Information Technology Reference
In-Depth Information
and discharging of the storage capacitors) and it requires that the complete contents of its
memory be refreshed with power many times a second (because the tiny capacitors loose
their charge over a short time). This power refresh is thus wasteful of electrical power and
leads to heat dissipation.
The microprocessor is the main controller of the computer. It only understands binary
information and operates on a series of binary commands known as machine code. It fetches
binary instructions from memory, decodes these instructions into a series of simple actions
and carries out the actions in a sequence of steps. A system clock synchronises these steps.
To access a location in memory the microprocessor puts the address of the location on the
address bus. The contents at this address are then placed on the data bus and the microproc-
essor reads the data from the data bus. To store data in memory the microprocessor places
the data on the data bus. The address of the location in memory is then put on the address bus
and data is read from the data bus into the memory address location.
The classification of a microprocessor relates to the maximum number of bits it can proc-
ess at a time, that is their word length. The evolution has gone from 4-bit, 8-bit, 16-bit, 32-bit
and to 64-bit architectures.
2.1.1 Bus specification
The basic specification of a computer can be determined by analysing the performance of the
busses within the system. Each bus performs a specific function and is suited to the devices
that connect to it. The basic specifications for busses include:
•
Data rate
(in bytes per second or bits per second). This defines the maximum amount of
data that can be transferred, at a time. For example, the ISA bus has a maximum data
rate of 16 MB/s, Gigabit Ethernet has a maximum data rate of 125 MB/s, and the local
bus which connects a PC processor to local memory can have a data rate of over
800 MB/s (64 bits at 100 MHz).
•
Maximum number of devices which connect to the bus
. The number of devices which
connect to a bus can have a great effect on its performance as they all provide an electri-
cal loading on the bus and the more that connect to the bus, the greater the overhead of
bus arbitration will be. Standard SCSI only allows a maximum of seven devices to be
connected to the bus, whereas Ethernet can allow thousands of devices to connect to the
bus.
•
Bus reliability
. This defines how well the bus copes with any errors which occur on the
bus. Some busses, especially in industrial environments, can be susceptible to externally
generated noise. A good bus should be able to detect if it has received data which has
been corrupted by noise (or was sent incorrectly).
•
Data robustness
. This is the ability of the bus to react to faults within the bus or from
the malfunctioning of connected devices. Busses such as the CAN bus can isolate incor-
rectly operating devices.
•
Electrical/physical robustness
. This is the ability of the bus to cope with electrical
faults, especially due to short-circuits and power surges. Problems can also be caused by
open circuit electrical connections, although these tend not to cause long term damage to
the bus. The physical robustness of a bus is also important, especially in industrial or
safety critical situations.
•
Electrical characteristics
. This involves the basic electrical parameters of the bus, such
as the range of voltage levels used, electrical current ranges, short-circuit protection sys-
