Technical The BIOS and CMOS

In the left margin you will find links to the most common pages found in the BIOS setup. My experience is mainly with AMI, Award and Phoenix although I have worked with many branded machines, such as Dell, Compaq, Packard Bell, Apricot for example and although their BIOS look very different the principles are very much the same.

The ROM BIOS Settings

These pages are mainly about Pentium motherboards. There are a lot of settings in our BIOS pages, some from older boards. Each page has comprehensive descriptions of many BIOS fields and options. Any new fields that are later added, will be from ATX motherboards, until that too becomes outdated.

There are two things in a computer referred to as BIOS. The ROM BIOS which this page is about and the DOS BIOS which does a similar job on a different level for DOS.

The ROM BIOS is a program that provides the most basic low level operations on the computer. The BIOS could be thought of as an interface to the hardware in the computer, somewhere between hardware and software - firmware. As the software is encoded in a type of ROM (Read Only Memory) its contents are not lost after powering off the PC.
There are two parts to the ROM BIOS program (except the original IBM PC which contained BASIC in a third part of the ROM BIOS). Some of the delay between switching on a PC and when it starts working is caused by the POST (Power On Self Test), programs that test the functionality and initialise the PC.

The second and more interesting part the routines, provide a list of services for use by a operating system and applications, such as disk input output, including error checking. All of the BIOS program is stored in 128 KB of memory E000 to F000 hex and the amount used depends on the complexity of the ROM BIOS software. Some expansion cards have their own ROM BIOS to control the cards own functions. The ROM BIOS is written to work with the computers hardware. The BIOS will for instance ensure that the right port sends the right commands to the right hardware and such things as timing involving close co-operation between programs and hardware.

The initialisation of the PC during POST creates interrupt vectors to the proper interrupt handling routines and sets up registers with parameters. Included in the initialisation are steps to tell the PC what hardware is present. The BIOS checks and initialise all equipment it knows about. The BIOS also checks for additions to the BIOS such as ACPI.
The boot routine tries to start DOS or any other operating system. The BIOS reads a boot record from the beginning of a disk then hands control of the PC to the boot record program that begins to load DOS into the PCs memory. Hardware interrupt handling and service handling routines carry out requests from programs and hardware, thus taking care of the PCs hardware separately from requests that programs make. Additions to the BIOS perform start-up and initialisation work then pass control back to the main BIOS. Now support has been added for new equipment. Upgrading the BIOS is possible using EEPROM or Flash RAM, chips that remember data stored in them until they receive an instruction to erase and replace the contents.

Power Management at the BIOS level watches the ports for activity and if unused for periods of time, system features either turn off or slow down.
Power Management through ACPI (Advanced Configuration and Power Interface) allows the operating system to control power in the system. For ACPI to fully function the device drivers and applications used by the operating system must be ACPI aware.

Plug and play (pray) enabled the PC to expand in the market. Limiting some of the difficulty to use a PC with standards such as plug and play meets needs of non technical users. Any level of plug and play compliance makes upgrading in general less problematic. A fully plug and play compliant system means the application must be able to request system resources and dynamically adjust requirements, the operating system should be capable of managing the available resources so conflicts are eliminated and support automatic allocation, installation or configuration of device drivers. The BIOS must isolate and interrogate devices, reporting or reconfiguring conflicts and support reconfiguration requests from the operating system. Also the expansion cards used in the PC must be plug and play. PCI card slots reserve configuration space and define specific addressing. To overcome the lack of space on ISA slots an algorithm is combined on a plug and play card to report presence and configuration detail in a plug and play manner.

The plug and play sequence in a full plug and play system functions as power on, plug and play devices required to boot activate using defaults, devices not required to boot are inactive. Prior to POST the BIOS will isolate a plug and play card, assign it a handle then read the resource data, then for the activated devices the resource assignments are checked for conflicts before activating the device. Other inactive devices are configured or left in an inactive state then POST then Boot.

Cache and Shadow options in the BIOS are often best set to the default. However most graphics cards should not have caching or shadowing enabled as unnecessary cycles are used to read the cache to determine if the requested data is in high speed memory before a read from, write to or flush to system RAM is made, wasting value cycles in the process. The CPU internal cache should under almost all situations be enabled and with today's Pentium III CPU cache clocking at the speed of the CPU bus, much higher system performance is maintained. External cache should also be enabled in most systems. Caching uses the portions of the motherboards secondary cache to maintain instructions or data previously read or written to the system RAM using high speed algorithms. Caching devices has more of an impact on a DOS only system and the real performance impact can only be accurate when proper benchmarking and diagnostics are adhered. Changing only one fields option and re running the benchmarks is the only way to be sure of any impact the setting has on system performance.

Shadowing copies code from the slower ROM to faster system RAM and will use upper memory. Most graphics cards do not perform optimally if shadowing of the video BIOS is enabled. Shadowing may or may not improve performance in a DOS only system.