Question

Q2.Discuss Random access method and Direct access methods in details. Do a comparison and discuss which method is better.

Answer 1

DIRECT MEMORY ACCESS-

DMA is a procedure where the device(input output) is given permission to transmit or receive data straight from or to the main memory. The Central processing unit is bypassed so that memo. operations can be fastened up. This entire procedure is controlled by DMAC, full form- Direct Memory Access Controller.

In DMA, data is filled in tracks which have read and write heads. With this data, we are able to access the track in a orderly manner. Examples where it is used- Magnetic disks.

In DMA, CPU lets go the bus upto some clock cycles.

DMAC has 3 modes-

1) TRANSPARENT

2) CYCLE STEALING

3) BURST MODE

DMA is basically a part of harware, helping us to access memory without any involvement of CPU.

CPU gives the starting location and count of words of the block which gets transported to/from memo. to DMAC. Bus gets vacant for DMAC so that data can be transmitted.

RANDOM ACCESS METHOD-

There are mainly two types of RA methods-

1) ALOHA- Pure/slotted.

2) CSMA- Better version of Aloha. Full form- Carrier sense multiple access. Types- CSMA/CD and CSMA/CA.

This method( Random access)  is also known as contention method. No bus terminal is given priority over any other terminal. No permission method is present for a particular terminal to transmit data. The protocol present for a terminal chooses whether the data is to be sent or not. If the medium is busy, the data can't be transferred and vice-versa.

No particular time for a terminal to transfer data. It is done randomly.

There are no guidelines of which terminal will transmit next, but terminals race on having the access to medium. No terminal has control over other terminals at all.

Comparison between them- There is no substantial difference in data acess speed. However, they are implemented in different ways.

Random access method is executed through Random access memory wiring.

Direct access method makes use of secondary memory.

So, it is true that there is no substantial difference between the two methods, but RAM has a slight edge in terms of speed over Direct access method.