KarbosGuide.com. Module 2b1
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
 Data on the motherboard
1
The buses 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 typical Pentium board. We will
look at buses, chip sets and CPUs. Here is an
illustration of some of the motherboard "logic." You
can print it:
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 Record. This is
located in the very first sector on either hard disk
(C) or floppy drive A. From the MBR it reads the
boot-strap which points to the location of the
startup files of the Operating System.
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:
About the System Bus
In this module, you can read about the following
subjects, which add to our tour of the PC:
 PC buses, an intro ,  The system bus ,  66 MHz
bus ,  100 MHz bus
Introduction to the PC buses
The PC receives and sends its data from and to buses.
They can be divided into:
 The system bus, which connects the CPU with RAM
 I/O buses, 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 buses, as you can see
in this illustration. It is not completely correct, since
the architecture is much more complex, but it shows
the important point, that the I/O-buses usually derive
from the system bus:
The data flow on the motherboard
On the motherboard, you will find the CPU, which
is the "brain" of the PC and the buses. The buses
are the nerve system of the motherboard. They
connect the CPU to all the other components.
There are at least three buses, which you can see
below. You can read more about those on the
following pages.
You see the central system bus, which connects the
CPU with RAM. A bridge connects the I/O buses with
the system bus and on to RAM. The bridge is part of
the PC chip set, which will be covered in module 2c.
KarbosGuide.com. Module 2b1
2
3 different I/O buses
The I/O buses 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 buses:
 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 buses will be described later. Here,
we will take a closer look at the PC's fundamental
bus, from which the others are branches from.
CPUs in the
80486 family
System bus
width
System bus
speed
80486SX-25
32 bit
25 MHz
80486DX-33
32 bit
33 MHz
80486DX2-50
32 bit
25 MHz
80486DX-50
32 bit
50 MHz
80486DX2-66
32 bit
33 MHz
80486DX4-100
32 bit
40 MHz
5X86-133
32 bit
33 MHz
66 MHz bus
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 buses branch off from
it.
The system bus is on the motherboard. 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 motherboard. The faster the system
bus gets, the faster the remainder of the electronic
components must be..
The following three tables show different CPUs and
their system buses:
Older CPUs
System bus
width
System bus
speed
For a long time all Pentium based computers ran at 60
or 66 MHz on the system bus, which is 64 bit wide:
CPUs in the
Pentium family
System
bus width
System bus
speed
Intel P60
64 bit
60 MHz
Intel P100
64 bit
66 MHz
Cyrix 6X86
P133+
64 bit
55 MHz
AMD K5-133
64 bit
66 MHz
Intel P150
64 bit
60 MHz
Intel P166
64 bit
66 MHz
Cyrix 6X86
P166+
64 bit
66 MHz
Pentium Pro 200
64 bit
66 MHz
8088
8 bit
4.77 MHz
8086
16 bit
8 MHz
80286-12
16 bit
12 MHz
Cyrix 6X86
P200+
64 bit
75 MHz
80386SX16
16 bit
16 MHz
Pentium II
64 bit
66 MHz
80386DX25
32 bit
25 MHz
100 MHz bus
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:
The speed of the system bus has increased in 1998.
Using PC100 SDRAM a speed of 100 MHz is well proven
and the use of RDRAM will give us much higher
speeds.
However the rise from 66 MHz to 100 MHz has the
greatest impact on Socket 7 CPUs 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 its own speed independent of the system
bus.
KarbosGuide.com. Module 2b1
3
With the K6 the increase of system bus speed
gives a vastly improved performance since the
traffic between L1 and L2 cache crosses the
system bus.
133 MHz
Intel's 820 and 815 chipsets to be used with
Pentium III work with 133 MHz RAM as well as
several VIA chipsets do.
In AMD's Athlon the system bus architecture was
changed; it is not really a system bus any longer.
Hence Athlon chipsets may work with many types
of RAM.
Processor
Chip set
System
bus
speed
CPU
speed
Intel
Pentium II
82440BX
82440GX
100
MHz
350,
400, 450
MHz
AMD K6-2
Via
MVP3ALi
Aladdin
V
100
MHz
250,
300, 400
MHz
Intel
Pentium II
Xeon
82450NX
100
MHz
450, 500
MHz
Intel
Pentium III
i815
i820
133
MHz
600, 667
MHz and
up
AMD Athlon
VIA
KT133
and
others
200
MHz
600 1000
MHz
With the 100 MHz bus, we dicovered that
motherboards have to be well constructed with
good power supply and many capacitors.
Newer buses
As mentioned under AMD Athlon, "system bus" is
not that relevant a term looking at modern
motherboards. The bus to RAM becomes separated
from the other buses and this design opens up for
better bandwidth between the CPU and the RAM.
Intels use of Rambus RAM working at 400 MHz as
well as PC2100 RAM on non-Intel boards follows
this trend.
The DDRAM operates with interfaces working at
200, 266 and 333 MHz.