Memory Concept

Digital Principles and Computer Organization:

Chapter 11: Memory

 

Memory Concept

• Memories are made up of registers. Each register in the memory is one storage location also called memory location.

• Each memory location is identified by an address. The number of storage locations can vary from a few in some memories to hundreds of thousand in others. Each location can accommodate one or more bits.

• Generally, the total number of bits that a memory can store is its capacity. Most of the types the capacity is specified in terms of bytes (group of eight bits).

• Each register consists of storage elements (flip–flops or capacitors in semiconductor  memories and magnetic domain in magnetic storage), each of which stores one bit of data. A storage element is called a cell.

• The data stored in a memory by a process called writing and are retrieved from the memory by a process called reading. Fig. 11.1.1 illustrates in a very simplified way the concept of write, read, address and storage capacity for a generalized memory.

• As shown in Fig. 11.1.1 a memory unit stores binary information in groups of bits called words. A word in memory is an entity of bits that moves in and out of storage as a unit. A word having group of 8–bits is called a byte. Most computer memories use words that are multiples of eight bits in length. Thus, a 16–bit word contains two bytes, and a 32–bit word is made of 4–bytes.

• The communication between a memory and its environment is achieved through data lines, address selection lines, and control lines that specify the direction of transfer.

• Fig. 11.1.2 shows the block diagram of memory unit. Then data lines provide the information to be stored in memory and the k address lines specify the particular word chosen among the many available. The two control inputs specify the direction transfer.

• When there are k address lines we can access 2^k memory words. For example, if k = 10 we can access 2¹⁰ = 1024 memory words.

Illustrative Examples

Example:  1

A bipolar RAM chip is arranged as 16 words. How many bits are stored in the chip?

Solution :

16 + 8 = 128 bits

one word = 8 bits.

Example: 2

How many address bits are needed to operate a 2 K × 8 ROM?

Solution :

2 K memory locations = 2048 locations

Since 2¹¹ = 2048, we need 11 address lines.

Example: 3

How many locations are addressed using 18 address bits?

Solution :

The number of locations addressed = 2¹⁸ = 262144.

 

1. Characteristics of Memory

Table 11.1.1 lists the key characteristics of memory systems.

Location –

CPU

Internal (main)

External (Secondary)

Capacity –

Word size

Number of words

Unit of transfer –

Word

Block

Access method –

Sequential access

Direct access

Random access

Associative access

Performance –

Access time, Cycle time, Transfer rate

Physical type –

Semiconductor

Magnetic surface

Physical characteristics –

Volatile / nonvolatile

Organisation –

Erasable / nonerasable

Physical characteristics :

Volatile/Nonvolatile: If memory can hold data even if power is turned off, it is called nonvolatile memory; otherwise it is called volatile memory.

Erasable/Nonerasable: The memories in which data is once programmed cannot be erased are called nonerasable memories. On the other hand, if data in the memory is erasable then memory is called erasable memory.

Table 11.1.2 shows the characteristics of some common memory technologies.

Characteristics of some common memory technologies

Distinguish between volatile and non–volatile memories

Volatile memory

1. Volatile memory requires constant power to maintain the stored information.

2. Volatile memory is typically used only for primary storage.

3. Type of volatile memories are :

i) Random Access Memory (RAM)

ii) Static RAM (SRAM)

iii) Dynamic RAM (DRAM)

iv) Fast Page Mode DRAM (FPM DRAM)

v) Extended Data Output RAM (EDO RAM)

vi) Synchronous DRAM (SDRAM)

vii) Double Data Rate (DDR) SDRAM

Non–volatile memory

1. Non–volatile memory can hold the stored information even when there is no power supply to the memory or storage device.

2. Non–volatile memory is typically used for the task of secondary storage or long–term persistent storage.

3. Examples of non–volatile memory include read–only memory, flash memory, most types of magnetic computer storage devices (e.g. hard disks, floppy disk drives and magnetic tape), optical disc drives, and early computer storage methods such as paper type and punch cards.

• The processor of a computer can usually process instructions and data faster than they are fetched from the memory unit. The memory cycle time, then is the bottleneck in the system. One way to avoid this bottleneck is to use a cache memory. Cache memory is a small, fast memory that is inserted between the larger, slower main memory and the processor. It usually holds the currently active segments of a program and their data.

• In most modern computers, the physical main memory is not as large as the address space spanned by an address issued by the processor. Here, the virtual memory technique is used to extend the apparent size of the physical memory. It uses secondary storage such as disks, to extend the apparent size of the physical memory.

 

2. Classification of Primary Memory

• memory devices can be classified based on following parameters :

■ Principle of operation

■ Physical characteristics

■ Mode of access and

■ Terminology used for fabrication.

• Fig. 11.1.3 shows the classification of memory.

• Broadly semiconductor memories as classified as and non–volatile memories .Volatile memories can retain their state as long as power is applied. On the other hand, non–volatile memories can hold data even if power is turned off.

• Read/Write Memories (RWMs) are those memories, which allows both read and write operations. They are used in applications where data has to change continuously. They are also used for temporary storage of data. ROM memories allow only read operation. They are used to store monitor programs and constants used in the program.

• The volatile memories which can hold data as long as power is ON are called Static RAMs (SRAMs). Dynamic RAMs (DRAMs) stores the data as a charge on the capacitor and they need refreshing of charge on the capacitor after every few milliseconds to hold the data even if power is ON.

• EPROM and EEPROM are erasable memories in which the stored data can be erased and new data can be stored.

• The semiconductor memories are also classified as bipolar and MOS memories depending upon the type of transistors used to construct the individual cell.

 

3. Classification of Secondary Memory

• A primary memory is costly and has a limited size. This memory is mainly used for storing the currently processing data.

• Secondary storage is used to store data and instructions (programs) when they are not being processed.

• The devices those are used as secondary storage are non–volatile and have a larger storage capacity. Also, they are less expensive as compared to primary storage devices.

• However, they are slower in comparison. The examples are hard disks, floppies, CD–ROMs, magnetic tapes etc. This type of memory is also called secondary memory, auxiliary memory or peripheral storage.

• Fig. 11.1.4 shows the classification of secondary storage devices. They can be categorized broadly according to their access types as sequential and random (direct).

 

Review Questions

1. Define a memory cell. Give an example.

2. Define a 'memory location' and a 'cell'.

3. Write a short note on semiconductor memories.

4. State various characteristics of memory devices and explain in brief any two.

5. Distinguish between volatile and non–volatile memories.

6. Explain the key characteristics of computer memory systems in details.

7. Explain the various characteristics of memory system.

8. Classify memories on the basic of principle of operation, physical characteristics, mode of access and fabrication technology.

9. Give the classification of semiconductor memories.

10. Which memory is called volatile? Why?

11.30 Explain static memory.

12. Give the classification of primary memory.