Give classification of memory based on the method of access. Also discuss construction and working of magnetic disk and various components of disk access time.
Give classification of memory based on the method of access. Also discuss construction and working of magnetic disk and various components of disk access time.
Quick answer
A magnetic disk has one or more platters made of aluminium or glass, coated with magnetic material. They are fixed on a spindle which rotates at a constant speed, such as 7200 rpm. Each surface is divided into concentric circles called tracks. Each track is divided into sectors, and a sector is the smallest unit that can be read or written (often 512 bytes). The tracks at the same position on all surfaces form a cylinder. A read/write head is placed over each surface, and all heads are fixed on one arm assembly that moves together. A disk controller drives the arm and moves data.
Classification by method of access
| Type | How data is reached | Example |
|---|---|---|
| Random access | any location in the same time, using its address | RAM, ROM, cache |
| Sequential access | data is reached in order, passing all earlier data | magnetic tape |
| Direct (semi-random) access | the head goes to the nearby area directly, then reads in order | magnetic disk |
| Associative access | a word is found by its content, not by its address | cache tag memory, CAM |
Construction of a magnetic disk
Working
To write, the head sends a current through its coil, which magnetizes a tiny spot on the surface in one of two directions, standing for 1 and 0. To read, the moving magnetized spots induce a small current in the head, which is sensed as bits. To reach a sector, the arm moves the heads to the right cylinder. Then the disk turns until the sector is under the head. Then the data is transferred.
Components of disk access time
$$T_{access} = T_{seek} + T_{rotation} + T_{transfer} + T_{controller}$$
- Seek time is the time to move the head to the required track. It is the largest part, a few milliseconds.
- Rotational latency is the time for the needed sector to come under the head. On average it is half a rotation, $\frac{60}{2 \times \text{rpm}}$ seconds. For 7200 rpm it is $60/(2 \times 7200) = 4.17$ ms.
- Transfer time is the time to read the sector bytes, equal to the data size divided by the transfer rate.
- Controller overhead is the small time used by the disk controller for commands and checks.
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