5/10/2021
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`How BIOS Works
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`The Wayback Machine- http://web.archive.org/web/20141003015022/http://flint.cs.yale.edu:80/feng/cos/resources/BIOS/
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`How BIOS Works
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`Whatnew:
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`Whatare the interested properties? - Feedback from BIOSand Device Driver hackers
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`The Machine Model
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`What is BIOS
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`Every computer with a motherboard includes a special chip referred to as the BIOS or ROM BIOS(Read Only Memory Basic Input/Output System).
`The BIOS includesinstructions on how to load basic computer hardware. The BIOSalso includes a test referred to as a POST (Power On SelfTest)
`which will ensure that the computer meets requirements to boot up properly. If the computer does not pass the POST youwill receive a combination of
`beepsindicating what is malfunctioning within the computer.
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`The BIOS has 4 main functions:
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`POST- Test computer hardware insuring hardware is properly functioning beforestarting process of loading Operating System.
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`Bootstrap Loader- Processof locating the operating system. If capable Operating system located BIOSwillpass the controltoit.
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`BIOS- Software / Drivers which interfaces between the operating system and your hardware. When running DOSor Windowsyouare using complete
`BIOSsupport.
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`CMOSSetup - Configuration program. Which allows you to configure hardware settings including system settings such as computer passwords,time,
`and date.
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`Whatrole does the BIOS play with during the system boot up?
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`How does system bootstrap
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`The system BIOSis whatstarts the computer running when youturn it on. The following are the steps that a typical boot sequence involves.
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`1. The internal powersupply turns on andinitializes. The power supply takes sometimeuntil it can generate reliable powerfor the rest of the
`computer, and havingit turn on prematurely could potentially lead to damage. Therefore, the chipset will generate a reset signal to the processor
`(the sameas if you held the reset button down for a while on your case) until it receives the Power Goodsignal from the power supply.
`2. When thereset button is released, the processorwill be ready to start executing. When the processorfirst starts up, it is suffering from amnesia;
`there is nothingat all in the memory to execute. Of course processor makers knowthis will happen, so they pre-program the processor to always
`look at the sameplace in the system BIOS ROM forthestart of the BIOS boot program. This is normally location FFFFOh,right at the end of
`the system memory. Theyputit there so that the size of the ROM can be changed without creating compatibility problems. Since there are only
`16 bytes left from there to the end of conventional memory, this location just contains a "jump"instruction telling the processor where to go to
`find the real BIOSstartup program.
`3. The BIOS performs the power-onself test (POST). If there are any fatal errors, the boot process stops. POST beep codes can be foundin this
`area of the TroubleshootingExpert.
`4. The BIOS looks forthe video card.In particular, it looks for the video card's built in BIOS program andrunsit. This BIOS is normally foundat
`location C000h in memory. The system BIOS executes the video card BIOS, whichinitializes the video card. Most modern cards will display
`information on the screen about the video card. (This is why on a modern PC youusually see something on the screen about the video card
`before you see the messages from the system BIOS itself).
`5. The BIOS then looks for other devices' ROMsto see if any of them have BIOSes. Normally, the IDE/ATA hard disk BIOS will be found at
`C8000h and executed. If any other device BIOSes are found, they are executed as well.
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`6. The BIOS displays its startup screen.
`7. The BIOS does moretests on the system, including the memory count-up test which you see on the screen. The BIOS will generally display a
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`text error message on the screen if it encounters an errorat this point; these error messages and their explanations can be foundinthis part ofthe
`TroubleshootingExpert.
`8. The BIOS performs a "system inventory"of sorts, doing more tests to determine whatsort of hardwareis in the system. Modern BIOSes have
`many automatic settings and will determine memory timing (for example) based on what kind of memory it finds. Many BIOSescan also
`dynamically set hard drive parameters and access modes, and will determine these at roughly this time. Some will display a message on the
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`screen for each drive they detect and configure this way. The BIOS will also now search for and label logical devices (COM and LPTports).
`9. If the BIOS supports the Plug and Play standard, it will detect and configure Plug and Play devicesat this time and display a message on the
`screen for each oneit finds. See here for more details on how PnP detects devices and assigns resources.
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`web.archive.org/web/20141003015022/http://flint.cs.yale.edu/feng/cos/resources/BIOS
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`Patent Owner, Bot M8 LLC - Ex. 2057, p. 1
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`Patent Owner, Bot M8 LLC - Ex. 2057, p. 1
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`How BIOS Works
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`10. The BIOS will display a summary screen about yoursystem's configuration. Checking this page of data can be helpful in diagnosing setup
`problems, althoughit can be hard to see because sometimesit flashes on the screen very quickly before scrolling off the top.
`11. The BIOS begins the search for a drive to boot from. Most modern BIOSescontain a setting that controls if the system should first try to boot
`from the floppy disk (A:) orfirst try the hard disk (C:). Some BIOSeswill even let you boot from your CD-ROM drive or other devices,
`depending on the boot sequence BIOSsetting.
`12. Having identified its target boot drive, the BIOS looks for boot informationto start the operating system bootprocess. Ifit is searching a hard
`disk,it looks for a master boot record at cylinder 0, head 0,sector 1 (the first sector on the disk); if it is searching a floppydisk, it looksat the
`same address onthe floppy disk for a volumebootsector.
`13. If it finds whatit is looking for, the BIOS starts the process of booting the operating system, using the information in the boot sector. At this
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`point, the code in the boot sector takes over from the BIOS. The DOSbootprocess is described in detail here. If the first device that the system
`tries (floppy, hard disk, etc.) is not found, the BIOS will then try the next device in the boot sequence, and continue until it finds a bootable
`device.
`14. If no boot device at all can be found, the system will normally display an error message and then freeze up the system. What the error messageis
`dependsentirely on the BIOS,and can be anything from the rather clear "No boot device available" to the very cryptic "NO ROM BASIC-
`SYSTEM HALTED". This will also happen if you have a bootable hard diskpartition but forget to set it active.
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`BIOSisa piece of program. Whenthe system starts, the register EIP is initialized to FFFFO to execute the JMPinstruction there, whichleads to the
`execution of the system BIOS code.
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`BIOSwill initialize other devices; initialize the interrupt vector; find other BIOS programsand run them.
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`BIOS Extension
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`Shadowing
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`A technique used to increase a computer's speed by using high-speed RAM memory in place of slower ROM memory (RAMis aboutthree times as
`fast as ROM). On PCs, for example, all code to control hardware devices, such as keyboards, is normally executed in a special ROM chipcalled the
`BIOS ROM.However,this chip is slower than the general-purpose RAM that comprises main memory. Many PC manufacturers, therefore, configure
`their PCs to copy the BIOS code into RAM when the computer boots. The RAM usedto hold the BIOS codeis called shadow RAM.
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`Links
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`Relationship between BIOS and Device Driver
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`web.archive.org/web/20141003015022/http://flint.cs.yale.edu/feng/cos/resources/BIOS
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`2/2
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`Patent Owner, Bot M8 LLC- Ex. 2057, p. 2
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`Patent Owner, Bot M8 LLC - Ex. 2057, p. 2
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