Explain the following with the help of an example or a diagram, whichever is appropriate: 8m jun2006
(i) Parity bit
(ii) Floating point notation
(iii) Refresh rates in video controllers
(iv) An l/O channel
An error bit changes from 0 to 1 or 1 to 0. One of the simplest error detection codes is called parity bit.
Parity bit: – A parity bit is an error detection bit added to binary data such that it makes the total number of 1’s in the data either odd or even. For example, in a seven bit data 0110101 an 8th bit, which is a parity bit, may be added. If the added parity bit is even parity bit then the value of this parity bit should be zero, as already four 1’s exists in the 7-bit number. If we are adding an odd parity bit then it will be 1, since we already have four 1 bits in the number and on adding 8th bit (which is a parity bit) as 1 we are making total number of 1’s in the number (which now includes parity bit also) as 5, an odd number.
Similarly in data 0010101
Parity bit for even parity is 1
Parity bit for odd parity is 0
Floating point notation
Floating-point number representation consists of two parts. The first part of the number is a signed fixed-point number, which is termed as mantissa, and the second part specifies the decimal or binary point position and is termed as an Exponent. The mantissa can be an integer or a fraction. Please note that the position of decimal or binary point is assumed and it is not a physical point, therefore, wherever we are representing a point it is only the assumed position.
The following figure shows a format of a 16-bit and 32-bit floating-point number.
Refresh rates in video controllers
A special circuit called the Video Controller scans the video memory one row at a time and reads data value at each address sending the data out in a serial data stream.This data is displayed by a process called Scanning where the electron beam is swept across the screen one-line-at-a-time and left-to-right. This is controlled by a vertical and a horizontal field generated by electromagnets — one moving the beam horizontally and another vertically.
An I/O channel
I/O channels – an independent processor for Input/Output
The I/O channel represents an extension of the DMA concept. An I/O channel has the ability to execute I/O instructions, which gives complete control over the I/O operation. With such devices, the CPU does not execute I/O instructions. Such instructions are stored in the main memory to be executed by a special-purpose processor in the I/O channel itself. Thus, the CPU initiates an I/O transfer by instructing the I/O channel to execute a program in memory.