Primary Memory (Main Memory)-
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; the CPU can access them directly and quickly.
Functions of Primary Memory
• Holding data and instructions for the task assigned by input device before processing.
• Holding intermediate results while processing.
• Holding data and information after processing unless they are transferred to output unit.
• Holding programs and instructions from secondary storage device.
• Holding system programs like operating system.
Types of Primary Memory-
• Random Access Memory (RAM)
• Read Only Memory (ROM)
Random Access Memory-
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 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. RAM is considered “random access” because you can access any memory cell directly. RAM is also named direct access memory because the operating system, application programs and data currently in use are stored in RAM directly accessed by the CPU/processor.
The Random Access Memory (RAM) is sub divided into-
• Dynamic Random Access Memory (DRAM)
• Static Random Access Memory (SRAM)
Dynamic Random Access Memory-
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-
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 does not require refreshing. It can store information till the power supply is “on”. It is faster and expensive in comparison to DRAM and primarily used in 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 startup. During the computer startup 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 startup (booting) operation. The contents of ROM cannot be written or erased by the user. ROM also performs the BIOS (Basic Input Output System) functions.
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 “one time programmable device”. It is made as a blank memory. 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 ultra violet rays of some specific wavelength falls into EPROM’s chip glass panel, the bits of memory come back to 1 and the chip enables 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 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 computer system’s BIOS (Basic Input Output System).
Secondary Memory-
It is also known as external memory or auxiliary memory or storage memory. It is a non-volatile memory which is used to store instructions and data permanently unless erased by the user or the till the damage of the storage device. These kinds of memories are less expensive and have much larger storage capacity than primary memory. Secondary storage devices are: hard disk, pen drives, CD (Compact disks), DVD’s (Digital Versatile Disc), memory card, etc.