Comparison between CD and DVD,cache memory and external memory devices

Comparison between CD and DVD

1. Data on CD is written in the form of small dots.
2. The difference between CD and DVD is that DVD uses more concentrated laser beam. This allows it to read very small dots. We can save more data on DVD because the size of the dots is reduced.
3. CD can hold 640-700 Megabytes of information on a single disc whereas DVD can hold 4.7 Gigabytes of information on a single layer disc and 8.5 Gigabytes on a dual layer disc.
4. CD is thinner than DVD.
5. CD is cheaper than DVD.

Formatting: Before the computer can use a disk to store data, the disk surface must be magnetically mapped so that the computer can go directly to a specific point on it without searching through data. While formatting a disk, the disk drive creates a set of magnetic concentric circles called tracks. The tracks are numbered from outermost circle to the innermost.

Sector: Each track on a disk is also split into smaller parts called sectors. A sector is the smallest unit with which any disk drive can work.

Working mechanism of CD: The surface of the CD contains land and pit. Land represents binary 1 and pit represents binary 0. During CD reading process a beam of light is emitted which strikes the surface of CD and gets reflected. The reflection is captured by a lens in the CD drive. The difference in reflection from land and pit is identified to determine 1 and 0. The land is actually in a crystalline form and pit is in amorphous form. The size of land and pit is smaller in DVD compared to CD.

Cache memory: Cache memory is a holding area in which the data and instructions most recently called from RAM by the processor are temporarily stored. It is a type of very fast memory that is used to improve the speed of a computer. It balances the difference in operating speed of RAM and CPU. It is 5-10 times faster than RAM. It acts as an intermediate store between the CPU and memory and works by storing the most frequently and recently used data and instructions.

When a program is running and the CPU needs to read a data or program instruction from RAM, the CPU checks first to see whether the data is in cache memory. If the data is not there, the CPU reads the data from RAM in registers, but it also loads a copy of the data into cache memory. The next time the CPU needs that same data, it finds it in the memory and saves the time needed to load the data from RAM. If the CPU finds required data or program instruction in the Cache, then it is cache hit and if the CPU does not find required data or program instruction in the cache, then it is cache miss. The ratio of hit and miss in a present computer is 9:1. Cache is further divided into L1 Cache and L2 Cache. L1 Cache contains more frequently required data compared to L2 Cache.

External memory devices

1. Zip disk: It is a magnetic memory. It is the modification of floppy disk. It has the storage capacity of 100MB and 250MB. It requires Zip disk drive for its operation.
2. Jaz disk: It is also a magnetic memory. It has the storage capacity of 2GB. It also requires separate Jaz disk drive for its operation.
3. Super disk: It is also a magnetic memory. It has the storage capacity of 120MB. It requires super disk drive for its operation. Both floppy and super disk can be used in a super disk drive.
4. Pen drive: It is a flash memory. It is a semiconductor memory. It is mainly used for transferring data. It has faster read/write speed compared to magnetic memory. Pen drive has a storage capacity of few MB-GB. A pen drive is a plug and play device. A pen drive can be connected to the computer through the USB port.
5. Portable hard disk: It is a magnetic hard disk. It can be connected to the computer through the USB port. It is also a plug and play device. It is mainly used for storing the backup. It has the storage capacity from few GB-TB.
6. SD Card: SD card stands for Secured Digital Card. It is a tiny memory unit which makes the storage feature portable in various devices like camera, smartphones, tablets, etc. It has the capability to store movies, songs, pictures and documents. Its capacity varies with its price.

Reference

Ghishing, Er.Ashim. Computer Science. Kathmandu, Nepal: Benchmark , 2008.