The Memory Unit is the part of the computer that holds data and instructions for processing. Although it is closely associated with the CPU, in actual fact it is separate from it.
Memory associated with the CPU is also called primary storage, primary memory, main storage, internal storage and main memory.
Types of Memory
There are two main types of memory and they are:
- Primary memory (Main memory)
- Secondary memory (Auxiliary storage devices)
Primary memory (Main Memory): Primary memory is the memory that is directly accessed by the CPU to store and retrieve information. Most of the time, primary memory is also referred to as the RAM (Random Access Memory). It is a volatile memory, which loses its data when the power is turned off. Primary memory is directly accessible by the CPU through the address and memory bus and it is constantly accessed by the CPU to get data and instructions. Furthermore, computers contain a ROM (Read Only Memory), which holds instructions that are executed often such as the startup program (BIOS). This is a non volatile memory that retains its data when the power is turned off. Since the main memory is accessed often, it needs to be faster. But they are smaller in size and also costly.
The main memory is divided into two parts:
- Read Only Memory (ROM)
- Read Access Memory (RAM)
Read Only Memory (ROM): This is the permanent place of storage of instructions and programs for starting, testing and controlling of the operations of the computer. This is a pre-installed instruction by the manufacturers.
The content of the ROM is a non-volatile storage; the content can only be read but cannot be altered. Therefore is regarded as Read Only Memory
Random Access Memory (RAM): This takes a dominant part of the main memory. The storage of currently worked application programs and data is carried out on it. The content of RAM is over-written by incoming ones after actions on the previous ones have been completed. This stores temporarily, as the content on it is destroyed when switched off. It is therefore said to be a volatile storage.
Secondary memory (Auxiliary storage devices): Secondary memory is a storage device that is not accessible directly by the CPU and used as a permanent storage device that retains data even after power is turned off. CPU accesses these devices through an input/ output channel and data is first transferred in to the primary memory from the secondary memory before accessing.
i. Hard Disk: Hard disks are normally fixed permanently inside the computer, but in the recent times there are external hard disks of high capacity as well. The first hard disk was made by IBM in 1973. It was a 30MB monster with 16-inch platters called Winchester. The storage capacities of hard disk have increased recently that it can hold up to 800 GB of data.
ii. Flash drive/Disk: These are old storage device that are used to make backups or duplication of files. These come in various sizes such as 3 ½ or 3.5 inch type and 5 ¼ or 5.25 inch type.
iii.Compact Disk (CD) /Digital Video Disk (DVD): CD/DVD drive is another storage that can read from or write to a CD disc. It comes in different types such as CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-R, DVD-RW etc.
Differences between Primary and Secondary Memory
1. Primary memory is the memory that is directly accessed by the CPU to store and retrieve information, whereas the secondary memory is not accessible directly by the CPU.
2. Primary memory is accessed using address and data buses by the CPU, while secondary memory is accessed using input/ output channels.
3. Primary memory does not retain data when the power is turned off (volatile) while secondary memory retains data when the power is turned off (non-volatile).
4. Furthermore, primary memory is very fast compared to the secondary memory and has a lower access time.
5. But, primary memory devices are more costly compared to secondary memory devices. Due to this reason, usually a computer comprises of a smaller primary memory and a much larger secondary memory.
Tests and Exercises
1. The primary memory can also be regarded as ________
a. Control unit b. Main memory c. ALU d. All of the above. Answer: Main memory
2. Which of the following is also classified as temporary memory?
a. Primary Memory b. Secondary memory c. Micro chips d. None Answer: Primary memory
3. The main memory type that cannot store information when the computer is switched off is
a. ROM b. RAM c. Auxiliary storage devices d. All Answer: RAM
4. Which of the following secondary memory has the highest storage capacity?
a. Compact disk b. Flash disk c. Hard disk d. None of the above. Answer: Hard disk
5. The main memory categories that take the dominant part is known as ______
a. RAM b. ROM c. EPROM d. EEPROM. Answer: ROM
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