Memory

When we receive some instruction or information we retain them in our memory. Similarly a computer stores the instructions for solving a problem, the data to be processed, the intermediate results and the final results until they are displayed. This storage unit s known as computer memory or store. It consists of chips either on the motherboard or on a small circuit board attached to the motherboard. This electronic memory allows the CPU to store and retrieve data quickly.

Memory Two types of built-in-memory: Permanent memory. Some memory chips retain the data they hold, even when the computer is turned off. This type of permanent memory is called nonvolatile. Non-Permanent memory. Other chips – in fact, most of the memory in a microcomputer-lose their contents when the computers power is shut down. This type of nonpermanent memory is called volatile.

Nonvolatile Memory Nonvolatile chips hold data even when the computer is unplugged. During normal use, the data in these chips is only read and used- not changed- so the memory is called read only memory (ROM). Specifically, chips that cannot be changed are called programmable read only memory (PROM). PROM chips are often found on hard drives and printers. They contain the instructions that power the devices. These instructions, once set, never need to be changed.

Nonvolatile Memory When a computer is turned on, it must know how to start. ROM contains a set of start-up instructions called the basic input output system (BIOS) for a computer. FLASH MEMORY Flash memory is a special type of nonvolatile memory. It is often used in portable digital devices for storage. Digital cameras, portable MP3 players, USB “keychain” storage devices all use flash memory.

Volatile Memory Volatile memory requires to store data. The volatile memory in a computer is called random access memory (RAM). RAM’s job is to hold programs and data while they are in use. Physically, RAM consists of chips on a small circuit board. Single inline memory modules (SIMMs) and dual inline memory module (DIMM) chips are found in desktop computers. While the smaller, small outline DIMM (SO-DIMM) chips are found in laptop computers.

RAM Random Access Memory It is also called main memory or direct access memory or read/write memory. A program must be loaded into RAM before execution. Data and instructions can be read and modified. It play an important role in processing of computer.

Types of RAM SRAM (Static Random Access Memory) Retains contents as long as power is maintained. Faster than DRAM. SRAM is more expensive. CPU does not wait to access data. It is usually used to build a very fast memory known as cache.

Types of RAM DRAM (Dynamic RAM) Must be constantly refreshed. Requires an electric current to maintain its electrical state. The electrical charge of DRAM will decrease with time that may result in lose of data. Used for most PC memory because of size and cost.

Read Only Memory (ROM) Programs and data that are permanently recorded at the factory. The instructions can only be read and cannot be deleted. Cannot be changed by the user. When the computer is switched on,the instructions in the ROM are automatically loaded into the memory of the computer.

Types of ROM PROM Programmable ROM This form of ROM is initially blank. The user can write data and programs on it. The user can write data and instructions on it only once.

EPROM Stands for Erasable Programmable Read Only Memory It is initially blank The user can write data and programs on it. The data and program written on it can be erased using ultraviolet rays.

EEPROM Stands for Electronically Erasable Programmable Read Only Memory It is initially blank The user can write data and programs on it. The content of EEPROM can be modified easily. The data and program written on it can be erased using electrical pulses.

Cache Memory Small block of very fast temporary memory Speed up data transfer It is located inside or close to CPU chip. It is faster than RAM Instructions and data used most frequently or most recently.

Cache Step 1 Processor requests data or instructions Step 2 Go to address in main memory and read Step 3 Transfer to main CPU and cache Next processor request Look first at cache Go to memory PROCESSORPROCESSOR RAMRAM Cache

Types of Cache Internal cache – Level 1 (L1) – Built into microprocessor – Up to 128KB or 256 KB External cache – Level 2 (L2) – Separate chips – 256KB or 512 KB – SRAM technology – Cheaper and slower than L1 – Faster and more expensive than memory

Measuring Storage Capacity KB – kilobyte 1024 bytes Some diskettes Cache memory MB – megabyte Million bytes RAM GB – gigabyte Billion bytes Hard disks CDs and DVDs TB – terabytes Trillion bytes Large hard disks