COMPUTER MEMORY SYSTEM

computer hardware 

System Memory: Memory is major part of computers that categories into several types. Memory is best storage part to the computer users to save information, programs and etc. The computer memory offer several kinds of storage media some of them can store data temporarily and some them can store permanently. Memory consists of instructions and the data saved into computer through Central Processing Unit (CPU).

Memory is the best essential element of a computer because computer can’t perform simple tasks. The performance of computer mainly based on memory and CPU. Memory is internal storage media of computer that has several names such as majorly categorized into two types, Main memory and Secondary memory.

1. Primary Memory / Volatile Memory.

2. Secondary Memory / Non Volatile Memory.

Read Only Memory (ROM): The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories during manufacture. A ROM, stores such instructions that are required to start a computer. This operation is referred to as bootstrap. The different types of ROM are given below.

MROM (Masked ROM): The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kinds of ROMs are known as masked ROMs which are inexpensive. The MROM cannot be programmed by the user.

PROM (PROGRAMMABLE READ ONLY MEMORY): A variation of ROM chip is programmable read only memory. ROM chip are supplied by the computer system manufacture and it is not possible for a customer to modify the programs stored inside the ROM chip. However, it is possible for a user to customize a system by converting his own programs to micro programs and storing them in a PROM chip. Once the users programs are stored in a PROM chip, they can usually be executed in a fraction of the time previously required. PROMs are programmed to record information using a special facility known as a pro-programmer. However, once the chip has been programmed, the recorded information can not be changed, i.e. the PROM becomes a ROM and it is only possible to read the stored information. PROM is also non-volatile storage, i.e. the stored information remains intact even if power supply of computer is switched off.

EPORM (ERASABLE PROGRAMMABLE READ ONLY MEMORY):

Once information is stored in a ROM or a PROM chip it can not be changed or altered. However, there is another type of memory chip called erasable programmable read only memory that overcomes this problem. As the name implies, it is possible to erase information stored in an EPROM chip. The chip can be reprogrammed to store new information using a special prom-programmer facility. Information stored in an EPROM chip is erased by exposing the chip for some time to ultraviolet light. When an EPROM is in use, information can only be read and the information remains on the chip until it is erased. EPROMs are mainly used by R & D personnel because they frequently change the micro programs to test the efficiency of the computer system with new programs.

EEPROM (ELECTRONICALLY ERASABLE PROGRAMMABLE ROM):

It is also called flash BIOS, in this type of memory high voltage electric pulses are used to erase information content in the ROM. New information can be recorded in ROM by special software program although there is a limit to the number of times that can be programmed. E2PROM chip allows user to upgrade their BIOS. The EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (millisecond). In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of re-programming is flexible but slow.

Flash memory: It is a non-volatile computer memory that can be electrically erased and reprogrammed. Examples are memory cards, chips, pen drives, and USB flash drives etc. flash memory costs very less than byte-programmable EEPROM. It is very portable in nature.

Random Access Memory (RAM):

RAM stands for Random Access Memory and it is a read/write memory. Information can be written into and read from a RAM. It is a volatile memory. It retains the stored information as long as it is supplied with power. When power supply of computer is switched off or interrupted the stored information in the RAM is lost. RAMs of various capacities are available for example, 1K, 4K, 16K, 64K, 1M, 4M and so on.

There are two important types of RAMs that are Static RAM and Dynamic RAM. Static RAMs retain stored information as long as power supply is on . But a Dynamic RAM loses its stored information in a very short time even though the power supply is on. Therefore, Dynamic RAMs have to be refreshed periodically, generally every 2 millisecond. The Dynamic RAMs are cheaper and have high packing density and moderate speed. They consume less power. They are used where large capacity memories are needed. Static RAMs are costlier and consume more power. They do not need refreshing circuits. They have higher speed than Dynamic RAMs. Static RAM and Dynamic RAM are also written as SRAM and DRAM. RAM comes in module packaging of SIMM's and DIMM's Remember SIMM's and DIMM's are not memory chips. They are packaging modules.

DRAM (Dynamic RAM) DRAM is dynamic random access memory. (This is what most people are talking about when they mention RAM.) When you expand the memory in a computer, you are adding DRAM chips. You use DRAM to expand the memory in the computer because it’s a cheaper type of memory. Dynamic RAM chips are cheaper to manufacture than most other types because they are less complex. Dynamic refers to the memory chips’ need for a constant update signal (also called a refresh signal) in order to keep the information that is written there. If this signal is not received every so often, the information will bleed off and cease to exist. Currently, the most popular implementations of DRAM are based on synchronous DRAM and include SDRSDRAM, DDR, DDR2, DDR3, and DRDRAM

Types of DRAM Chips:

1. Asynchronous DRAM: Asynchronous DRAM (ADRAM) is characterized by its independence from the CPU’s external clock. Asynchronous DRAM chips have codes on them that end in a numerical value that is related to (often 1⁄10 of the actual value of) the access time of the memory. Access time is essentially the difference between the time when the information is requested from memory and the time when the data is returned. Common access times attributed to asynchronous DRAM were in the 40- to 120-nanosecond (ns) vicinity. A lower access time is obviously better for overall performance.

2. Fast Page Mode (FPM) DRAM: FPM was the traditional RAM for PC's, before the EDO was introduced. It is mounted in SIMM modules of 2, 4, 8, 16, or 32 MB. Typically, it is found in 60 ns or 70 ns versions. 60 ns is the fastest and the one to use. You cannot mix different speeds on the same Pentium mainboard.

3. EDRAM (enhanced dynamic random access memory) (Enhanced DRAM)

4. EDO DRAM Memory (Extended Data Output): 72 pin SIMM configuration. EDO memory should say EDO right on the chip. EDO RAM are usually sold in 60 ns versions. EDO memory is faster because it doesn't produce as many wait states before holding a new electrical charge.

SYNCHRONOUS DRAM (SDRAM) :Synchronous DRAM ( SDRAM ) shares a common clock signal with the computer’s system bus clock, which provides the common signal that all local-bus components use for each step that they perform. This characteristic ties SDRAM to the speed of the FSB and hence the processor, eliminating the need to configure the CPU to wait for the memory to catch up.

Originally, SDRAM was the term used to refer to the only form of synchronous DRAM on the market. As the technology progressed, and more was being done with each clock signal on the FSB (Front side bus), various forms of SDRAM were developed. Today, we use the term single data rate SDRAM (SDR SDRAM) to refer to this original type of SDRAM.

SDR SDRAM

With SDR SDRAM, has 168 pins .every time the system clock ticks, 1 bit of data can be transmitted per data pin, limiting the bit rate per pin of SDRAM to the corresponding numerical value of the clock’s frequency. With today’s processors interfacing with memory using a parallel data-bus width of 8 bytes (hence the term 64-bit processor), a 100MHz clock signal produces 800MBps. That’s megabytes per second, not megabits. Such memory modules are referred to as PC100, named for the true FSB clock rate they rely on. PC100 was preceded by PC66 and succeeded by PC133, which used a 133MHz clock to produce 1066MBps of throughput.

DDR SDRAM

The current technology is called DDR (double data ram) and there are three types. Any particular computer system can only use one of the three types. The types are:

·         DDR

·         DDR2

·         DDR3

Double data rate was double data rate (DDR) memory in a 184-pin and currently systems use DDR2 and DDR3 (both of which use 240-pin modules, but are not interchangeable). Double data rate (DDR) SDRAM produces its name by doubling the transfer rate of ordinary SDRAM. This obtains twice the transfer rate at the same FSB clock frequency. It’s the increasing clock frequency that generates heating issues with newer components, so keeping the clock the same is an advantage. The same 100MHz clock gives a DDR SDRAM system the impression of a 200MHz clock in comparison to an SDR SDRAM system. For marketing purposes and to aid in the comparison of disparate products (DDR vs. SDR, For example, with a 100MHz clock and two operations per cycle, motherboard makers will market their boards as having an FSB of 200MHz. Multiplying this effective rate by 8 bytes transferred per cycle, the data rate is 1600MBps. Because DDR made throughput a bit trickier to compute, the industry began using this final throughput figure to name the memory modules instead of the actual frequency, which was used when naming SDR modules. This makes the result seem many times better (and much more marketable), while it’s really only twice (or so) as good, or close to it.

DDR SPEED:-

Memory clock speed	I/O Clock speed	DDR Speed	Transfers/second	Transfer Rate	Module Name
100	100	DDR-200	200 Million	1600MB/s	PC-1600
133	133	DDR-266	266 Million	2100 MB/s	PC-2100
166	166	DDR-333	333 Million	2664 MB/s	PC-2700
200	200	DDR-400	400 Million	3200 MB/s	PC-3200

DDR2 Speed:-

Memory clock speed	I/O Clock speed	DDR Speed	Transfers/second	Transfer Rate	Module Name
100	200	DDR2 -400	400 Million	3200 MB/s	PC2-3200
133	266	DDR2-533	533 Million	4266 MB/s	PC2-4200
166	332	DDR2-667	667 Million	5333 MB/s	PC2-5300
200	400	DDR2-800	800 Million	8400 MB/s	PC2-8400

DDR3 SPEED:-

Memory clock speed	I/O Clock speed	DDR Speed	Transfers/second	Transfer Rate	Module Name
100	400	DDR-800	800 Million	6400MB/s	PC3-6400
133	533	DDR-1066	1066 Million	8533 MB/s	PC3-8533
166	667	DDR-1333	1333 Million	10667 MB/s	PC3-10667
200	800	DDR-1600	1600 Million	12800 MB/s	PC3-12800

6. RAMBUS RAM (RDRAM): RAMBUS is a future RAM type. Intel and others have great expectations from this type. RDRAM will have a bus speed of 600 MHz vs. SDRAM of 100 MHz.

2. SRAM (Static RAM): Static random access memory (SRAM) doesn’t require a refresh signal like DRAM does. The chips are more complex and are thus more expensive. However, they are considerably faster. DRAM access times come in at 40 nanoseconds (ns) or more; SRAM has access times faster than 10ns. SRAM is classically used for cache memory.

Types of SRAM Chips:

1.      VRAM (Video Random Access Memory): This is RAM optimized for graphics cards. VRAM and WRAM are both dual-ported memories that allow the microprocessor to draw into memory two chips at the same time. VRAM chips are larger than DRAM and can boost video performance by 60%.

2.      WRAM (Windows Ram): makes it easier to allow blocks or windows of memory to be addressed with just a few commands.

TYPES OF SECONDARY MEMORY

FLOPPY DISK 

A floppy disk, also called a floppy, diskette or just disk, is a type of disk storage composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic enclosure lined with fabric that removes dust particles. Floppy disks are read and written by a floppy disk drive (FDD).

Floppy disks, initially as 8-inch (200 mm) media and later in 5¼-inch (133 mm) and 3½-inch (90 mm) sizes, were a ubiquitous form of data storage and exchange from the mid-1970s into the late 2000s.

By 2007, computers were rarely manufactured with installed floppy disk drives; 3½-inch floppy disks can be used with an external USB floppy disk drive, but USB drives for 5¼-inch, 8-inch, and non-standard diskettes are rare to non-existent. These formats are usually handled by older equipment.

The floppy disk (or a 3½ floppy for today's standard) is a removable magnetic storage medium. Floppy disks are used for moving information between computers, laptops or other devices. Some early digital cameras, electronic music instruments and older computer game consoles use floppy disks. Floppy disks are inserted in to a floppy disk drive or simply floppy drive to allow data to be read or stored.

The floppy drive

 A floppy drive (often called drive “A” on a computer) can be connected to or installed in a personal computer (PC). It is the part of a computer that reads and writes the disk that can be removed. A disk, often called a floppy disk, is used to store files and carry them from one computer to another by using the floppy Drive. The floppy drive reads the disk (or diskette) and the user is then able to open and change the files saved on the disk.

                                                         

    Fig- A 3^1/2 inch floppy disk          Fig - A USB floppy drive. This one can be   plugged into        any modern computer

Capacities :

Hard Disk Drive: Hard Disk is present inside the system unit. Hard disk is the main secondary storage device that stores large amount of data permanently. Hard disk is also called fixed disk. Hard disk stores operating system software of a computer, data and other applications software. Hard disk consists of many metal disks or platters coated with magnetic material that store data in tracks, which is further divided into sectors. The various capacities of Hard disks are available in the market. Nowadays hard disk of 1TB (Terabyte) is available in the market.

MAGNETIC TAPE: A magnetic tape transport consists of the electrical, mechanical, and electronic components to provide the parts and control mechanism for a magnetic-tape unit. The tape itself is a strip of plastic coated with a magnetic recording medium. Bits are recorded as magnetic spots on the tape along several tracks. Usually, seven or nine bits are recorded simultaneously to form a character together with a parity bit. Read/write heads are mounted one in each track so that data can be recorded and read as a sequence of characters.

Magnetic tape unit can be stopped, stated, to move forward or in reverse, or can be rewound. However, they cannot be started or stopped fast enough between individual characters. For this reason, information is recorded in blocks referred to as records. Gaps of unrecorded tape are inserted between records where the tape can be stopped. The tape starts moving while in a gap and attains its constant speed by the time it reaches the next record. Each record on tape has an identification bit pattern at the beginning and end. By reading the bit pattern at the end of the record, the control recognizes the beginning of the gap. A tape unit is addressed by specifying the record number and the number of characters in the record. Records may be of fixed or variable length.

Optical Disc Drive:

An optical disc drive (ODD) is a disk drive that uses laser light or electromagnetic waves for reading or writing data to or from optical discs (normally CD-ROM). A CD-ROM is a pre-pressed optical compact disc which contains data. The name is an acronym which stands for "Compact Disc Read-Only Memory". Computers can read CD-ROMs, but cannot write to CD-ROMs which are not writable or erasable.

·         Compact Disk: Compact Disk/Digital Video Disk stores huge amount of data permanently. The Compact Disk Drive (CD-ROM) is required to read data from a compact disk. A CD-RW drive is used to write data on blank CDs and delete, modify or add data on an already written CD-RW (CD-Rewritable). Standard CDs have a diameter of 120 millimeter’s (4.7 in) and can hold up to about 80 minutes of uncompress audio or about 700 MB of data. The Mini CD has various diameters ranging from 60 to 80 millimeter’s (2.4 to 3.1 in); they are sometimes used for CD singles, storing up to 24 minutes of audio, or delivering device drivers.

 

·         DVD Drive: DVD is required to read the data from DVD (Digital Versatile Disk). DVD Writer allows you to read as well as write data on DVDs. The capacity of a DVD is 7 GB. DVD is portable disk that can be used for transferring data and programs from one computer to another computer. Since CDs and DVDs are reliable storage devices they are also used for keeping backup copy of data and programs. DVD disks are similar to CD disks except that they can store a lot more data. DVD has several formats and compatibility issues. DVD also introduces layers and sides, where more than one dye layer is formed on the disk, and sides, where the disk can hold data on both sides.

Each layer of a DVD can hold 4.7GB of data. The possibilities are:

Number of sides	1	1	2	2
Number of layers	1	2	1	2
Disk capacity	4.7GB	8.5GB	9.4GB	17GB

DVD formats:

1	DVD-ROM	Digital versatile disks are read only. These disks are mechanically stamped and can be manufactured as single or double layer and as single sided or double sided.
2	DVD-R	These are single or dual layer disks and can be single or double sided. They can be written to only once.
3	DVD-RW	These are the same as DVD-R only that they can be written to several times.
4	DVD+R	These are single or dual layer disks and can be single or double sided. They can be written to only once.
5	DVD+RW	These are the same as DVD+R only that they can be written to several times.

 

A number of new high capacity formats have come onto the market. These are:

1	HD-DVD	High Definition DVD uses a new laser technology and the result is that each layer can now hold 15GB of data instead of the 4.7GB capacity on standard DVD. This gives a total capacity of a double sided and double layered disk as 60GB.
2	Blu-Ray	This has a higher capacity than HD-DVD as a single layer can hold 25GB of data. A dual layer disk can hold 50GB. Technically, a double sided and double layered disk can hold 100GB of data.

 

Solid-state drive (SSD): A solid-state drive (SSD, also known as a solid-state disk) is a solid-state storage device that uses integrated circuit assemblies as memory to store data persistently. SSD technology primarily uses electronic interfaces compatible with traditional block input/output (I/O) hard disk drives (HDDs), which permit simple replacements in common applications. Additionally, new I/O interfaces, like SATA Express and M.2 have been designed to address specific requirements of the SSD technology.

Card reader drive: These drives have replaces floppy disk drives in the majority of new computer systems. They can read media cards from most digital cameras:

• Compact flash:

• Microdrive:

• Smart media:

• SD memory card:

• Memory stick/Duo/Pro:

• xD picture card:

Pen Drive: Pen drive is also called Flash Drive. Pen drive is a portable device used for transferring data and programs from a computer to another computer. You can use a pen drive to copy data, songs, games and programs from one computer to another computer. Different storage capacities of pen drives (4 GB to 32GB) are available in the market.