3.2 MEMORY AND STORAGE DEVICES
Memory is an important component of a computer where all the data and information are stored in the form of binary digits (combination of 0‟s and 1‟s) and retrieved whenever necessary. There are two main functions of the memory:
To store programs, data and information into the computer.
To store the results of computation.
A computer system uses a variety of devices for storing the instructions and data. When you want to execute a computer program, the program has to be in memory. Any input data needed for processing by that program should also be in memory. All the intermediate results and outputs from the program are stored in the memory until the machine is turned off. The storage devices of a computer system are ranked according to the following criteria:
1. Access time: This is the time required to locate and retrieve stored data from the storage unit in response to a program instruction. That is the time interval between the read/write request and the availability of the data. A fast access time is always preferred.
2. Storage capacity: It is the amount of data that can be stored in the storage unit. A large capacity is preferred.
3. Cost per bit of storage: It is the cost of a storage unit for a given storage capacity. Low cost per bit of storage is always preferred. The final goal is to minimize this cost.
Based on above mentioned criteria, at present the following three kinds of memory system are commonly used in modern computers:
Thus from above discussions, we can summarize the following points:
Secondary memory cannot be accessed directly by the CPU. First the information of these memories (which is needed by the CPU for current processing) is transferred to the main memory and then the information can be accessed as the information of main memory. Hard-disk and floppy disks are the most common secondary memories used in computers.
Secondary storage systems must offer large storage capacities, low cost per bit and medium access times. Magnetic media (such as floppy disks and hard disks) have been used for such purposes for a long time. But audio and video media, either in compressed form or uncompressed form, require higher storage capacity than the other media forms and the storage cost for such media is significantly higher.
Optical storage devices offer a higher storage density at a lower cost. A CD-ROM can be used as an optical storage device. Many software companies offer both operating system and application software on CD-ROMs today. This technology has been the main catalyst for the development of multimedia in computing because it is used in multimedia external devices such as video recorders and digital recorders (Digital Audio Tape) which can be used for multimedia systems.
Removable disk, tape cartridges are other forms of secondary storage devices used for back-up purposes having higher storage density and higher transfer rate.
There is another type of high speed memory, known as Cache memory, which is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate. Cache memory is a relatively small, high speed memory that stores the most recent used instructions or data. It acts as a high-speed buffer between main memory and the CPU.The cache memory is placed in between CPU and main memory. Access time is the time it takes a device or program to locate information and make it available to the computer for further processing. Cache memory access time is about 0.5 to 2.5 ns which is much less than that of the main memory. The access time of main memory is about 50-70 ns. Because of its very high cost, the capacity of the cache memory deployed is 2 to 3 percent of that of the main memory. The access time of mass storage devices such as hard disks are measured in milliseconds (ms). The most common memory hierarchy is shown in Figure 3.3 :
Now let us start with the memory organization of primary storage. A primary or internal storage section is basic to all computers. Figure 3.4 compares the different types of memory in terms of capacity, access speed, cost per bit of storage as follows:
All the memory devices can be categorized into three main categories:
Semiconductor (or Main) memory
Magnetic memory
Optical memory
The Figure 3.5 illustrates the storage cost, speed and capacity of these memories. Note that cost increases with faster access speeds but decreases with access capacity.
You can note down the following points from the Figure 3.5
Semiconductor memories are used mainly for primary storage. It stores programs and data which are currently needed by the CPU.
The semiconductor memory is an electronic, static device. There are no moving parts in it. Some examples of semiconductor memory are RAM, ROM etc.
The semiconductor memory is faster, compact and lighter. It consumes less power.
The magnetic and optical memories are slow compared to semiconductor memory.
But they are cheaper than semiconductor memory. They are not static devices. They are either in the form of a rotating disk or tape. All computers contain both semiconductor as well as magnetic memory. The examples of magnetic memory are Hard-disk, floppy disk, magnetic disk and tapes. The Figure 3.6 shows a relationship between the access-time and capacity of various types of memory.
Optical recording techniques have been recently used to store data on the surface of a coated disk. Information is written to or read from an optical disk using a laser beam. An example of this kind of serial access memory is a CDROM (Compact Disk Read-Only Memory). Only one surface of an optical disk is used to stored data. An optical disk has very high storage capacity, up to 20 GB. It is relatively inexpensive and has a long life of at least 15-20 years. Better optical recording methods which records data on multiple layers on a disk surface have been recently introduced. This storage device is known as DVD-ROM (Digital Versatile Disk Read-Only Memory). The main drawback of the optical disk system is its slow average access time. Table 3.3 shows the some characteristics of the discussed various memory technologies.
Note that there are two basic methods of accessing information from various memory devices :
Sequential or serial access, or
Direct or Random access
A Sequential-access memory device reads data in sequence.
In other words, information on a serial device can only be retrieved in the same sequence in which it is stored. Data is recorded one after another in a predetermined sequence (such as in numeric order) on a storage medium. Sequential processing is quite suitable for such applications like preparation of monthly pay slips, or monthly electronic bills etc., where each address needs to be accessed in turn. If you are working with a sequential access device and information is stored at the last address, then data stored at the last address cannot be accessed until all preceding locations in the sequence have been traversed. That is locating an individual item of data requires searching the recorded data on the tape until the desired item is located.
A sequential-access memory such as magnetic tape is organized by arranging memory cells in a linear sequence. These do not have unique storage address that can be directly addressed. Instead, data is presented serially for writing and is retrieved serially during a read.
In case of a random access device the information is available at random, i.e., any location in the device may be selected at random. So any location in the device can be accessed in approximately equal time in any order. In other words, we can say that each storage position (1) has a unique address and (2) can be individually accessed in approximately equal time without searching through other storage positions. Magnetic disk and CDROM are typical random access storage devices. Any data record stored on a magnetic or optical disk can be accessed directly in approximately the same time period. The Figure 3.8 shows sequential versus direct access storage:
Basic Storage Fundamentals
Data is processed and stored in a computer system through the presence or absence of electronic or magnetic signals in the computer‟s circuitry (ie. RAM) or in the media it uses (i.e. magnetic Disk). This is called a “two-state” or Binary representation of data. Transistor and other semiconductor circuits are either in conducting or in non-conducting states. For Magnetic media, such as magnetic disk or tapes, these two states are represented by having magnetized spots whose magnetic fields have one of two different directions or polarities.
For any electronic circuits, the conducting (ON) state represents the number 1, while the non-conducting (OFF) state represents the number 0.This is so only for positive logic. One can always have the reverse convention that we call negative logic. For magnetic media, the magnetic field of a magnetized spot in one direction represents a 1 while magnetism in the other direction represents a 0.
The smallest element of data is called a bit, which can have a value of either 0 or 1. The capacity of a memory chip is usually expressed in terms of bits. A group of 8-bits is known as a byte, which represents one character of data in most computer coding schemes. Thus, the capacity of a computer‟s memory and secondary storage devices is usually expressed in term of bytes. Computer codes such as ASCII (American Standard Code for Information Interchange) use various arrangements of bits to form bytes that represent the numbers 0 to 9, the letters of the alphabet, and many other characters.
3.2.1 Semiconductor (Main) Memory
All computers except very small computers contain both semiconductor as well as magnetic memory.
All modern computers use semiconductor memory as its main memory (or primary memory). Semiconductor memory is known as Random access memory (RAM) because any part of the memory can be accessed for reading and writing.
It stores programs and data which are currently needed by the CPU.
Another part of main memory is Read Only Memory (ROM). ROMs are those memories on which it is not possible to write the data. They can only be read.
Thus RAM and ROM memories are used as the main memory of the computer.
The Main memory holds the programs and data required by the CPU for carrying out its operations.
The primary (main) storage is a semiconductor device that is built using integrated circuits. The data is stored in binary form in main memory. Numeric as well as non-numeric data can be represented in binary form. With two binary digits, we can represent 4 different characters. With three binary digits, we can represent 8 different characters. Computers internally use eight binary digits to represent characters and digits (A binary digit is referred to as a bit and 8 bits are called a byte). 256 characters can be represented by a byte.
The capacity of a computer‟s memory is usually expressed in terms of bytes. Computer codes such as ASCII (American Standard Code for Information Interchange) use various arrangements of bits to form bytes that represent the numbers 0 to 9, the letters of the alphabet, and many other characters.
Storage capacities are frequently measured in Kilobytes (KB), Megabytes (MB), Gigabytes (GB), or Terabytes (TB). Table 3.4 summarizes the commonly used names with abbreviations and number of bytes for these storage capacities.
Types of Main Memory
Memory can be of various types like Random Access Memory (RAM) and Read-Only Memory (ROM). Figure 3.9 summarizes the different types of main memory.
RAM (Random Access Memory)
The Read and write memory (R/W memory) of a computer is called a RAM. The user can write information into RAM and read information from it. It is called random access since any memory location can be accessed in a random manner for reading and writing. The access time is the same for each memory location. It usually refers to “temporary” memory, which means that when the system is shut down, the memory is lost.
Random Access Memory (RAM) is really the main store and is the place where the program and software we load gets stored. When the CPU runs a program, it fetches the program instructions from the RAM and carries them out. Similarly, if the CPU needs to store the final results of calculations, it stores them in RAM. Thus, the CPU can both READ data from RAM and WRITE data into the RAM.
There are two important types of RAMs:
Static RAM (or SRAM)
Dynamic RAM (or DRAM)
Static RAMs retain stored information only as long as the power supply is on whereas a Dynamic RAM loses its stored information in a very short time (a few milliseconds) even though the power supply is on.
Dynamic RAMs are cheaper and consume less power whereas Static RAMs are costlier and consume more power. Static RAMs have a higher speed than dynamic RAMs.
Dynamic RAM is cheaper and so is used for main memory. Static Ram is faster and so is used in cache memory.
Dynamic RAM requires the data to be refreshed periodically in order to retain the data while SRAM does not need to be refreshed.
Both static and dynamic RAMs use CMOS technology. CMOS devices consume less power. Static RAMs hold information in a flip-flop circuit consisting of two cross coupled inverters. In a RAM the memory cell must be associated with a read and write facility. Six (6) transistors are needed per memory cell in a static RAM. Dynamic RAMs required fewer transistors per memory cell. The following are commonly used RAM chips:
EDO (Extended Data Output RAM): In an EDO RAM any memory access stores 256 bytes of data into latches. The latches hold next 256 bytes of information, so that in most programs which are sequentially executed, the data are available without wait states.
SDRAM (Synchronous DRAM) and SGRAM (Synchronous Graphics RAM): These RAM chips use the same clock rate as the CPU uses. As a result the memory chips remain ready to transfer data when the CPU expects them to be busy. SDRAM is often used as mass storage whereas SGRAM is used as a high end graphics memory.
Dual-Ported DRAM: These types of RAM allow one to access two memory locations simultaneously. Sometimes it is also called video RAM (or VRAM). WRAM (Window RAM) is a special version of VRAM, which is commonly used in PCs running WINDOWS and WINDOWS applications.
SIMM and DIMM: These stand for single-Inline and Double Inline Memory Modules. These are small printed circuit cards, on which several DRAM memory chips are placed. Such cards are plugged into the system board of the computer.
ROM (Read Only Memory)
A Read-Only memory (ROM) is a non-volatile memory, i.e., the information stored in it is not lost even if the power supply goes off. Thus a Read Only Memory (ROM) is one in which information is stored permanently.
Unlike RAM, the information from ROM can only be READ and it is not possible to WRITE fresh information to it. That is, the CPU can only fetch or READ instructions from ROM. This is the reason why it is called ROM. Computers almost always contain a small amount of Read-Only memory (ROM). It is much cheaper compared to RAMs when produced in large volumes.
ROM is used for storing a special set of instruction, which the computer needs when it starts up (boots up).
The contents of ROMs are decided by the manufacturers. The contents are permanently stored in a ROM at the time of manufacture.
From the programming mode point of view, we have
Masked-programmed
User-programmed
ROMs in which contents are written at the time of IC manufacture are called mask-programmed ROMs. PROM, EPROM and EEPROM or any other kind of PROM are user programmable ROMs. If we simply write (or say) ROM it means masked programmed.
An example of a ROM is the Toshiba mask ROM, TCS 534000.
PROM (Programmable ROM)
A variation of ROM chip is programmable read only memory (PROM). A PROM is a memory chip on which data can be written only once.
ROM chips are supplied by computer manufacturer and it is not possible for a user to modify the programs stored inside the ROM chip. However, in case of PROM, it is possible for a user to customize a system by storing own program in a PROM chip.
Once a program has been written on to a PROM chip, the recorded information cannot be changed i.e., the PROM becomes a ROM and it is only possible to read the stored information.
PROM is also a non-volatile memory i.e. the stored information remains even if power is switched off.
The basic difference between PROM and a ROM is that a PROM is manufactured as blank memory, whereas a ROM is programmed during the manufacturing process.
To write data on a PROM chip, you need a special device called a PROM programmer or a PROM burner. The process of programming a PROM is sometimes called burning the PROM.
3.2.2 Magnetic Memory
In the above section we have seen various types of semiconductor RAMs. These high speed semiconductor storage devices (i.e. RAMs) are expensive. So we need some inexpensive media for storage. Also semiconductor memory has the following limitations:
1) Limited Capacity: Semiconductor (primary) memory of today‟s computers is not sufficient, since most of the data processing organizations deal with a large volume of data.
2) Volatile Memory: Semiconductor memory is volatile in nature. But there is always a need to store data on a permanent basis.
Thus there is a need of additional memory, that is inexpensive, non-volatile in nature and has large capacity. Magnetic material is inexpensive and long lasting, so it is an ideal choice for us. Magnetic memory is a permanent non-volatile, type of memory. Now-a-days, we are not using floppy disk. A modern computer uses the following two types of magnetic memory:
(i) Magnetic Disks: Hard disks and Floppy disks.
(ii) Magnetic Tapes : Magnetic disks are the most common form of secondary storage because they provide fast access and high storage capacities at a reasonable cost.
Storage Mechanism: Magnetic disk drives contain metal disks that are coated on both sides with an iron oxide recording material. Several disks are mounted together on a vertical shaft which typically rotates the disks at speeds of 3600 to 7600 revolutions per minute (rpm). Electromagnetic read/write heads are positioned by access arms between the slightly separated disks to read and write data on concentric, circular tracks. Data are recorded on tracks in the form of tiny magnetized spots to form the binary digits of common computer codes. Thousands of bytes can be recorded on each track, and there are several hundred data tracks on each disk surface, which provides billions of storage positions for your software and data.
There are basically two types of magnetic disk arrangements, one having a removable disk cartridge and other having a fixed disk unit. Removable disk devices are popular because they are transportable and can be used as backup copies of your data.
Data Organizations: A magnetic disk is a surface device, which stores data on its surface. Its surface is divided into circular concentric tracks. The number of tracks on a disk range up to 800. Each track is divided into sectors (normally 10-100). These sectors can be either fixed or variable length sectors. The division of track into equal sized blocks or pages is set by the Operating system during disk formatting. The number of bytes stored in each sector is kept the same.
The numbers vary but there are often 200 or more ranging up to 800 is sectors per track. Magnetic disks are semi-random devices. A track on a disk is selected in a random fashion, but data is written to or read from a sector in serial fashion. Hard-Disk Drives (HDD)
Hard disks are on-line storage devices.
The term online means that the device (hard-disk) is permanently connected to the computer system and when the computer is on, the device (hard-disk) is available to store information or to retrieve information.
HDD stores programs, data, operating system, compiler, assemblers, application programs etc.
Storage Organization in HDD
HDD contains magnetic disks, access arms and read/write heads into a sealed, air filtered enclosure. This technique is known as Winchester technique.
Winchester disk is another name for “hard disk drive”. There are two stories behind the name Winchester disks; one is that the disk was developed at IBM‟s facility at Winchester, New York State; that had 30MB of fixed storage and 30MB of removable storage; the other is that the first model number was given as 3030, which is also the model number of the well-known Winchester Rifle popular in the Wild West. Although modern disk drives are faster and hold more data, the basic technology is the same, so Winchester has become synonymous with hard disk.
Thus Winchester disk is a sealed “hard disk” having rotation speed typically 7200 rpm. A disk has 5000 to 10,000 concentric tracks per centimeter and about 100,000 bits per centimeter around circumference. Figure 3.12 illustrates a portion of Winchester disk.
The read/write head reads data from the disk and writes data to the disk. A disk is mounted (or stacked) on the disk drive, which has the motor that rotates it. Hard-disks together with read/write heads, access mechanism and driving motor constitute a unit called hard-disk-drive (HDD) unit. The whole unit is fixed.
Hard disk is also known as platter. It can not be removed or inserted into a HDD unit. Some disks have a single platter e.g. floppy disk.
To increase the storage capacity several hard-disks (platters) are mounted (stacked) vertically, normally at a distance of an inch. This is known as disk pack or multi-platter configuration.
A set of corresponding tracks in all surfaces of a disk pack (i.e. the tracks with the same diameter on the various surfaces) is called a cylinder (see Figure 3.13). Here the concept of cylinder is very important because data stored on the same cylinder can be retrieved much faster than if it were distributed among different cylinders.
Relationship among Capacity, density and speed Suppose a HDD (or disk pack) having n plates, has: m=2n= total number of recording surfaces t= tracks per surface p= Sectors per track s=bytes per sector, π=3.14 then
Storage capacity of the disk=(m*t*p*s) bytes
If d is the diameter of the disk, the density of the recording is:
Density=(s*p)/(π*d) byte/inches
There are several disk drives (C,D,F etc.) in a computer, which are connected to a disk controller. The controller converts instructions received form the computer (software) to electrical signals to operate disks. The Disk controller accepts commands from the computer and positions the read/write head of the specified disk for reading or writing.
For reading or writing operations on a disk pack, the computer must specify the drive number, cylinder number, surface number, and sector number. The drive number must be specified, because a controller normally controls more than one drive. Table 3.6 shows a disk address format for a disk controller of 8 drives, each disk pack having 250 cylinders, 12 surface and 256 sectors.
Access time on a magnetic disk
Magnetic disks are semi-random devices. A track on a disk is selected in random fashion, but data is written to or read from a sector in serial fashion. In order to access information from a disk, the disk address of the desired data has to be specified. The disk address is specified in terms of track number, surface number and the sector number. Information is always written from the beginning of a sector and can be read only from the track beginning.
As soon as the read/write command is received by the disk controller, the read/write heads are first positioned onto the specified track number (or cylinder) by moving the arm assembly in the proper direction. The time required to position the read/write head over proper track is called the seek time. Seek time (Ts): The time required to move the read/write head on a specific (address) track.
Seek time varies depending on the position of the arm assembly when a read/write command is received.
Seek time will be maximum, if the arm assembly is positioned on the outer most track and the track to be reached is the inner most one and it will be zero if the arm assembly is already on the desired track.
The average seek time is thus specified for most systems which is generally between few milliseconds to fractions of a second.
Note that seek time is associated only with movable-head system. For a fixed-head system, it is always 0 because there is a head for each track and no head movement is required for accessing a particular track.
Once the heads are positioned on the desired track, the head on the specified surface is activated. Since the disk is continuously rotating, this head should wait for the desired data (specified sector) to come under this head. This rotational waiting time i.e. time required to bring the needed data (i.e. starting position of the addressed sector) under the read/write head is called the latency time.
Latency Time (tL) or Search time: Time required to bring the needed data under the R/W head. Latency time is also a variable and depends on the following two parameters:
Distance of the desired data from the initial position of the head on the specified track.
Rotational speed of the disk.
The average seek time is thus normally specified for most systems which is generally of the order of 10 to 15 milliseconds. The total access time for a disk is equal to the seek time plus the latency time.
The total access time for a disk is equal to the seek time plus the latency time.
The average access time for most disk systems is usually between 10 to 100 milliseconds. Pen Drive
Now-a-days a Pen Drive is available as a very convenient and flexible data storage medium which can store up to 256 GB data. It can be used for the same purposes as floppy-disks or CD-ROMs. Pen Drives are a smaller, faster, durable and more reliable storage medium. Compared to floppy disks or CD-ROMs it has thousands of times more data storage capacity. It is a portable USB flash memory device. It is integrated with a USB (Universal Serial Bus) interface. It can be used to quickly transfer data from one system to another. The pen drive derives its name from the fact that many of these devices resemble a small pen or pencil in shape and size. Flash drives implement the USB mass storage device class so it is possible for modern operating systems to read and write from them without installing the device driver software. Some computers can even boot up from flash drives.
Magnetic Tapes A Magnetic tape is a sequential access type secondary storage device. It is used for backups in servers, workstations, and large computers. The main advantages of magnetic tapes are that they are cheaper and since these are removable from the drive, they provide unlimited storage capacity (20 GB to 150 GB).
The read/write heads of magnetic tape drives record data in the form of magnetized spots on the iron oxide coating of the plastic tape. Magnetic tape devices include tape reels and cartridges in mainframes and midrange systems, and small cassettes or cartridges for PCs. The main drawback of magnetic tapes is that they store information sequentially. A file or some particular information stored on a magnetic tape cannot be accessed directly on random basis as is possible in the case of hard-disks or floppy disks. These devices are slower, but due to their low cost, they are still widely used for massive data warehouse and other business storage requirements. The storage capacity of a tape is measured by multiplying its length and data recording density. Data recording density is the amount of data that can be stored on a given length of tape. That is,
Storage Capacity = data recording density * length
3.2.3 Optical Memories
Optical memories or Optical disks are alternate mass storage devices with huge capacity (up to 20 GB). Information is written to or read from an optical disk using a laser beam. Only one surface of an optical disk is used to stored data. An optical disk is relatively inexpensive, and has a long life of at least 15 years. Since the read/write head does not touch the disk surface, there is no problem of disk wear or head crash. The main draw back of the optical disk system is its slow average access time. Here, we will discuss 3 types of optical disks:
1. CD-ROM (Compact-Disk Read Only Memory)
2. WORM (Write Once Read many) or CD-R (CD-Recordable).
3. Erasable Optical Disk
4. DVD-ROM, DVD-R and DVD-RAM
(1) CD-ROM
CD-ROM technology uses 12-centimeter (4.7-inch) compact disks (CDs) similar to those used in stereo music systems. Each disk can store more than 600 MB. That is approximately equivalent to 400 1.44 MB floppy disks or 300,000 double-spaced pages of text.
First of all a master disk is prepared. On a master disk, a laser records data by burning permanent microscopic pits in a spiral track to represent 1. From a master disk, CD-ROMs are produced on mass scale. Then CD-ROM disk drives use a laser device to read the binary codes formed by those pits.
For reading the data a laser beam of lower intensity is employed. A laser system needs 25mW for writing whereas only 5mW are needed for reading.
CD-ROMs use long spiral tracks to store data serially, as shown in Figure 3.15. The track is divided into blocks of same size as shown in the figure. A CD-ROM disk rotates at a variable speed so that the pits are read by the laser at a constant linear speed. The speed of the disk is adjusted in such a way that the track passes under the read/write head at a constant linear velocity.
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