Information Technology Reference
In-Depth Information
2
Busses, Interrupts and PC Systems
2.1
Busses
The part that makes computers operate and allows devices to be easily plugged in is the com-
puter bus, which allows the orderly flow of data between one device and another. The PC,
and other computer systems, has an amazing number of different types of interfaces and bus
systems, these include the PC bus, ISA bus, PCI bus, CAN bus, AGP bus, games port, paral-
lel port, serial port, and so on.
The main elements of a basic computer system are a central processing unit (or micro-
processor), memory, and I/O interfacing circuitry. These connect by means of three main
buses: the address bus, the control bus and the data bus. A bus is a collection of common
electrical connections grouped by a single name. Figure 2.1 shows a basic system. External
devices such as a keyboard, display, disk drives can connect directly onto the data, address
and control buses or through the I/O interface circuitry.
Data Bus
I/O
Interfacing
Circuitry
CPU or
Micro-
processor
Memory
(RAM or
ROM)
Control
Lines
External
Devices
Address Bus
Figure 2.1 Block diagram of a simple computer system
Electronic memory consists of RAM (random access memory) and ROM (read only
memory). ROM stores permanent binary information, whereas RAM is a non-permanent
memory and loses its contents on a loss of power. Applications of this type of memory in-
clude running programs and storing temporary information. RAM is normally made up of
either DRAM (Dynamic RAM) or SRAM (Static RAM). DRAM uses a single capacitor and
a transistor to store a single bit of data, whereas SRAM uses six transistors, arranged as a
flip-flop device, to store a single bit of data. DRAM has the advantage that more memory
can be crammed onto a microchip (as only one transistor is required for each bit stored).
DRAM, though, has two major disadvantages: it is relatively slow (because of the charging
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