Electronics Online

I/O busses connect the CPU to all other components, except RAM. Data are moved on the busses from one component to another, and data from other components to the CPU and RAM. The I/O busses differ from the system bus in speed. Their speed will always be lower than the system bus speed. Over the years, different I/O busses have been developed. On modern Pentium PC’s, you will find at least two significant busses, and one less significant:

• The ISA bus, which is an older low speed bus.
• The PCI bus, which is a new high speed bus.
• The USB bus (Universal Serial Bus), which is a new low speed bus.

As mentioned earlier, I/O busses are really derivatives from the system bus. On the system board it ends in a controller chip, which forms a bridge to the two other busses. The essential in modern PC’s is fast busses. Let us compare the two primary I/O busses:

Clearly,

The PC receives and sends its data from and to busses. They can be divided into:

• The system bus, which connects the CPU with RAM
• I/O busses, which connect the CPU with other components.

The point is, that the system bus is the central bus. Actually, it connects to the I/O busses, as you can see in this illustration:

You see the central system bus, which connects the CPU with RAM. A bridge connects the I/O busses with the system bus and on to RAM. The bridge is part of the PC chip set.

During start-up. the BIOS programs are read from a ROM chip. BIOS is abbreviation of Basic Input Output System and those are programs, which are linked to specific hardware systems. For example, there is a BIOS routine, which identifies how the PC reads input from the keyboard.

BIOS is a typical link in the IBM compatible PC design. The BIOS programs control hardware, the user (programmer) controls hardware via a call to BIOS.

BIOS typically occupy 1 MB, and the programs are saved ROM chips on the system board.

During start-up, BIOS is read from ROM chips. That information is supplemented with the system data saved in CMOS. Furthermore, there is BIOS code on the expansion cards. The expansion cards are external hardware, as interpreted by the system board, and the BIOS code, which is linked to the expansion card, must be included in the configuration.

Therefore, this expansion card ROM is read during start-up, and the program code is woven together with other BIOS data. It is all written into RAM, where it is ready for the operating system, as you can see here:

Otherwise, the BIOS routines are not always in use.