An illustrated Guide to the PC System BUS
Click & Learn. Module 2b. WWW.MKDATA.DK
About the System Bus
In this module, you can read about the following subjects, which add to our tour of the PC:
●The boot process, the last step in the PC start-up
●The data flow on the system board
●Introduction to the PC busses
●The system bus, which is essential circuitry on the system board
●66 MHz bus
●100 MHz bus
The boot process
The last step in the PC start-up is reading the operating system. The start-up program is instructed to find the Master boot sector. The boot sector is the very first sector on either hard disk (C) or floppy drive A.
By default, the PC will look for a boot sector in floppy drive A. That is why the PC "drops dead" if there is a different diskette in A drive. If there is no diskette in A drive, the start-up program will search for the boot sector on hard drive C. When the boot sector is found, a small program segment (boot-strap) is read from there. The boot-strap then takes over control of the PC. The start-up program has done its job. Now DOS, Windows, or another operating system takes control.
Read more about boot sectors, etc. in module 6a, which deals with file systems.
Here is an illustration of the start-up process:
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An illustrated Guide to the PC System BUS
The data flow on the system board
On the system board, you will find the CPU, which is the "brain" of the PC and the busses. The busses are the nerve system of of system board. They connect the CPU to all the other components. There are at least three busses, which you can see below. You can read more about those on the following pages.
●The busses are the PC's expressways. They are "wires" on the circuit board, which transmit data between different components. One "wire" can move one bit at a time.
In the following text, we start from a modern Pentium board. We will look at busses, chip sets and CPU's. Here is an illustration of the system board "logic," which you can print out.
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An illustrated Guide to the PC System BUS
Here, you can switch to read about what data really are. Or click to move to the page about the CPU. Or read more about data, and how they are saved in the PC file systems. Or just continue here.
Introduction to the PC busses
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
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An illustrated Guide to the PC System BUS
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, which will be
covered in module 2c.
3 different I/O busses
The I/O busses move data. They connect all I/O devices with the CPU and RAM. I/O devices are those components, which can receive or send data (disk drives, monitor, keyboard, etc. ). In a modern Pentium driven PC, there are two or three different I/O busses:
●The ISA bus, which is oldest, simplest, and slowest bus.
●The PCI bus, which is the fastest and most powerful bus.
●The USB bus, which is the newest bus. It may in the long run replace the ISA bus.
The three I/O busses will be described later. Here, we will take a closer look at the PC's fundamental bus, which the others are branches from:
The system bus
The system bus connects the CPU with RAM and maybe a buffer memory (L2-cache). The
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An illustrated Guide to the PC System BUS
system bus is the central bus. Other busses branch off from it.
The system bus is on the system board. It is designed to match a specific type of CPU. Processor technology determines dimensioning of the system bus. At the same time, it has taken much technological development to speed up "traffic" on the system board. The faster the system bus gets, the faster the remainder of the electronic components must be..
The following three tables show different CPU's and their system busses:
We see, that system bus speed follows the CPU's speed limitation. First at the fourth generation CPU 80486DX2-50 are doubled clock speeds utilized. That gives the CPU a higher internal clock frequency. The external clock frequency, used in the system bus, is only half of the internal frequency:
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An illustrated Guide to the PC System BUS
66 MHz bus
All the first Pentium based computers ran at 60 or 66 MHz on the system bus, which is 64 bit wide:
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An illustrated Guide to the PC System BUS
100 MHz bus
The speed of the system bus will increase in 1998. Using PC100 SDRAM a speed of 100 MHz is well proven and later the use of RDRAM will give us much higher speeds.
However the raise from 66 MHZ to 100 MHZ will have the greatest impact on socket7 CPU's and boards. In the Pentium-II modules 70-80% of the traffic is inside the SEC-module, holding both L1 and L2 cache. And the module has it own speed independent of the system bus. But with the K6 the increase of system bus speed will give a vastly improved performance since the traffic between L12 and L2 cache crosses the system bus.
Read more about I/O busses in module 2c
Read more about the system boards chip set in module 2d
Read more about RAM in module 2e
To overview
Last revised: 19 May 98. Copyright (c) 1996, 1997, 1998 by Michael B. Karbo. WWW.MKDATA.DK.
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An illustrated Guide to the PC System BUS
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