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| What Is The BIOS? |
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Computing Basics July 1994 • Vol.5 Issue 7 |
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What Is The BIOS? | |
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If you've ever lain awake in the middle of the night, pondering this great unknown, worry no more. Here is the straight scoop on BIOS, that mysterious creature at the heart of a computer. BIOS stands for Basic Input/Output System. It's a set of software instructions that performs necessary functions for the PC. More precisely, the BIOS is "firmware," says James Deutch, field applications engineer at Phoenix Technologies Ltd. It's software that's inside the hardware, encoded on a chip. Most often, BIOS comes on a ROM (read-only memory) chip seated on a PC's motherboard, or main circuit card. The chip may be an EPROM (erasable, programmable, read-only memory) or other kind of chip. The chip "looks like any other chip," says Dick Holmberg, software engineer at American Megatrends Inc. (AMI), "and usually holds 64KB of programs and data" (although 128KB and 256KB chips are becoming common).  Three Major Responsibilities The BIOS chip's read-only nature means it's permanent; it doesn't go away when you turn off the computer. It's also there when you turn on (boot) the computer, which points to BIOS' first function. "When the power is first applied to the machine," says Holmberg, "the BIOS essentially enables and turns on and configures all the hardware, all the chips inside the computer. "The second responsibility is to run tests to insure that everything's operating correctly." The BIOS literally may test a hundred things, including the memory chips, the keyboard, the video display, the disk drives and other peripherals, and internal components such as the timer and controllers. With its "Testing memory" message, the BIOS at work is what you first see when you boot your PC. It checks each component to "make sure it's there," says Deutch, "and make sure it's working." For instance, the BIOS sends the keyboard a command and looks for an acknowledgment, then asks it for an ID and looks for a response. The ID tells the BIOS whether you have a regular or extended keyboard so it can interpret your keystrokes correctly. Together, the startup and test functions make up the POST (Power On Self Test) services. Once they're complete, the PC is ready to run. But the BIOS' work isn't over. It also supplies run -time services while the computer is working. Now the BIOS acts as the interface, the "traffic cop," between you and the operating system (DOS or OS/2 or Unix). For instance, when you type dir on a DOS machine, says Holmberg, "the BIOS provides a service to read and write information from the hard drives and floppy drives." DOS doesn't need to know what type of equipment is installed physically on the computer. It can just ask the BIOS to retrieve a piece of information, and it will. "Likewise," says Holmberg, "the BIOS provides a set of services for displaying text on the monitor. DOS doesn't have to understand or know whether the system has a VGA card or an older CGA card or a monochrome card. DOS can simply give the BIOS something to display on the screen and the BIOS will do it." In short, says Deutch, input goes from "the keyboard, to the BIOS, to the operating system." And output goes from "the operating system to the BIOS to whatever the device is." The operating system doesn't have to know the peculiarities of every chip set, hard drive, or video adapter because they're all programmed into the BIOS. Therefore, concludes Deutch, "everybody's application will work on everybody's hardware." The BIOS doesn't handle all input/ output�only the "basic" variety, as its name implies. Windows and Windows applications that use fancier graphics do the processing themselves. The BIOS also doesn't handle devices such as SCSI ("scuzzy") hard drives or CD-ROM drives, which is why these come with their own BIOSes. From the Ground Up Although other computers had their versions of BIOS, IBM developed the first PC BIOS in 1979. According to Ray Bridenbaugh, marketing manager at AMI, this BIOS was the controlling program in the IBM PC that made it "PC-compatible." Because IBM kept it proprietary, building a comparable machine was difficult. Some companies (Compaq, for one) wrote their own BIOSes, but few matched IBM's. The computers suffered compatibility problems and the compatible market remained small. But in 1984, says Alan Painter, a Phoenix sales representative, "Phoenix provided the first commercially available PC-XT BIOS." Companies such as AMI followed suit, Painter says, "and that's when the PC-compatible market was created." How does a BIOS come about? A PC manufacturer such as IBM, Samsung, or NEC will go to a BIOS developer and give it a set of requirements. The nature of the BIOS will depend on such variables as the chip set and basic features of the motherboard. "Realize that the BIOS is not just one product," says Bridenbaugh. "We have probably a couple hundred different versions," each optimized for a particular hardware configuration. With specifications in hand, the developer creates BIOS routines. Phoenix, for one, employs more than 150 engineers writing BIOSes in a "clean room environment." This means the engineers developing an IBM-style BIOS "have never seen the IBM BIOS software," says Painter. "Engineers that have seen the IBM BIOS software are termed dirty engineers. They're no longer clean." Since these engineers can reveal trade secrets, even inadvertently, Phoenix won't hire them for the job. "We have a very, very strict recruiting phase that these guys go through," says Painter. "And then we have an auditing firm that audits by phone calls and so on. It's really something that's strictly enforced." Developers generally don't design BIOSes from scratch anymore. Instead, they build them in modules. They'll take a core program and then add blocks of code, depending on whether the BIOS is for a desktop or notebook PC, a 486 chip or the new Pentium chip, and so forth. "That's one reason a lot of OEMs [original equipment manufacturers] really like our BIOS," says AMI's Bridenbaugh. "We can adapt it very quickly and maintain compatibility." As the BIOS is being developed, it goes through comprehensive testing, both simulated and live. It's run on all the popular operating systems and on the ones in progress. The developer also tests the BIOS with a variety of applications, from spreadsheets to Flight Simulator. Fitting It All In Once a BIOS is finished, the developer generates a "rommable image." This means a BIOS that will fit inside and run from an EPROM or other chip. Using a mechanism called an EPROM programmer, the developer loads the rommable image onto chips supplied by a company such as Intel. "Then on the manufacturing line," says Painter, the PC producers can "just stuff the boards with this rommable image." Considering the hundreds of POST and run -time services a BIOS may perform, how do developers fit all the code into a mere 64KB chip? With great difficulty. PC manufacturers continue to demand functionality, says Painter, so "we have to get more and more creative on how we can add new features into this finite amount of space." One way is with compression and shadowing. Rather than load the BIOS code straight into a ROM chip, developers compress it first. Then, when the POST executes, the BIOS is copied to RAM (random-access memory) and expanded. The RAM is "volatile" (it exists only while the computer is on), but there is much more of it than ROM, so more commands can be executed. Not only that, but processes in RAM run faster because the PC doesn't have to read the chip constantly. Once the POST is done, that portion of the BIOS is no longer needed. Phoenix has pioneered the technique of reducing the BIOS' "footprint," or amount of code left in memory. "We're able to dispose of the code that is specific to the Power On Self Test," says Deutch, the code "that is not used by the run-time services." As much as 40KB of the 64KB BIOS is discarded, leaving only a 24KB "shadow BIOS"� "so we can minimize the amount of space that we take in the BIOS area itself," says Deutch, "and we're actually able to free up some space." Whither BIOS? Techniques such as shadowing are necessary because of all the features being crammed into BIOS. And with the continuing growth in the PC world, that need is only going to increase. To keep abreast of the latest trends, BIOS developers meet constantly with such industry leaders as Intel and Microsoft. Together they plot the future of the PC. One development is the PC's bus, which is the internal component that shuttles data back and forth between the central processing unit and the disks, printer, and other devices. The newest bus is the PCI (Peripheral Component Interconnect), which is replacing the old EISA standard. PCI is "a very high-performance, very high-speed 32-bit (and 64-bit eventually) bus," says Holmberg. "It is the standard now and you're going to see more and more of it. Even Apple is building it into several Macintosh machines." A PCI committee meets regularly to "test compatibility of different boards on different platforms," says Painter. "On one side you've got the card manufacturers, and on the other side you've got the platform OEMs like NEC and DEC and IBM." The BIOS developers are involved too, because as the bus changes, so must the BIOS to communicate with it. Another trend, one facilitated by PCI, is plug-and-play. "That's where you can basically open your system," says Bridenbaugh, "plug in a peripheral card like a video card, close the system, and you don't have to do any configuration software or any kind of jumper settings" to set it up. Adds Painter, "the computer reconfigures itself completely on the fly, transparent to the user." Plug-and-play is "done in cooperation between the operating system and the BIOS," says Bridenbaugh, "so the BIOS is extremely active in the plug-and-play environment." New memory cards based on the PCMCIA (Personal Computer Memory Card International Association) standards are driving BIOS changes. Originally there was just one type of PCMCIA card, but now "there are three sizes and there's a fourth on the horizon," says Painter. "Type 1 are the memory cards, Type 2 are a little fatter, so those could be memory cards, or they could also be LAN cards and FAX cards, and then Type 3 is rotating media. So companies like Seagate and Western Digital are putting out hard drives in PCMCIA format." With increasing environmental concerns, power management is becoming important, too. Any part of the computer not in use is just generating excess heat and wasting energy. This is especially true with notebook computers. With their limited battery power, conserving energy wherever possible is critical. APM (Advanced Power Management) BIOSes can put a component "to sleep" until it's needed, making smaller and smaller notebooks possible. New chips. New buses. New input and output devices. As long as PCs exist, BIOSes will continue evolving to keep them working.  by Robert Schmidt
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