 hex. Thus, the Checksum Byte ensures that running an "8-bit Checksum operation" on every byte in the BIOS chip will produce a result of zero (0). [ Note: An 8 KiB file full of only FF bytes would also have an 8-bit Checksum of zero, but its 32-bit Checksum would be 1FE000; which is the sum of all 8,192 of its bytes added together (2,088,960 decimal).] In summary, an 8-bit Checksum Byte can be computed as follows (we also show in the table below all checksum data for the original IBM PC's three different BIOS chip versions):

1. When you have finished coding the BIOS, insert a Checksum Byte of 00 (or simply proceed without one).
2. Use a program or utility to compute the 8-bit Checksum of these bytes.
3. Subtract this checksum from 100 hexadecimal; for example, in the table below: 100h - 15h = EB.
4. The result is the 8-bit Checksum Byte for the BIOS; ensuring its final 8-bit Checksum is zero.

If you wish to compute the 8-bit (or 16- or 32-bit) Checksum of a file, you can easily do so using HxD, as shown here:

What Most Can Find Ain't All There Is!
(Or: There's more to modern BIOS code than most have imagined!)

Although you could boot-up your computer with a DOS boot diskette or a CD-ROM disc so there's no chance of its memory being altered by anything but 16-bit real DOS before dumping its contents, even then there's no assurance (without prior research) this will provide you with a copy of all the bytes actually contained in its "BIOS chip(s)". Why? Because the BIOS code may include features that are never retained in memory; for example, a PC company's splash screen that often appears on the display when a computer is first turned on. Or, for example, this familiar EPA ("Energy Star") Logo:

To see just how sophisticated modern BIOS code has become; using compression and possibly even encrypting its machine code, we've decided to delve into some of the methods used by BIOS manufacturers for our readers. We want you to see how much BIOS code has changed over the decades.

If you have a particular interest in this topic, feel free to write to us about your own PC's BIOS for help in obtaining a copy of what's actually stored in its BIOS chip(s) and/or a BIOS upgrade file.

¹[Return to Text] On page 8 of the ASUS® CUSL2-C, Intel® 815EP ATX Motherboard, USER'S MANUAL, Revision 1.04 E639, Copyright ©2000 by ASUSTeK Computer Inc., it states: "Equipped with three Dual Inline Memory Module (DIMM) sockets to support PC100/PC133-compliant SDRAMs (available in 64, 128, 256, 512MB densities) up to 512 MB." [Emphasis ours.] Note: The Black Pearl Edition (CUSL2-CBP) was no different; its User Manual has the same note on page 12.

²[Return to Text] On page 9 of the Intel® 815 Chipset Family:, 82815EP and 82815P, Memory Controller Hub (MCH), March 2001, Document Reference Number: 290693-002, Copyright © 2000,2001 Intel Corporation, under "Integrated SDRAM Controller" it lists: "32 MB to 512 MB using 16Mb/64Mb/128Mb/256Mb technology". And on page 50, it quite clearly states: "The maximum supported main memory capacity is 512 MB." [Emphasis ours.]

³[Return to Text] On page 12 of the GA-G33M-DS2R/, GA-G33M-S2, LGA775 socket motherboard for Intel® Core™ processor family/, Intel® Pentium® processor family/Intel® Celeron® processor family, User's Manual, Rev. 1003, 12ME-G33MD2R-1003R, ©2007 by GIGABYTE™ you'll find: "4 x 1.8V DDR2 DIMM sockets supporting up to 8 GB of system memory." [Emphasis ours.] The fact that GIGABYTE included four sockets on this board, means we'll only need to purchase DIMMs of 2 GB each when moving from a 32 to 64-bit OS to take full advantage of that 8 GiB of memory.

A Few Relevant Facts concerning some early Intel® Processors
Processor ( )	Bits per Bus		Addressable Memory	Initial Clock Speed	Introduced
Processor ( )	Data	Address	Addressable Memory	Initial Clock Speed	Introduced
8080	8	16	64 KiB	2 MHz	April 1974
8086	16	20	1 MiB	4.77 MHz	June 1978
8088	8	20	1 MiB	4.77 MHz	July 1979
80186	16	20	1 MiB	6 MHz	JAN 1982
80188	8	20	1 MiB	6 MHz	JAN 1982
80286	16	24	16 MiB	6 MHz	FEB 1982
80386	32	32	4 GiB	16 MHz	OCT 1985

BIOS Checksum Data for U33 of the 5150 IBM® Personal Computer
ROM Version P/N (Release Date)	Original Checksum		Checksum Byte	Final Checksum
ROM Version P/N (Release Date)	8-Bit	32-Bit	Checksum Byte	8-Bit	32-Bit
5700051 (04/24/81)	15	E8515	EB	00	E8600
5700671 (10/19/81)	65	E8565	9B	00	E8600
1501476 (10/27/82)	89	EAF89	77	00	EB000

A History of the PC's BIOS

Where is a PC's BIOS code stored? Various types of BIOS chips.

How to Identify your PC's Chipset

The Location of a PC's First Instruction

Release Date

How to Save BIOS code from the First 1 MiB of a PC's Memory

BIOS Checksum Bytes

What Most Can Find Ain't All There Is!
(Or: There's more to modern BIOS code than most have imagined!)

A History of the PC's BIOS

Where is a PC's BIOS code stored? Various types of BIOS chips.

How to Identify your PC's Chipset

The Location of a PC's First Instruction

Release Date

How to Save BIOS code from the First 1 MiB of a PC's Memory

BIOS Checksum Bytes

What Most Can Find Ain't All There Is! (Or: There's more to modern BIOS code than most have imagined!)

What Most Can Find Ain't All There Is!
(Or: There's more to modern BIOS code than most have imagined!)