Lesson 5: Computer Hardware – Memory Unit

In computing, memory refers to the physical devices used to store programs (sequences of instructions) or data (program state or information) on a temporary or permanent basis for use in a computer or other digital electronic device. The device that performs such activities is referred to as Memory Unit.

Unit 1: Types of Memory
There are two major types of memory:1. Primary memory (sometimes referred to as main memory).
2. Secondary memory (can also be called auxiliary storage devices).

Primary Memory
Primary storage is called ‘primary’ because it is the main memory that is accessible to the CPU. It is used to store data that are currently being used. Primary memory is broadly divided into two categories:

i. Random Access Memory (RAM)
ii. Read Only Memory (ROM)

(i) Random Access Memory (RAM)
Random Access Memory (RAM) is a type of chip used in primary storage memory. It is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required.  Any data actively operated on is also stored there in uniform manner. It is also a temporary storage, holding software instructions and short-term working memory for the processor. RAM can be increased in most computers by using the expandable memory slots,

(ii) Read Only Memory (ROM)
Read Only Memory (ROM) are chip that cannot be written on or erased by the computer user without special equipment. While using ROM, contents are not lost when power to the computer is no longer available. It is simply not volatile. Since it does not need power, and cannot be rewritten, the only things put on ROM are starting (booting) instructions.

Secondary Memory
Computer storage is often referred to as secondary memory. It is where data is more permanently stored. The secondary memory is usually available in the form of floppy disk storage media, hard disk, Compact Disk (CD), Digital Video Disk (DVD), pen drive (i.e. mass storage devices), or memory chips.

Floppy Disk
A 5 and \frac{1}{4} inch floppy disk typically stores 1.44 megabytes (MB) of data. The data on the floppy disk is organised in terms of tracks and sectors.

Hard Disk
Hard disks can have large capacity, something like 60-400 Gigabytes (GB) or higher. Hard disk is usually much faster compared to floppy disk.

Compact Disk (CD)
CD can store up to 750 MB of data. Information on CD ROM is organised in terms of a spiral track. The recordable CD (CD-R) disks are usually written once and read many times except for re-writable CDs (CD-RW) or the multisession recorded CDs.

Digital Versatile Disk (DVD)
A Digital Versatile Disk-originally called digital video disk (DVD) is an optical disc storage media format, invented and developed in 1995. DVDs offer higher storage capacity than compact discs while having the same dimensions. A DVD can store 4.6 Gigabytes of information.

USB Flash Drive
A USB (Universal Serial Bus) flash drive is a data storage device that consists of flash memory with an integrated Universal Serial Bus (USB) interface. USB flash drives are typically removable and rewritable, and physically much smaller in physical size than a floppy disk.

USB flash drives are often used for the same purposes for which floppy disks or CD-ROMs were used. They are smaller, faster, have thousands of times more capacity, and are more durable and reliable because of their lack of moving parts. USB drives with USB 2.0 support can store more data and transfer faster than a much larger optical disc drives.

Unit 2: Differences between Primary and Secondary Memory
1. The primary memory or the main memory is part of the main computer system while secondary memory is the auxiliary that is not necessarily part of the main computer system.
2. The processor or the CPU directly stores and retrieves information from primary memory while the processor needs a device to store or retrieve information from secondary memory.
3. This primary memory is accessed by CPU in random fashion while secondary is not at all time in random fashion.
4. Primary memory is much faster and more cost effective. But the secondary memory is much slower and less costly.
5. Secondary memory stores the data permanently unless it is erased while primary memory is volatile, in the case of RAM.
6. The secondary memory is usually available in the form of floppy disk storage media, hard disk, CD, DVD, pen drive (i.e. mass storage devices), memory chips, while primary memory is internal and comes in form of RAM and ROM.

Unit 3: Units of Storage
Data in a computer is represented or stored in a series of bits. Since the birth of computers, bits have been the language that controls the processes that take place inside the computer. The unit of storage of data in computer is as follows:

(i) Bits
A bit is simply a 1 or a 0; a true or a false. It is the most basic unit of data in a computer. It is also called machine language. A bit (so called binary digit) is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states.

In computing, a bit can also be defined as a variable or computed quantity that can have only two possible values. These two values are often interpreted as binary digits, and are usually denoted by the Arabic numerical digits 0 and 1. The two values can also be interpreted as Logical values (true/false, yes/no), algebraic signs (+/-), activation states (on/off), or any other two-valued attribute.

(ii) Nibble
In computing, a nibble is a four-bit aggregation, or half an octet. As a nibble contains 4 bits, there are sixteen (16) possible values; so a nibble corresponds to a single hexadecimal digit (thus, it is often referred to as a “ha digit” or “hexit”).

A nibble is a small bite, which in this context is construed as “half a bite”. It is a storage unit that is in a sequence of 4 bits, i.e. it is equivalent to 4 bits. In essence, 1 nibble = 4 bits

The nibble is used to describe the amount of memory used to store a digit of a number stored in packed decimal format within an LBM mainframe. This technique is used to make computations faster and debugging easier. An 8-bit byte is split in half and each nibble is used to store one digit.

(iii) Byte
A byte is a unit of measurement of information storage, that equals ‘8 bits’. It can be used to represent letters and numbers. For example, the number 01000001 is 8 bits long, and represents the letter A in ASCII.  Historically, a byte is the number of bits used to encode a single character of text in a computer, and for this reason it is the basic addressable element in many computer architectures. A byte is the smallest memory space in a computer and it is equivalent to 2 nibbles or 8 bits.

(iv) Kilobyte (KB)
A kilobyte (KB) is a unit of data that equals 1024 bytes. The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo-means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 (210) bytes or 1000 (103) bytes, dependent upon context, in the fields of computer science and information technology.

(v) Megabyte (MB)
A megabyte is 1024 KB, that is, the kilobyte squared or 10242 10242 bytes. The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: 1048576 bytes (220) generally for computer memory; and one million bytes (106) generally for computer storage. In rare cases, it is used to mean 1000 x 1024 (1024000) bytes. It is commonly abbreviated as Mbyte or MB.

(vi) Gigabyte (GB)
A gigabyte is a unit of data storage worth a billion bytes—meaning either exactly 1 billion bytes (10243) or approximately 1.07 billion bytes. The prefix giga means 109 in the International System of Units (SI); therefore 1 gigabyte is 1000000000 bytes. The unit symbol for the gigabyte is GB or Gbyte.

(vi) Terabyte (TB)
A terabyte is 10244 and is defined as about one trillion bytes, or 1024 gigabytes. The prefix tera means 1012 in the International System of Units (SI), and therefore 1 terabyte is 1000000000000 bytes, or 1 trillion bytes, or 1000 gigabytes. The unit symbol for the terabyte is TB or TByte.

Conversion of Units – How to convert KB to MB
Technically, 1KB = 1024 Bytes (210); and 1 MB = 1024 KB (because the actual bytes are in the powers of 2 and hence will not be exact 1000).
Hence, 60MB = 61440KB
In normal usage, however, it can be used as 60000 KB.

Measurements of Data Speed
Today there are generally two ways of describing data transfer speeds: in bits per second, or in bytes per second. As explained above, a byte is made of 8 bits. Network engineers still describe network speeds in bits per second, while your Internet browser would usually measure a file download rate in bytes per second. A lowercase “b” usually means a bit, while an uppercase “B” represents a byte.

1. bps: known as bits per second, bps was the main way of describing data transfer speeds several decades ago. Bps was also known as the baud rate; therefore, a 600 baud modem was one which could transfer data at around 600bps.

2. Kbps: kilobits per second, or 1000 bits per second.

3. Mbps: 1,000,000 bits per second (usually used in describing Internet download/upload speeds).

4. Gbps: 1,000,000 kilobits per second or 1,000,000,000 bits per second. This term is most commonly heard in local area networks, where the close proximity of machines allows for lightning fast data transfer rates.

Exercises
1. Mention two types of main memory we have in a computer
2. How many bits are in a byte?
3. List three units of storage
4. If a computer hard disk has the memory space of 3200000 bytes, express this in megabytes.

Scroll Down to Select Page 2 for the next topic – Lesson 6: Logic Circuits.



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