Introduction to PC Bios

Introduction to the BIOs

Software Layer Model

Layer #	Layer
0	Hardware
1	BIOS
2	Operating System
3	Applications

BIOS stands for Basic Input/Output System and is software that manages hardware and allows the operating system to talk to the various components. The BIOS is also responsible for allowing you to control your computer's hardware settings, for booting up the machine when you turn on the power or hit the reset button and various other system functions.

The term BIOS is typically used to refer to the system BIOS, however, various other components such as video adapters and hard drives can have their own BIOSes hardwired to them. During the rest of this section, we will be discussing the system BIOS. The BIOS software lives on a ROM IC on the motherboard known as a Complementary Metal Oxide Semiconductor(CMOS). People often incorrectly refer to the BIOS setup utility as CMOS, however, CMOS is the name of the physical location that the BIOS settings are stored in.

Basic CMOS Settings:

Printer Parallel Port
- Unidirectional - Single direction communication.
- Bi-directional - Two directional communication. Used by HP printers.
- ECP(Extended Capability Port) - Same as Bi-directional but uses a DMA to bypass processor and speed up transfer.
- EPP(Enhanced Parallel Port) - Same as bi-directional and offers an extended control code set.
COM/Serial Port
- Memory Address - Each COM port requires a unique memory address.
- IRQ - Every COM port requires a unique IRQ to talk to the CPU.
- COM1 = IRQ4 and 03F8
- COM2 = IRQ3 and 02F8
Hard Drives
- Size - The Size is automatically detected by the computer.
Primary Master/Secondary Slave
- Each hard drive has a controller built in the drive that controls the drive.
- If two drives were on the same channel the adapter could get confused.
- By setting one as a master it tells it which is in charge.

BIOS services are accessed using software interrupts, which are similar to the hardware interrupts except that they are generated inside the processor by programs instead of being generated outside the processor by hardware devices.

BIOS routines begin when the computer is booted and are mad up of 3 main operations. Processor manufacturers program processors to always look in the same place in the system BIOS ROM for the start of the BIOS boot program. This is normally located at FFFF0h - right at the end of the system memory.

First, the Power On Self Tests(POST) are conducted. These tests verify that the system is operating correctly and will display an error message and/or output a series of beeps known as beep codes depending on the BIOS manufacturer.

Second, is initialization in which the BIOS looks for the video card. In particular, it looks for the video card's built in BIOS program and runs it. The BIOS then looks for other devices' ROMs to see if any of them have BIOSes and they are executed as well.

Third, is to initiate the boot process. The BIOS looks for boot information that is contained in file called the master boot record(MBR) at the first sector on the disk. If it is searching a floppy disk, it looks at the same address on the floppy disk for a volume boot sector. Once an acceptable boot record is found the operating system is loaded which takes over control of the computer.

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