A computer's basic input/output system (BIOS) is a program that's stored in nonvolatile memory such as read-only memory (ROM) or flash memory, making it firmware. The BIOS (sometimes called ROM BIOS) is always the first program that executes when a computer is powered up.
Here's what happens during the boot process (see steps in diagram below):
Power is turned on.
The CPU hands control over to the BIOS.
The BIOS runs a program called Power-On Self Test, which determines how much memory the computer has and then confirms that critical low-level hardware is operating correctly. Any errors are indicated by sequences of audible beeps. After this, the BIOS disables all configurable devices.
The BIOS identifies all of the computer's peripheral devices, such as hard drives and expansion cards. It first looks for plug-and-play devices and assigns a number to each, but it doesn't enable the devices at this time.
The BIOS locates the primary boot or initial program load (IPL) device. This is usually a storage device such as a hard drive, floppy drive or CD-ROM that holds the operating system, but it can be a network card connected to a server. The BIOS also locates all of the system's secondary IPL devices.
The BIOS builds a system resource table, assigning conflict-free resources according to which devices it found and the configuration data stored in nonvolatile RAM.
It selects and enables the primary input (keyboard) and output (monitor) devices, so that if trouble occurs during the boot process, the BIOS can display a recovery screen and allow the user to select a stored configuration of system settings that are known to work. The BIOS captured these settings the last time the computer booted successfully, and it stores them in nonvolatile RAM.
It scans for non-plug-and-play devices, including the Peripheral Component Interconnect (PCI) bus, and adds data from their ROMs to its resource table.
The BIOS resolves device conflicts and configures the chosen boot device.
It enables plug-and-play devices by calling their option ROMs with appropriate parameters.
It starts the bootstrap loader. If, for some reason, the default IPL fails to load the operating system, the BIOS tries the next IPL device in the list.
The IPL device loads the operating system into memory.
- The BIOS hands over control to the operating system, which may make other resource assignments.
The BIOS also contains a setup program with which the user can configure hardware-based settings such as computer passwords, the time and the date. Because the BIOS configures a primary input and output device during the boot process, a user can run the setup program and adjust device settings, perhaps even selecting a different IPL device, such as a second hard drive, when the computer fails to boot.
A major change in PC BIOS functions occurred in 1995 with the advent of Windows 95. The new operating system featured plug-and-play functionality, which not only simplified the job of adding expansion cards but also helped to define a consistent mechanism to let the BIOS recognize and configure the devices in a system.
Early systems assumed that a device would always require the same resources - a disk controller's interrupt number and range of I/O addresses, for example. It was believed that these would never change or that they were static in nature and thus needed to be assigned only once.
However, plug-and-play technology gives the BIOS the freedom to modify the interrupt number and I/O addresses that the disk controller uses to avoid resource conflicts.
With Universal Serial Bus and IEEE 1394 connections, devices can be hot-plugged. In other words, they can appear or disappear without warning.
This means that the BIOS must store system resource information for every device the system has ever known about, and do it in a dynamic way so that system resources, such as interrupt number, address range or device identity, can be reassigned without requiring a reboot.
Thompson is a training specialist at Metrowerks Inc. Contact him at thompson@metrowerks.com.
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Upgrading a BIOS
When a computer is to be upgraded with new hardware, such as a larger hard drive, more memory or a new video card, it's often discovered that the computer's BIOS doesn't support all the capabilities of the new hardware. Perhaps the most dramatic evidence of this occurred when hard drives grew beyond 4GB and then 8GB in size. At that time, it was quite possible to install, say, a 12GB hard drive and then discover that the computer could use only the first 8GB.
The answer to the problem is to upgrade the BIOS chip. Most computers made in recent years can have their systems updated by a flash program, which installs new instructions and capabilities. Normally, the information and files needed to do so are available on the Web site of the computer or motherboard maker.
Unfortunately, it's critical to understand how drastic a step a BIOS upgrade can be. Before you do it, it's a good idea to back up all data from the hard drive. Also, see if there's a recovery jumper switch that will allow you to recover the original BIOS. Although a BIOS upgrade is usually trouble-free, it's possible for the process to damage or destroy the BIOS chip and thus make the computer unusable.