(A) Primary Memory-
Computer memory is used to store information (data and instructions) in a digital computer. A memory is divided into several small parts called memory cells. Each memory cell or location has a unique address in the form of machine language (0/1).
The computer memory is divided into main and auxiliary memory. Main memory also called primary memory and secondary memory also called storage memory. Main memory holds instructions and data when a program is in execution, while auxiliary or secondary memory holds data and programs not currently in execution and acts as a storage medium. Primary memory holds instructions and data currently being processed by the CPU- the intermediate results produced during the execution of the process. The instructions and data remain in primary memory which can be accessed by the CPU directly and quickly.
Types of Primary Memory-
• Random Access Memory (RAM)
• Read-Only Memory (ROM)
Random Access Memory (RAM)-
It is a volatile or temporary memory. Its contents get lost once the power supply is switched off. It is also called read/write (R/W) memory. It allows CPU to read as well as write data and instructions over it. It temporarily holds software, programs /instructions before and after processing of the data. RAM is used during the execution of instructions.
The Random Access Memory is further divided into-
(i) Dynamic Random Access Memory (DRAM) (ii) Static Random Access Memory (SRAM).
Dynamic Random Access Memory (DRAM)-
It makes use of a transistor and a capacitor to form a memory cell standing for a single bit of data. DRAM is not able to retain information for a longer time even if the power supply is on because it needs refreshing frequently. It is slower and less expensive than SRAM.
Static Random Access Memory (SRAM)-
In SRAM flip flops are used for retaining memory. Typically, it uses four to six transistors for each memory cell. It does not have a capacitor and hence does not require refreshing. It can store information until the power supply is “on”. It is faster and expensive in comparison to DRAM and primarily used in the processor’s cache memory.
Read-Only Memory (ROM)
It is a non-volatile memory whose contents are not lost even the electric power is off. It stores instructions for computer start-up. During the computer start-up operation, the main memory (RAM) of the computer is empty because of its volatile nature, so that the computer gets instructions from the ROM to start-up (booting) operation. The contents of ROM cannot be written or erased by the user only trained programmers and technicians can alter or write or erase the contents of ROM.
Types of Read-Only Memory
• Programmable Read-Only Memory (PROM)
• Erasable Programmable Read-Only Memory (EPROM)
• Electronically Erasable Read-Only Memory (EEPROM)
Programmable Read-Only Memory (PROM) -
The data stored in PROM cannot be modified and therefore it is also known as a ‘one-time programmable device’. It is made as a blank memory as a result when you want to write on a PROM chip, a ROM burner is required to write data onto a PROM chip.
Erasable Programmable Read-Only Memory (EPROM)-
Unlike programmable ROM; erasable programmable ROM can write/modify data more than once. When ultraviolet rays of some specific wavelength fall into EPROM’s chip glass panel, the bits of memory come back to 1 and the chip enables them to write.
Electronically Erasable Read-
Only Memory (EEPROM)- Electrically Erasable Programmable ROM is as same as Erasable Programmable ROM, but the same work of erasing the stored data is performed with an electric current. EEPROM is a type of non-volatile memory and used in computers and other electronic devices. It used to store relatively small amounts of data and also allows individual bytes to be erased and reprogrammed. It stores a computer system’s BIOS (Basic Input Output System).
(B) Secondary Memory
Secondary memory is where programs and data are kept on a long-term basis. Common secondary storage devices are the hard disk and optical disks. A hard disk might have a storage capacity of one terabyte (TB). A hard disk is slow in accessing speed compared to the main memory. Secondary memory has enormous storage capacity, especially hard disk. Some of the secondary storage devices are explained below-
Hard Disk Drive (HDD)- Hard disks are stored permanently inside their drive, which is normally installed into the computer box where you cannot see any part of it. Hard Disks can store a lot of data and are used to store most of the information on computers. The advantage of HDD is cost and capacity.
Floppy Disk Drive (FDD)- Floppy disks are small disks that you can pop in and out of their drive. All you can see of their drive is a slit in the computer box with a button and a light. You can put in different disks and then take them out and give them to someone so that you can transfer data between your computers. FDD's are now obsolete. They can store only about 1.44 MB, which is way too little.
Compact Disk (CD)- The most popular kind of optical disk is the Compact Disc (CD). CDs can still be put into and taken out of their drive, making them good for buying programs and for transferring data nowadays. The normal CDs that you buy with programs on them are CD-ROMs. They are called so because like ROM, they are read-only.
Digital Versatile Discs (DVD)- DVDs are another kind of Optical disc that works identically to CDs; however, they can store much more information than CDs. DVDs can store usually about 4.3 GB, but dual-layered DVDs can store up to about 9 GB and DVD-RAM can store about the same amount. They are sometimes called Digital Video Discs because of their extensive use in video and films.
Flash memory- Nowadays, one can get memory cards that are called flash memory or, more properly, EEPROMs. These are cards that operate just like ROM that isn’t read-only, so they don’t lose their information when the power is cut.