MEMORY DEVICES

INTRODUCTION Memory is the most essential part of a computer. Without memory there would be no computer, as we know it today. It is used for storing both instructions to be executed and data. This presentation has been developed after an intensive research on Memory Devices.

TYPES OF MEMORY MAIN MEMORY o Random Access Memory (RAM) o Read Only Memory (ROM) CACHE MEMORY SECONDARY MEMORY o Optical Media Devices o Magnetic Media Devices

MAIN MEMORY Directly or indirectly connected to the CPU via a memory bus Comprises of two buses: an address bus and a data bus The CPU firstly sends a number through an address bus, a number called memory address, that indicates the desired location of data. Then it reads or writes the data itself using the data bus. Additionally, a memory management unit(MMU) is a small device between CPU and RAM recalculating the actual memory address, for example to provide an abstraction of virtual memory or other task. Broadly, the main memory is of two types- o Random Access Memory (RAM) o Read Only Memory (ROM).

RANDOM ACCESS MEMORY In RAM, the memory cells can be assessed for information transfer from any desired location, that is, the processing of a word in memory is the same and requires an equal amount of memory. It is the fastest main memory technology. It requires constant power to maintain the stored information, therefore, it is volatile. A 1GB DDR RAM memory module

BLOCK DIAGRAM OF RAM Memory unit 2 k words N bits per word K address line read write n data input lines n data output lines Control lines

TYPES OF RAM The RAM chips are of two types- o Dynamic RAM(DRAM) A form of volatile memory which also requires the stored information to be periodically re-read and re-written, or refreshed, otherwise it would vanish. o Static RAM (SRAM) A form of volatile memory similar to DRAM with the exception that it never needs to be refreshed.

Dynamic RAM A type of RAM that stores each bit of data in a separate capacitor within an integrated circuit. Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically. Its advantage is its structural simplicity: only one transistor and a capacitor are required per bit, compared to four transistors in SRAM. This allows DRAM to reach very high density.

DRAM STORAGE

Row address Array of storage cell circuits Regenerator circuits Column address Read/write circuitry to control input/output

TYPES OF DRAM DRAM chips are available in various designs:  EDODRAM ( Extended Data Out DRAM )  SDRAM ( Synchronous DRAM)  RDRAM ( Rambus DRAM )  DDRDRAM ( Double Data Rate DRAM )

EDODRAM (Extended Data Out DRAM) Its cells keep the data valid until it receives an additional signal. It has a dual-pipeline architecture that allows the memory controller to simultaneously read new data while discharging the old. A pair of 32 MB EDO DRAM modules

SDRAM (Synchronous DRAM) SDRAM has a synchronous interface, meaning that it waits for a signal before responding to control inputs and is therefore synchronized with the computer's system bus. This allows the chip to have a more complex pattern of operation than asynchronous DRAM which does not have a synchronized interface. Pipelining means that the chip can accept a new instruction before it has finished processing the previous one

RDRAM (Rambus DRAM) It is a type of synchronous DRAM, designed by the Rambus Corporation. It is fairly fast and has tried to address some of the complex electrical and physical problems involved with memory.

DDRDRAM (Double Data Rate DRAM) Unlike SDRAM, it can do two operations per cycle thereby doubling the memory bandwidth over the corresponding single- data-rate SDRAM

Static Random Access Memory (SRAM) It is a type of memory in which, memory refreshing is not required. It uses flip-flops to store binary information. As it takes up more space than DRAM, it is used for specialized applications. It is much easier to use and has shorter read-write cycles compared to DRAM.

READ ONLY MEMORY It performs only read function not write function. So the data stored in ROM cannot be modified. It comes with special internal electronic fuses that can be programmed for a specific configuration. Once this pattern is established it stays in the unit. Thus, ROM is non-volatile.

TYPES OF ROM Programmable read-only memory (PROM)  This device uses high voltages to permanently destroy or create internal links (fuses or antifuses) within the chip. Consequently, a PROM can only be programmed once. Erasable programmable read-only memory (EPROM)  It can be erased by exposure to strong ultraviolet light (typically for 10 minutes or longer), then rewritten with a process that again requires application of higher than usual voltage. Electrically erasable programmable read-only memory (EEPROM)  It allows its entire contents (or selected banks) to be electrically erased, then rewritten electrically, so that they need not be removed from the computer (or camera, MP3 player, etc.).

CACHE MEMORY It is a high speed storage mechanism. Can be either a reserved section of main memory or an independent storage device. It speeds up access to data and instructions stored in RAM. MEMORY CACHE-  It is a portion of memory of SRAM instead of the slower DRAM. By keeping as much of the information as possible in high speed SRAM, it avoids accessing the slower DRAM. DISK CACHE-  It works under the same principle, but uses conventional main memory (DRAM) instead of high speed SRAM. It improves the computers performance a lot as accessing data from RAM is much faster than from hard-disk.

TYPES OF CACHE MEMORY Level 1 (L1) cache  Built inside the CPU.  It works at half CPU clock speed. Level 2 (L2) cache  Built external to CPU, in the motherboard.  It works at the motherboard bus speed. Nowadays both L1 and L2 are integrated in the CPU to reduce access time and further improve system performance.

The whole idea of memory cache is to keep staging more instructions and data in a high- speed memory closer to the CPU. MEMORY CACHE

DISK CACHE Disk caches are usually just a part of main memory made up of common dynamic RAM (DRAM) chips.

SECONDARY STORAGE DEVICES These devices are used to store large amount of data permanently. It differs from primary storage in that it is not directly accessible by the CPU. So they need more access time and thus are much slower. Per unit, it is typically also an order of magnitude less expensive than primary storage. Consequently, modern computer systems have an order of magnitude more secondary storage than primary storage and data is kept for a longer time there (such as in hard disk). It is broadly of two types- 1) MAGNETIC MEDIA and 2)OPTIC MEDIA.

MAGNETIC MEDIA DEVICES Magnetic storage uses different patterns of magnetization in a magnetizable material to store data It is a form of non-volatile memory. The information is accessed using one or more read/write heads. HARD DISKS and FLOPPY DISKS are such devices.

HARD DISKS It stores information on one or more continuously spinning disks which are coated with magnetic material. Information is recorded by magnetic heads called access arms. These days, hard disks have storage capacity between 80 to 300 GB. ACCESS ARM

FLOPPY DISK (DISKETTE) It is made of a flexible substance called Mylar. It has a magnetic surface for recording data. It stores upto 1.44 MB of data. It cannot include graphics or pictures within it. All floppy disks must be formatted before data can be written on it.

OPTIC MEDIA DEVICES Optic devices are generally circular disc which can contain data encoded in bumps on a special material on one of its flat surfaces. The encoding pattern follows a continuous, spiral path covering the entire disc surface and extending from the innermost track to the outermost track. The data is stored on the disc with a laser or stamping machine, and can be accessed when the data path is illuminated with a laser diode in an optical disc drive These are broadly of two types- 1) CDs and 2) DVDs.

COMPACT DISK (CD) CDs are very cheap and store upto 700 MB of data. They are of three types-  CD-ROM (CD Read Only Memory)  CD-R (CD Recordable)  CD-RW (CD Rewritable)

DIGITAL VIDEO DISK (DVD) It is of the same size as a CD but stores 15 times as much information, is 20 times faster than it. It can hold 17 GB of data. It comes in three varieties- DVD-ROM (DVD Read Only Memory) DVD-R (DVD Recordable) DVD-RW (DVD Rewritable)

NEW AGE MEMORY DEVICES FLASH MEMORY-  is a non-volatile memory  It is a specific type of EEPROM  primarily used in memory cards and USB flash drives BLU-RAY DISK-  Supersedes DVDs  Uses blue-violet laser to read the disc  stores almost six times more data than on a DVD