Memory Revisited

The different types of memories available for a computer are shown in Fig. 1.4.

1. Primary Memory

All modern computers use semiconductor memory as primary memory. One of the important semiconductor memories used in desktop computers is Random Access Memory (RAM). Here “random access” means that any storage location can be accessed (both read and write) directly. This memory is faster, cheaper, and provides more storage space in lesser physical area. These very large-scale integrated semiconductor memory chips are mounted on pluggable printed circuit board  (PCBs). Enhancement or replacement of memory with such PCB memory modules is easy. These characteristics have made semiconductor memory more popular and attractive. The only drawback of semiconductor memory is that it is volatile, i.e., it loses its contents whenever power is switched off. RAM holds the data and instructions waiting to be processed by the processor. In addition to data and program’s instructions, RAM also holds operating system instructions that control the basic functions of a computer system. These instructions are loaded into RAM every time the computer is turned on, and they remain there until the computer is turn off. There are two types of RAM used in computer systems– dynamic and static.

Dynamic RAM (DRAM) is a type of RAM that employs refresh circuits to retain its content in its logic circuits. Each memory cell in DRAM consists of a single transistor. The junction capacitor of the transistor is responsible for holding the electrical charge that designates a single bit as logical 1. The absence of a charge designates a bit as logical 0. Capacitors lose their charge over time and therefore need to be recharged or refreshed at pre-determined intervals by a refreshing circuitry.

A more expensive and faster type of RAM, Static RAM (SRAM), does not require such type of refreshing circuitry. It uses between four to six transistors in a special ‘flipflop’ circuit that holds a 1 or 0 while the computer system is in operation. SRAM in computer systems is usually used as processor caches and as I/O buffers. Printers and liquid crystal displays (LCDs) often use SRAM to buffer images. SRAM is also widely used in networking devices, such as routers, switches, and cable modems, to buffer transmission information. Both dynamic and static RAM are volatile in nature and can be read or written to. The basic differences between SRAM and DRAM are listed in Table 1.3.

Table 1.3 Static RAM versus dynamic RAM

There are several popular types of dynamic RAM used in computers. They are SDRAM (Synchronous Dynamic RAM), RDRAM (Rambus Dynamic RAM) and DDR RAM (Double Data Rate RAM). The SDRAM used to be the most common type of RAM for personal computers. It was reasonably fast and inexpensive. It is no more used in the present day for personal computers as much improved RAMs are available now. The RDRAM was developed by Rambus Corporation and is its proprietary technology. It is also the most expensive RAM and is used mostly in video interface cards and highend computers that require fast computation speed and data transfer. RDRAMs are preferred for high-performance personal computers. The DDR RAM is a refinement of SDRAM. DDR stands for Double Data Rate. It gives faster performance by transmitting data on both the rising and the falling edges of each clock pulse. DDR 2, DDR3 are other higher-speed versions of DDR RAM. Another type of RAM, termed Video RAM (VRAM), is used to store image data for the visual display monitor. All types of video RAM are special arrangements of dynamic RAM (DRAM). Its purpose is to act as a data storage buffer between the processor and the visual display unit.

 There is a persistent mismatch between processor and main memory speeds. The processor executes an instruction faster than the time it takes to read from or write to memory. In order to improve the average memory access speed or rather to optimize the fetching of instructions or data so that these can be accessed faster when the CPU needs it, cache memory is logically positioned between the internal processor memory (registers) and main memory. The cache memory holds a subset of instructions and data values that were recently accessed by the CPU. Whenever the processer tries to access a location of memory, it first checks with the cache to determine if it is already present in it. If so, the byte or word is delivered to the processor. In such a case, the processor does not need to access the main memory. If the data is not there in the cache, then the processer has to access the main memory. The block of main memory containing the data or instruction is read into the cache and then the byte or word is delivered to the processor. There are two levels of cache.

Level 1 (Primary) cache This type of cache memory is embedded into the processor itself. This cache is very fast and its size varies generally from 8 KB to 64 KB.

Level 2 (Secondary) cache Level 2 cache is slightly slower than L1 cache. It is usually 64 KB to 2 MB in size. Level 2 cache is also sometimes called external cache because it was external to the processor chip when it first appeared.

Read Only Memory (ROM)

It is another type of memory that retains data and instructions stored in it even when the power is turned off. ROM is used in personal computers for storing start-up instructions provided by the manufacturer for carrying out basic operations such as bootstrapping in a PC, and is programmed for specific purposes during their fabrication. ROMs can be written only at the time of manufacture. Another similar memory, Programmable ROM (PROM), is also non-volatile and can be programmed only once by a special device. But there are instances where the read operation is performed several times and the write operation is performed more than once though less than the number of read operations and the stored data must be retained even when power is switched off. This led to the development of EPROMs (Erasable Programmable Read Only Memories). In the EPROM or Erasable Programmable Read Only Memory, data can be written electrically. The write operation, however, is not simple. It requires the storage cells to be erased by exposing the chip to ultraviolet light, thus bringing each cell to the same initial state. This process of erasing is time consuming. Once all the cells have been brought to the same initial state, the write operation on the EPROM can be performed electrically. There is another type of Erasable PROM known as Electrically Erasable Programmable Read Only Memory (EEPROM). Like the EPROM, data can be written onto the EEPROM by electrical signals and retained even when power is switched off. The data stored can be erased by electrical signals. However, in EEPROMs the writing time is considerably higher than reading time. The biggest advantage of EEPROM is that it is non-volatile memory and can be updated easily, while the disadvantages are the high cost and the write operation takes considerable time.

Note:

• RAM holds raw data waiting to be processed as well as the program instructions for processing that data. It also stores the results of processing until they can be stored more permanently on secondary storage media. Most important point to be noted is that RAM holds operating system instructions which are loaded at start-up and time to time as and when required.
•  Dynamic RAM is less expensive, consumes less electrical power, generates less heat, and can be made smaller, with more bits of storage in a single integrated circuit. Static RAM provides faster access with lower bit density and are more expensive than dynamic RAM.
• ROM contains a small set of instructions that tell the computer how to access the hard disk, find the operating system, and load it into RAM. After the operating system is loaded, the computer can accept input, display output, run software, and access data.
• The programmable read-only memory (PROM) is nonvolatile and can be reprogrammed only once by a special write device after fabrication. An erasable programmable ROM (EPROM) can be erased by ultraviolet (UV) light or by high-voltage pulses.

2.Secondary Memory

There are four main types of secondary storage devices available in a computer system:

• Disk drives
• CD drives (CD-R, CD-RW, and DVD)
• Tape drives
• USB flash drives

Hard disk, floppy disk, compact disk (CD), Digital Versatile Disk (DVD) and magnetic tapes are the most common secondary storage mediums. Hard disks provide much faster performance and have larger capacity, but are normally not removable; that is, a single hard disk is permanently attached to a disk drive. Floppy disks, on the other hand, are removable, but their performance is far slower and their capacity far smaller than those of hard disks. A CD-ROM or DVD -ROM is another portable secondary memory device. CD stands for Compact Disc. It is called ROM because information is stored permanently when the CD is created. Devices for operating storage mediums are known as drives. Most of the drives used for secondary memory are based on electro-mechanical technology. Mechanical components move much more slowly than do electrical signals. That’s why access to secondary memory is much slower than access to main memory.

The floppy disk is a thin, round piece of plastic material, coated with a magnetic medium on which information is magnetically recorded, just as music is recorded on the surface of plastic cassette tapes. The flexible floppy disk is enclosed inside a sturdier, plastic jacket to protect it from  damage. The disks used in personal computers are usually 3½ inches in diameter and can store 1.44 MB of data. Earlier PCs sometimes used 5¼ inch disks. The disks store information and can be used to exchange information between computers. The floppy disk drive stores data on and retrieves it from the magnetic material of the disk, which is in the form of a disk. It has two motors one that rotates the disk media and the other that moves two read-write heads, each on either surface of the disk, forward Floppy Disk Drive or backward.

A hard disk is a permanent memory device mounted inside the system unit. Physically, a hard disk consists of one or more metal (sometimes aluminum) platters, coated with a metal oxide that can be magnetized. The platters are all mounted on a spindle, which allows them to spin at a constant rate. Read/write heads are attached to metal arms and positioned over each of the platter surfaces. The arms can move the read/write heads radially inwards and outwards over the surfaces of the platters (see Fig. 1.5). Data and programs are stored on the hard disk by causing the write heads to make magnetic marks on the surfaces of the platters. Read heads retrieve the data by sensing the magnetic marks on the platters. The surface of each platter is divided into concentric rings called tracks. The tracks form concentric circles on the platter’s surface. Each track is divided into a certain number of sectors. A sector is capable of generally 512 bytes or sometimes 1,024 bytes of data. The head is mounted on an arm, which moves or seeks from track to track. The vertical group of tracks at the same position on each surface of each platter is called a cylinder. Cylinders are important, because all heads move at the same time. Once the heads arrive at a particular track position, all the sectors on the tracks that form a cylinder can be read without further arm motion. The storage capacity of a hard disk is very large and expressed in terms of gigabytes (GB). The data that is stored on the hard disk remains there until it is erased or deleted by the user.

The hard disk drive provides better performance and become mandatory for computer systems for the following reasons:

• Higher capacity of data storage
• Faster access time of data
•  Higher data transfer rates
• Better reliability of operation
•  Less data errors or data loss

A CD is a portable secondary storage medium. Various types of CDs are available: CD-R and CD-RW. CD-RW drives are used to create and read both CD-R and CD-RW discs. Once created (i.e. when it has been “burned”), data stored on CD-R (CDRecordable) disc can’t be changed. On the other hand, a CDRewritable (CD-RW) disc can be erased and reused. This disk is made of synthetic resin that is coated with a reflective material, usually aluminum. When information is written by a CD-writer drive, some microscopic pits are created on the surface of the CD. The information bit on a CD-ROM surface is coded in the form of ups and downs (known as pits and dumps), created by infrared heat. There is one laser diode on the reading head. The bits are read by shining a low - intensity laser beam onto the spinning disc. The laser beam reflects strongly from a smooth area on the disc but weakly from a pitted area. A sensor receiving the reflection determines whether each bit is a 1 or a 0 accordingly.

CDs were initially a popular storage media for music; they were later used as general computer storage media. Most personal computers are equipped with a CD-Recordable (CD-R) drive. A CD-Rewritable (CD-RW) disc can be reused because the pits and flat surfaces of a normal CD are simulated on a CDRW by coating the surface of the disc with a material that, when heated to one temperature becomes amorphous (and therefore non-reflective) and when heated to a different temperature becomes crystalline (and therefore reflective).

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