Title: Know what computer memory is
Course: Module 1, Concepts of Information and Communication Technology (ICT)

Classifying different types of memory

The information being used by a computer is kept in its memory. Computers store the data in their memory in a range of forms and they use their various memory devices in different ways. There are several ways you can classify the types of memory.

Temporary and Permanent Memory
- Any data in the computer’s temporary memory will be lost when the computer is switched off, while the data in the computer’s permanent memory is retained. Temporary memory (RAM) is sometimes referred to as volatile memory and permanent (ROM) as non-volatile memory.
- A computer’s RAM is its main store of temporary memory.
- ROM, hard disks, floppy disks, Zip disks and CD-ROMs are all examples of permanent memory or storage devices.

Primary and Secondary Memory
- RAM is also known as primary memory storage because it is generally the first place a processor uses for data. When you run a piece of software, the data which the computer is going to need to access quickly to run the software is loaded into RAM.
- The other memory stores on the computer are sometimes referred to as secondary memory.

Random Access Memory (RAM)

RAM is a type of memory in a computer that can be read from or written to. It is volatile memory, which means it loses its data when the power is switched off.
- RAM acts as a temporary storage for any programs that are running and the data they are using. It also stores the data being transferred to and from various different parts of the computer, including the contents of the screen.
- There are many different types of RAM with various different features. RAM is fitted in a number of different locations in a computer. Two examples are Static RAM (SRAM) which is used in the computer’s CPU and Synchronous Dynamic RAM (SDRAM) which is often used as the computer’s main memory and in graphics cards.

Main Memory
- Main memory is the largest amount of RAM installed on a computer’s motherboard. The data that is being used or that is most urgently required by components such as the CPU is loaded into the computer’s main memory.
- Every piece of software needs a certain amount of main memory to operate efficiently — this amount varies from one piece of software to another. Modern software usually requires more main memory than older software, so the amount of main memory installed in new computers is increasing all the time.

Cache

Cache
- Cache is a very fast type of RAM directly attached to particular components such as the CPU. It enables a certain amount of data to be copied from the main memory so the component can access it faster. Cache is usually a small amount of static RAM, which is much faster than SDRAM, however it is also far more expensive to produce.

- Data stored in a component’s cache is immediately available to the component — it doesn't have to access the main memory and copy the data across before it can be used.

- CPUs usually have a certain amount of cache built into them. AMD AthlonXP chips have a built-in cache of 512KB or more. Some motherboards also have cache installed on them, and modern hard disks usually have 8Mb of cache or more.

Read Only Memory (ROM)

ROM is memory that can be read from but not written to. Data and programs stored in ROM cannot be changed in the course of normal computer operations and are retained when the power is turned off.

Programmable Read-Only Memory (PROM)
- A PROM chip is a special component connected directly to the motherboard. It is used to store software that enables all the components in the computer to communicate, called the BIOS (See below).

- Once data is written to the PROM chip, this data cannot be changed. The PROM chip is usually pre-programmed by the manufacturer.

Erasable Programmable Read-Only Memory (EPROM)
- Like a PROM chip, this component is used to store BIOS software and usually comes pre-programmed by the manufacturer.

- Unlike a PROM chip, the data stored on an EPROM chip can be changed. For this reason, most modern computers use an EPROM chip in place of a PROM chip.

BIOS

Basic Input/Output System (BIOS)

BIOS stands for Basic Input/Output System. It is a program containing a set of basic instructions which tell the computer how to function.
- The BIOS is stored on a PROM or EPROM chip located inside the computer on the motherboard — the main printed circuit board on which the CPU is located.

- When a computer is first switched on, the BIOS software performs a self-diagnostic check to ensure that the computer is functioning properly.

- The BIOS software tells the computer how to control the exchange of data between the various parts of the computer, including the disk drives, the monitor, the printer, and ports.

- If the BIOS is stored on an EPROM chip, it is possible to update the BIOS software enabling a newer version to be used. BIOS updates are usually supplied by the manufacturer.

Units of computer memory measurement

Bits and Bytes
- The smallest component of computer data is the bit. Each bit can be in one of two states 1 or 0 with the 1 standing for On and the 0 standing for Off. Bits are combined in sets of eight to form characters, such as letters of the alphabet. For example, 01000001 is used to represent the letter A in binary code. This group of eight bits makes a byte.

- You may see hardware and software described as being a certain number of bits. This refers to how much information the hardware or software can process at any one time. For example, a 256-bit graphics card can process 256 bits of data at a time.

Megabytes
- A megabyte (MB) is often used to measure the storage capacity of a disk or the amount of main memory.

- A megabyte is 1,048,576 bytes (1024 kilobytes). 1MB is about as much data as 500 pages of double-spaced text.

Gigabytes
- A gigabyte (GB) is equivalent to 1,024MB. The storage capacity of most storage devices, such as hard drives, is measured in gigabytes.

- It is now becoming more common for main memory to be in the 1 Gigabyte region.

Terabytes
- A terabyte (TB) is equivalent to 1,024GB. Terabytes are more commonly found in mainframes and where extremely large data is stored e.g. Movie Studios post-production labs.

- A terabyte is about 1,000,000,000,000 bytes of information — that’s around 500,000,000 pages of double spaced text.

Files
- A file is a collection of information stored as a single unit. When they have been saved, any of the following may be regarded as a file:
  - A computer program.
  - Word-processed text.
  - A spreadsheet.
  - A computer graphic.

File sizes

The size of a file depends on the amount of data it contains. Let’s take the example of a database file:
- A database file is an organised collection of data that usually consists of a number of separate parts called records. Each record consists of a set of related data fields, which together can be treated as a unit. If you think of an entry in an address book as a record, then each part of that entry (i.e. name, job title, address, telephone number, etc) is a field.

- Data entered into a field is made up of characters, so each field will contain a certain number of bytes of data. As a given number of fields make up each record and a given number of records make up the whole file, you can see that the size of a database file depends on the following:

  - The number of records in the file.
  - The number of fields in each record.
  - The number of characters entered into each field.

- This determines the number of bytes of data that make up the file, and thus the file size. You can see that even with relatively few records, the size of a database file could be several thousand bytes (or several kilobytes).

Directory sizes

When you have a number of related files stored on your computer, you may want to store them in a directory.

You can think of a directory as being like a drawer in a filing cabinet.
- An empty directory does not contain any data. However, as a directory usually contains a number of files, the combined size of these files may mean the directory contains several thousand kilobytes (or many megabytes) of data.

- If you have several hundred directories, each containing a number of files, the total amount of space required to store all the data may run into several thousand megabytes (or several gigabytes). It is therefore easy to see how the space on storage devices, such as hard disks, can be used up.

- Since Microsoft Windows, however, directories have become widely known as folders.

Memory types

Which of the following are memory types?

- RAM
Feedback: Correct!
- ROM
Feedback: Correct!
- RIM
Feedback: Incorrect :-(
- RUM
Feedback: Incorrect :-(
- REM
Feedback: Incorrect :-(