io part1

introduction

Input–output (I/O) devices vary substantially in their characteristics. One distinguishing factor among input devices (and also among output devices) is their data processing rate, defined as the average number of characters that can be processed by a device per second. For example, while the data processing rate of an input device such as the keyboard is about 10 characters (bytes)/second, a scanner can send data at a rate of about 200,000 characters/second. Similarly, while a laser printer can output data at a rate of about 100,000 characters/second, a graphic display can output data at a rate of about 30,000,000 characters/second.

I/O protocol

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 information 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 availability 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.