Information Technology Reference
In-Depth Information
useful operation(s) while a huge amount of data is being transferred between the disk
and the memory. This is the essence of the direct memory access (DMA) mechanism
discussed in Section 8.4.
We begin our discussion by offering some basic concepts in Section 8.1.
8.1. BASIC CONCEPTS
Figure 8.1 shows a simple arrangement for connecting the processor and the memory
in a given computer system to an input device, for example, a keyboard and an output
device such as a graphic display. A single bus consisting of the required address, data,
and control lines is used to connect the system's components in Figure 8.1.
The way in which the processor and the memory exchange data has been
explained in Chapters 6 and 7. We are here concerned with the way the processor
and the I
O devices exchange data. It has been indicated in the introduction part
that there exists a big difference in the rate at which a processor can process infor-
mation and those of input and output devices. One simple way to accommodate this
speed difference is to have the input device, for example, a keyboard, deposit the
character struck by the user in a register (input register), which indicates the avail-
ability of that character to the processor. When the input character has been taken by
the processor, this will be indicated to the input device in order to proceed and input
the next character, and so on. Similarly, when the processor has a character to output
(display), it deposits it in a specific register dedicated for communication with the
graphic display (output register). When the character has been taken by the graphic
display, this will be indicated to the processor such that it can proceed and output the
next character, and so on. This simple way of communication between the processor
and I
/
O devices, called I
/
O protocol, requires the availability of the input and
output registers. In a typical computer system, there is a number of input registers,
each belonging to a specific input device. There is also a number of output registers,
/
Processor
Memory
System Bus
Input Device
(Keyboard)
Output Device
(Graphic Display)
Figure 8.1 A single bus system
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