### Measurement of Data

**Units of storage**

The capacity of memory or a storage medium is measured in bytes (unit of storage). A byte is made up of eight (8) bits. A bit is short for binary digit. Take for example the measurement of liquid, we say litre or the measurement of weight, we do so in grams.

It’s the same thing we mean when we say the measurement of storage. That is how much data can be stored in memory or on the storage medium at any one time.

** ****Types of Storage Units**

There are different types of storage unit:

- Bit (b)
- Byte (B)
- Kilobyte (KB)
- Megabyte (MB)
- Gigabyte (GB)
- Terabyte (TB)
- Word

A word is a fixed sized group of bits that are handled together by the system; the basic unit for accessing computer storage, for example, a computer that can access 32 bits of data in one operation, means that the computer can process 4 characters at a time.

The number of bits in a word is called the **word size**.

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**Example** 4.1: How many KB are in 2 MB?

**Answer**: 1000 KB = 1 MB

β KB = 2 MB

1 β = 2 * 1000

β = 2000/1

Therefore, β = 2000 KB (remember, these are approximate figures)

Let’s try another example.

**Example** 4.2: How many MB are in 2.8 GB?

**Answer**: 1 Gigabyte = 1000 megabytes

2.8GB = β MB

2.8 * 1000 = 1 β

2800/1 = β

Therefore, β = 2800 MB (again, these are approximate figures)

**Example** 4.3: How many characters can be stored on a 6 Gb secondary storage medium?

**Answer**: Approximately 6 billion characters or exactly 6 * 1,073,741,824 which equals to 6, 442, 450, 944 characters.

**1.1 ****Calculating disk storage capacity**

Disk storage capacity depends on a number of things:

- The number of recording surfaces (platter)
- The number of cylinder (tracks)
- The recording density (bytes per track)

For example, calculate the capacity of the following disk pack:

- Number of bytes per track = 8500
- Number of tracks per platter = 700
- Number of platters = 12

**Answer**: 8500 bytes per track * 12 platters * 700 tracks per platter would give a total storage of 71.4MB or approximately 70 MB (70 million characters).

**Exercise 4**

I). The unit of storage used to represent a character is:

(A). bit

(B). byte

(C). word

(D). kilobyte

II). Complete the following table:

(A). 1 KB = ____________________ bytes = ____________________bits

(B). 1 MB = ____________________bytes = ____________________ bits

(C). 1 GB = ____________________ bytes = ____________________ bits

(D). 1 word = ____________________ bits

(7 marks)

I) What is meant by units of storage?

___________________________________

(1 mark)

II) What is the storage capacity of a floppy disk, if it has 80 tracks, 18 sectors per track, 512 bytes per track sector and data are stored on both surfaces (double density disk)? Show workings. **N.B**. Data are stored in the track sector on the surface of a disk.

_____________________________________

(5 marks)

**[TOTAL 20 MARKS]**