The PC 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
Here is an illustration of the start-up process:
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.
The busses are the PC's expressways. They are "wires" on the circuit board, which transmit data between different components in the computer.
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,"
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 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.
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 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
66 MHz bus
All the first Pentium based computers ran at 60 or 66 MHz on the system bus, which is 64 bit
100 MHz bus
The speed of the system bus will increase in 1998. Using PC100SDRAM a speed of 100 MHz is
well proven and later the use of RDRAMwill 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.