Input–Output Design and Organization - Fundamentals of Computer Organization and Architecture - page 166

4. The instruction decoding will gate the data available on the data bus into the
input of a particular register in the CPU, normally the accumulator.

Output operations can be performed in a way similar to the input operation
explained above. The only difference will be the direction of data transfer, which
will be from a specific CPU register to the output register in the specified output
device. I

O operations performed in this manner are called programmed I / O.
They are performed under the CPU control. A complete instruction fetch, decode,
and execute cycle will have to be executed for every input and every output oper-
ation. Programmed I

/

O is useful in cases whereby one character at a time is to be
transferred, for example, keyboard and character mode printers. Although simple,
programmed I

/

O is slow.
One point that was overlooked in the above description of the programmed I

/

Ois

/

how to handle the substantial speed difference between I O devices and the pro-

cessor. A mechanism should be adopted in order to ensure that a character sent to
the output register of an output device, such as a screen, is not overwritten by the
processor (due to the processor's high speed) before it is displayed and that a char-
acter available in the input register of a keyboard is read only once by the processor.
This brings up the issue of the status of the input and output devices. A mechanism
that can be implemented requires the availability of a Status Bit (B in ) in the interface
of each input device and Status Bit (B in ) in the interface of each output device.
Whenever an input device such as a keyboard has a character available in its
input register, it indicates that by setting B in ¼

/

1. A program in the processor can
be used to continuously monitor B in . When the program sees that B in ¼ 1, it will

interpret that to mean a character is available in the input register of that device.
Reading such character will require executing the protocol explained above. When-
ever the character is read, then the program can reset B in ¼ 0, thus avoiding multiple

read of the same character. In a similar manner, the processor can deposit a character
in the output register of an output device such as a screen only when B out ¼

0. It is

only after the screen has displayed the character that it sets B out ¼ 1, indicating to

the program that monitors B out that the screen is ready to receive the next character.
The process of checking the status of I / O devices in order to determine their readi-

ness for receiving and or sending characters, is called software I / O polling.Ahard-

ware I / O polling scheme is shown in Figure 8.5.

/

Figure 8.5 Hardware polling scheme