Memory Organization in a Computer System

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Chapter 5
Memory Organisation

5.1 Introduction
The memory unit is an essential component in any digital computer since it is needed for storing programs and data. Main Memory The memory unit that communicates directly with the CPU is called the main memory. Auxiliary Memory Devices that provide backup storage are called auxiliary memory. The most common auxiliary memory devices used in computer systems are magnetic disks and tapes. The part of the computer system that supervises the flow of information between auxiliary and main memory is called the memory management system. 2 FP203 Computer

5.1.1 Main Memory

The main memory is the central storage unit a computer system. It is a relatively large and fast memory used to store programs and data during the computer operation.

What is the Main Memory???
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5.1.2 Memory Hierarchy in a Computer System
Fast and Expensive

Increasing performance and increasing cost

Slow and Inexpensive
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The Properties of the Memory Hierarchy

Memory Type
Register Cache Main Memory Disk Memory

Access Time
1 ns 5-20 ns 60-80 ns 10 ms


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Memory Organization in a computer system
Auxiliary Memory

Magnetic Tapes Magnetic Tapes

I/O Processor

Main Memory


Cache Memory


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5.1.3 Various types of semiconductor memory technologies
The principal technology used for the main memory is based on semiconductor integrated circuit such: RAM – Random Access Memory ROM – Read-only Memory

PROM – Programmable ROM
EPROM – Erasable PROM EEPROM – Electrically Erasable PROM Flash Memory – can be electrically rewritten tens of thousands of times (used extensively in digital video cameras and for control program in set-top cable television decoders and other devices.) 7 FP203 Computer Organisation

5.1.4 Various types of external memory
Magnetic Disk is a circular plate constructed of metal or plastic coated with magnetized material. The bits are stored in the magnetized surface in spots along concentric circle called track.

Magnetic 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.

Please find out the various types of RAID and optical memory??? 8 FP203 Computer Organisation

5.2 The concept of cache memory
The cache is used for storing segments of programs currently being executed in the CPU and temporary data frequently needed in the present calculation. By making the programs and data available at a rapid rate, it is possible to increase the performance rate of the computer. 9 FP203 Computer Organisation

5.2.1 Cache Memory
A special very-high-speed memory called a cache, is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate.


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5.2.2 Types of Cache Memory
The transformation of data from main memory to cache memory is called mapping. There are THREE (3) types of mapping procedures of practical interest when considering the organization of cache memory: Direct mapping Associative mapping Set-associative mapping 11 FP203 Computer Organisation

Direct Mapping
Direct mapped cache eliminates the need for an associative memory, by restricting where each main memory block can be placed in the cache. For a direct mapped cache, each main memory block can be mapped to only one slot, but each slot can receive more than one block. This scheme is called direct mapping, because each cache slot corresponds to an explicit set of main memory blocks. The disadvantage of direct mapping is that the hit ratio can drop considerable if two or more words whose addresses have the same index but different tags are accessed repeatedly. The performance of cache memory frequently measured in a terms of quantity called hit ratio.

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