What is the difference between BIOS and CMOS?
BIOS is non-volatile firmware used to perform hardware initialization during the booting process Originally, BIOS firmware was stored in a ROM chip on the PC motherboard. .. Once the expansion ROMs have registered using the BBS APIs, the user can select among the available boot options from within the BIOS's user . BIOS - Software / Drivers which interfaces between the operating system and your same place in the system BIOS ROM for the start of the BIOS boot program. In this case, it’s an image of the Operating System, a file that is written to the phone hardware that is ROM. BIOS stands for a Basic Input Output System, named so by IBM for its IBM PC. They are both stored in ROM (as Read-Only-Memory) in a phone or a computer hardware.
Now that the BIOS is separated out, anybody could write a BIOS for their machine, if it was based, and run this, so he started selling that separately under the company Digital Research that he formed and did quite well.
When a modern x86 microprocessor is reset, it starts in pseudo bit real modeinitializing most registers to zero. If the system has just been powered up or the reset button was pressed "cold boot"the full power-on self-test POST is run. This saves the time otherwise used to detect and test all memory.
The POST checks, identifies, and initializes system devices such as the CPURAMinterrupt and DMA controllers and other parts of the chipsetvideo display cardkeyboardhard disk driveoptical disc drive and other basic hardware. If the download was apparently successful, the BIOS would verify a checksum on it and then run it.
Post-boot, programs loaded can also call INT 19h to reboot the system, but they must be careful to disable interrupts and other asynchronous hardware processes that may interfere with the BIOS rebooting process, or else the system may hang or crash while it is rebooting. It loads and executes the first boot software it finds, giving it control of the PC.
Following the boot priority sequence in effect, BIOS checks each device in order to see if it is bootable. For a disk drive or a device that logically emulates a disk drive, such as a USB flash drive or perhaps a tape drive, to perform this check the BIOS attempts to load the first sector boot sector from the disk into RAM at memory address 0x If the sector cannot be read due to a missing or unformatted disk, or due to a hardware failurethe BIOS considers the device unbootable and proceeds to check the next device.
If the sector is read successfully, some BIOSes will also check for the boot sector signature 0x55 0xAA in the last two bytes of the sector which is bytes longbefore accepting a boot sector and considering the device bootable.
Note well that the BIOS does not interpret or process the contents of the boot sector other than to possibly check for the boot sector signature in the last two bytes; all interpretation of data structures like MBR partition tables and so-called BIOS Parameter Blocks is done by the boot program in the boot sector itself or by other programs loaded through the boot process and is beyond the scope of BIOS.
Nothing about BIOS predicates these data structures or impedes their replacement or improvement. Boot priority[ edit ] The user can control the boot process, to cause one medium to be booted instead of another when two or more bootable media are present, by taking advantage of the boot priority implemented by the BIOS.
For example, most computers have a hard disk that is bootable, but usually there is a removable-media drive that has higher boot priority, so the user can cause a removable disk to be booted, simply by inserting it, without removing the hard disk drive or altering its contents to make it unbootable. In most modern BIOSes, the boot priority order of all potentially bootable devices can be freely configured by the user through the BIOS configuration utility.
In older BIOSes, limited boot priority options are selectable; in the earliest BIOSes, a fixed priority scheme was implemented, with floppy disk drives first, fixed disks i. The BIOS in an early PC also usually would only boot from the first floppy disk drive or the first hard disk drive, even if there were two drives of either type installed. All more advanced boot priority sequences evolved as incremental improvements on this basic system.
Historically the BIOS would try to boot from a floppy drive first and a hard disk second. With the El Torito optical media boot standardthe optical drive actually emulates a 3. Optical disks are a special case, because their lowest level of data organization is typically a fairly high-level file system e.
Furthermore, the complexity of the medium makes it difficult to write a useful boot program in one sector, even though optical media sectors are typically bytes each, four times the standard byte size of floppy and legacy hard disk sectors.
Therefore, optical media booting uses the El Torito standard, which specifies a way for an optical disk to contain an image of a high-density 1.
How BIOS Works
Therefore, CD-ROM drives boot as emulated floppy disk drives; the bootable virtual floppy disk can contain software that provides access to the optical medium in its native format. Boot failure[ edit ] The behavior if the BIOS does not find a bootable device has varied as personal computers developed.
One model of the original IBM PC was available with no disk drive; a cassette recorder could be attached via the cassette port on the rear, for loading and saving BASIC programs to tape. Later computers would display a message like "No bootable disk found"; some would prompt for a disk to be inserted and a key to be pressed, and when a key was pressed they would restart the boot process. Unlike earlier BIOSes, modern versions are often written with the assumption that if the computer cannot be booted from a hard disk, the user will not have software that they want to boot from removable media instead.
Lately, typically it will only be a specialist computer technician who does that, only to get the computer back into a condition where it can be booted from the hard disk. This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. October Learn how and when to remove this template message The environment for the boot program is very simple: CS is always zero and IP is initially 0x7C Because boot programs are always loaded at this fixed address, there is no need or motivation for a boot program to be relocatable.
SP points to a valid stack that is presumably large enough to support hardware interrupts, but otherwise SS and SP are undefined. A stack must be already set up in order for interrupts to be serviced, and interrupts must be enabled in order for the system timer-tick interrupt, which BIOS always uses at least to maintain the time-of-day count and which it initializes during POST, to be active and for the keyboard to work. The keyboard works even if the BIOS keyboard service is not called; keystrokes are received and placed in the character type-ahead buffer maintained by BIOS.
The boot program must set up its own stack or at least MS-DOS 6 acts like it mustbecause the size of the stack set up by BIOS is unknown and its location is likewise variable; although the boot program can investigate the default stack by examining SS: SP, it is easier and shorter to just unconditionally set up a new stack. At boot time, all BIOS services are available, and the memory below address 0x contains the interrupt vector table. DRAM refresh for all system DRAM in conventional memory and extended memory, but not necessarily expanded memory, has been set up and is running.
All memory at and above address 0x can be used by the boot program; it may even overwrite itself. Code in these extensions runs before the BIOS boots the system from mass storage. An extension ROM could in principle contain an entire operating system or an application program, or it could implement an entirely different boot process such as booting from a network. The motherboard BIOS typically contains code to access hardware components necessary for bootstrapping the system, such as the keyboard, display, and storage.
BIOS - Wikipedia
Even devices built into the motherboard can behave in this way; their option ROMs can be stored as separate code on the main BIOS flash chipand upgraded either in tandem with, or separately from, the main BIOS.
An add-in card requires an option ROM if the card is not supported by the main BIOS and the card needs to be initialized or made accessible through BIOS services before the operating system can be loaded usually this means it is required in the bootstrapping process. This is normally location FFFF0h, right at the end of the system memory.
They put it 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 BIOS startup program. If there are any fatal errors, the boot process stops. The BIOS looks for the video card. In particular, it looks for the video card's built in BIOS program and runs it.
Most modern cards will display information on the screen about the video card. This is why on a modern PC you usually see something on the screen about the video card before you see the messages from the system BIOS itself.
If any other device BIOSes are found, they are executed as well.
The BIOS displays its startup screen. The BIOS does more tests on the system, including the memory count-up test which you see on the screen. The BIOS will generally display a text error message on the screen if it encounters an error at this point; these error messages and their explanations can be found in this part of the Troubleshooting Expert.
bios vs cmos vs system rom
The BIOS performs a "system inventory" of sorts, doing more tests to determine what sort of hardware is 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 BIOSes can also dynamically set hard drive parameters and access modes, and will determine these at roughly this time.
Some will display a message on the screen for each drive they detect and configure this way. If the BIOS supports the Plug and Play standard, it will detect and configure Plug and Play devices at this time and display a message on the screen for each one it finds.
See here for more details on how PnP detects devices and assigns resources. The BIOS will display a summary screen about your system's configuration.
Checking this page of data can be helpful in diagnosing setup problems, although it can be hard to see because sometimes it flashes on the screen very quickly before scrolling off the top. The BIOS begins the search for a drive to boot from. Most modern BIOSes contain a setting that controls if the system should first try to boot from the floppy disk A: Having identified its target boot drive, the BIOS looks for boot information to start the operating system boot process.