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.
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:
- 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.
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.
- 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.
For any electronic
circuits, the
conducting (ON)
state represents the
number 1, while
the non-conducting
(OFF) state
represents the
number 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.
👉Check Your Progress 1
1. State whether True or False
a) Primary memory is faster than secondary memory but that has a larger capacity.
True False
True False
True False
b) Primary memory is mainly used for bulk storage.
c) CD-ROM is a Random access storage device.
d) When we load software from a floppy, hard disk or CD-ROM, it is stored in
main
True False
e) In Random access memory any memory location can be accessed directly.
True False
f) Non-volatile memory means the stored data are lost when power goes off.
True False
2. Differentiate between the following:
a) Primary (main) memory versus secondary memory
……………………………………………..………………………..……………
……………………………………………………………………………………
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b) Different types of memory (in terms of access speed, storage capacity and cost
per bit storage).
……………………………………………..………………………..……………
……………………………………………………………………………………
………………………………………………………..………………………..…
c) A graph showing Capacity versus access time for different types of memory.
……………………………………………..………………………..……………
……………………………………………………………………………………
………………………………………………………..………………………..…
d) Random versus Sequential access.
……………………………………………..………………………..……………
……………………………………………………………………………………
………………………………………………………..………………………..…
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 Memory System 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.
(Storage
capacities are
frequently
measured in
Kilobytes
(KB),
Megabytes
(MB),
Gigabytes
(GB), or
Terabytes
(TB)
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.
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 Memory System 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.
- 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.
