Buses
Warning: some of the terminology
is used inconsistently within the
field
What is a bus?
A bus is a basically just wires through which
data travels from one part of a computer to
another.

Usually it’s implied the path is shared by a number
of parts
There is more than one bus and more than
one kind of bus




Data, address, control
System, expansion, local, external
USB, AGP, ISA, EISA, MCA, PCI, VESA
(Parallel, serial)
What does it carry?
In the simple architecture we’ve been
considering, data and addresses travel
on the same bus, while control
information traveled along individual
wires (not shared)

Saves on pins
In more complicated architectures, they
are separate
Data, address and control
buses
The data bus carries data and instructions
The address bus carries information about
where the data should go.
The control bus carries information from the
CPU to other parts of the computer, telling
what they should be doing

Some use control bus as a synonym for system
bus
Bus characteristics
The highway analogy: moving data
along the buses is like moving cars on
the highway.
Bus width (number of lanes)

How many bits are moving around in
parallel
Bus speed (speed limit)

How fast those bits are moving
Memory size
The width of the system’s address bus puts
an upper limit on the amount of memory
locations
For example, if the address bus width is 32,
then there are 232 (4,294,967,296) addresses
Note that instead of addressing individual
words, computers usually address individual
bytes, so that would mean 4 GB
Of course, most computers have a lot less
than 4 GB of memory, it’s just an upper limit
Bus speeds
Measured in MHz (millions of cycles per
second)
It doesn’t make much sense to have a
very fast processor speed and a slow
bus speed; they should be compatible

The bus speed is slower than the processor
speed and often limits the speed of the
computer
Multiple buses
A bus should not be too long; its speed
is determined in part by its length
Also slower devices do not need faster
(and more expensive) buses.
The computer should not be held back
by the slowest device
Solution: More than one bus
System bus
The system bus connects the CPU,
memory and other motherboard parts
This bus should be well coordinated
with the processor and memory access
speeds
Other buses must interface with the
system bus if they want to interact with
the processor
Frontside and backside buses
The bus within a processor that
connects the CPU with main memory.
It's used to communicate between the
motherboard and other components in
a computer system.
In contrast, a backside bus connects
the CPU to a Level 2 cache.
Expansion bus
The expansion bus connects the system bus
to the expansion slots (where cards are
inserted to expand the computer’s
capabilities)

This bus usually works at slower speeds
Early PCs used an expansion bus called the
ISA bus.
Most PCs today have a much faster PCI bus
but usually have an ISA bus for backward
compatibility.
Local bus
If a device or devices require a great
deal of speed (e.g. video), then one
solution is for the device to have its
own high-speed, direct (or nearly
direct) connection to the processor.
Such a connection is called a local bus
Can only support a few devices
ISA
Industry Standard Architecture (ISA) is
the bus used in early IBM PC and their
clones.
The AT version of the bus is called the
“AT” bus and became an industry
standard.
Worked at 8.33 MHz
Plug and Play
In 1993, Intel and Microsoft introduced
a version of the ISA called Plug and
Play ISA.
Plug and Play ISA enables the operating
system to do the configuring, instead of
the user setting switches and jumpers
PCI
Peripheral Component Interconnect, a local
bus standard introduced by Intel.
PCI is a 64-bit bus, though it is usually
implemented as a 32-bit bus.
It can run at clock speeds of 33 or 66 MHz.
At 32 bits and 33 MHz, it yields a throughput
rate of 133 MBps (Mega bits per second).
EISA and MCA
Between ISA and PCI were some short-lived
bus architectures
Extended Industry Standard Architecture
(EISA)
Micro Channel Architecture (MCA)
The principal difference between EISA and
MCA is that EISA is backward compatible with
the ISA bus, while MCA is not.
VLB
Short for VESA Local-Bus, a local bus
created by the Video Electronics
Standard Association (VESA).
33 MHz
Although it was used a lot in PCs made
in 1993 and 1994, PCI has become
more popular
external bus
A bus that connects a computer to
peripheral devices.
Two examples are the Universal Serial
Bus (USB) and IEEE 1394.
USB
Universal Serial Bus, a new external bus
standard that supports data transfer rates of
12 Mbps (12 million bits per second).
A single USB port can be used to connect up
to 127 peripheral devices, such as mice,
modems, and keyboards.
USB also supports Plug-and-Play installation
and hot plugging.
bus mastering
Refers to a feature supported by some
bus architectures that enables a
controller connected to the bus to
communicate directly with other devices
on the bus without going through the
CPU. Most modern bus architectures,
including PCI, support bus mastering
because it improves performance.
DMA
Direct Memory Access
Gives a peripheral device access to the
memory without going through the CPU
Speeds up data transfer
Three State Logic
A
E
Output
0
0
Z (High impedance)
0
1
0
1
0
Z (High impedance)
1
1
1
Tri-state buffer
In the high impedance state
In the high impedance state
In the “enabled” state
In the “enabled” state