Input–Output Design and Organization - Fundamentals of Computer Organization and Architecture - page 164

The main advantage of the shared I O arrangement is the separation between the

/

memory address space and that of the I O devices. Its main disadvantage is the need

to have special input and output instructions in the processor instruction set. The
shared I

/

O arrangement is mostly adopted by Intel.
The second possible I

/

O arrangement is to deal with input and output registers as
if they are regular memory locations. In this case, a read operation from the address
corresponding to the input register of an input device, for example, Read Device 6,is
equivalent to performing an input operation from the input register in Device #6.
Similarly, a write operation to the address corresponding to the output register of
an output device, for example, Write Device 9, is equivalent to performing an
output operation into the output register in Device #9. This arrangement is called
memory-mapped I / O. It is shown in Figure 8.3.
The main advantage of the memory-mapped I

/

O is the use of the read and write
instructions of the processor to perform the input and output operations, respectively.
It eliminates the need for introducing special I

/

/ O instructions. The main disadvantage

of the memory-mapped I O is the need to reserve a certain part of the memory address

/

space for addressing I O devices, that is, a reduction in the available memory address

space. The memory-mapped I / O has been mostly adopted by Motorola.

/

8.2. PROGRAMMED I / O

In this section, we present the main hardware components required for communi-
cations between the processor and I / O devices. The way according to which such

Address Bus

Data Bus

Processor

Memory

Control Lines

Input Device
(Keyboard)

Output Device
(Graphic Display)

Figure 8.3 Memory-mapped I / O arrangement