Memory Units And Memory Hierarchy Computer Science Essay

Memory administration is combination of several memory devices expeditiously. Memory devices are combined and organised based on their characteristic such as entree clip, velocity, storage capacity, physical size and cost per byte. Memory devices organised in such a manner that computing machine system should run at highest velocity, provide bulky storage capacity, smaller in size and less in a cost. Memory administration uses primary memory devices and secondary memory devices.

3.2 Memory Unit

Memory unit is storage unit of the computing machine system.

It shops content such as instructions ( codifications ) and informations for impermanent or lasting intent. It is used to shops content and to recover the content when needed by CPU or user. Memory unit includes two types of memory: primary ( chief ) and secondary. Direction or informations given by input devices is first acquiring stored in the primary memory. Further users saves processed ( executed ) informations in the secondary memory. Primary memory is volatile ( impermanent ) . It is faster than secondary memory. It includes RAM and ROM.

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It is straight accessible by CPU. Secondary memory is non-volatile ( lasting ) . It includes several secondary storage devices such as Hard Disk Drive ( HDD ) , Floppy Disk Drive ( FDD ) , CDROM, and DVD and so on. Figure 3.1 shows memory unit and communicating line with other units.

Note: Volatile memory loses its content when computing machine loses power. In non-volatile memory power loss of computing machine does non impact content.

Input Unit

Procedure Unit of measurement

Output Unit

Primary Memory

Secondary Memory

Memory Unit of measurement

Figure 3.1 memory unit and communicating lines with I/O unit and Process unit

3.3 Memory hierarchy

Memory administration includes assorted memory devices. These devises are structured in a hierarchy based on the features of memory devices. Memory hierarchy explains the sequence of accessing memory device while processing, in the computing machine system. Memory is structured in the hierarchy in such a manner that computing machine system should give first-class public presentation. Figure 3.2 shows Memory hierarchy. Hierarchy is made on the footing of features of memory, they are listed as follow:

Access Time - Time taken by memory to read or compose informations. It is measured in msecs ( MS ) or in nanoseconds ( N ) .

Memory size - It is sum of memory. Measured in KB, MB, GB or TB.

Cost per byte - It is cost needed to entree per byte.

Transportation bandwidth - It is velocity of the memory. It measured in MHz, GHz.

Unit of measurement of transportation - Data transportation rate. Measured in Kbps, Mbps or Gbps.

Increasing velocity and cost per spot dcreeasing sisize

Registers in CPU

Level one cache

Level two cache

Main memory

Virtual Memory

Increasing Size, diminishing velocity and cost

capacity

Secondary Storage devices

Figure 3.2 Memory Hierarchy

Memory hierarchy includes CPU registries on the top. Register provides fastest informations entree and it is one of the most expensive memory location.

Second and 3rd degrees are level-1 and level-2 cache severally. The degree one cache is little in size ( 4 KB to 32 KB ) though much larger than registry. It is fixed on CPU bit so user can non spread out.

The level-2 cache is present on some CPUs merely. It is much larger than level-1 cache. User can non spread out it as it is fixed on CPU bit but it is cheaper than level-1 cache.

Virtual memory is portion of disc thrust, which simulates to Random Access Memory ( RAM ) . Disk thrusts are less expensive ; so cost per spot is significantly less. Secondary storage devices are such as Hard Disk Drive ( HDD ) , Floppy Disk Drive ( FDD ) , Compact Disk Read Merely Memory ( CDROM ) , Digital Versatile Disk ( DVD ) and tape thrusts. It is bit slower.

3.4 Memory types

Basic intent of memory is to hive away content. Still memory is available in different types. Basic type of memory is primary memory and secondary memory. Types are made based on memory features and its function in the computing machine system. Primary memory is the first memory used by CPU, so it should be faster and easy accessible. To do primary memory as per the demand, memory administration includes RAM and ROM as primary memory. The chief usage of secondary memory is use to hive away user 's informations for good. Speed of secondary memory can be compromised, but storage capacity should be first-class. So memory administration includes HDD, FDD, and CDROM and so on as a secondary memory. Figure 3.2 shows types of memory.

Memory

Primary

Secondary

Random-access memory

Read-only memory

SRAM

Dram

SDRAM

DDRAM

RDRAM

Promenade

Erasable programmable read-only memory

EEPROM

Magnetic

Optical

FDD

HDD

Tape thrusts

Cadmium

Videodisk

Figure 3.3 Types of Memory

3.4.1 Primary Memory

Primary memory is entree straight by CPU. Primary divided in to two types RAM and ROM. Primary memory should be faster and easy accessible.

RAM - Random-access memory is a primary memory used in the computing machine. RAM is the chief memory of the computing machine system. It is volatile since its content is accessible merely every bit long as the computing machine is ON. The content of RAM is cleared every bit shortly as computing machine is powered OFF. It is at 3rd topographic point in the memory hierarchy. Operating system, application plan and running informations are loaded in the RAM. CPU straight accesses the RAM.

Types of RAM are as follows:

Inactive Random Access Memory ( SRAM ) - SRAM is made with several transistors, normally four to six, for each memory cell. It does non necessitate refresh clip as capacitances are non used. It is much faster and costlier than DRAM. Computer system usage SRAM as cache memory.

DRAM ( Dynamic Random Access Memory ) - It has memory cells with a mated transistor ; it uses capacitance, as capacitances easy leaks with as they stared discharging, so need periodic refresh. It is slower and chippers than SRAM. It is much denser ( shops many spots per bit ) . It requires less power and generates less heat as compared to the SRAM. DRAM is available in several signifiers such as SDRAM, so DDRAM to RDRAM.

Read Merely Memory ( ROM ) - Read-only memory is non-volatile primary memory. Data written on ROM can non cancel or overwrite. Data is burned on the ROM as microcode. ROM memory is used in the computing machine system to compose microcode for illustration BIOS plan.

Types of ROM are as follows:

Programmable read-only memory ( PROM ) - This type of memory is one clip written memory. Once plan is written can non be alteration.

Effaceable programmable read-only memory ( EPROM ) - Data written on EPROM can be erased. Ultraviolet is used to wipe out the informations written on PROM.

Electrically effaceable programmable read-only memory ( EEPROM ) - It is same as EPROM but to wipe out informations wholly it requires electrical signal. So non necessitate to take from computing machine. Datas can be erased or over written it the computing machine itself.

3.4.2 Secondary Memory

A secondary memory is non-volatile. Secondary memory used to salvage user 's informations for good. Devicess used as secondary memory are HDD, FDD, CD, DVD, Tape drives and so on. Secondary memory is at 6th topographic point in the memory hierarchy.

Devicess used as secondary memory are listed as follows:

Floppy Disk Drives ( FDD ) - Used as magnetic, portable memory device. It shops maximal 1.44 MB informations. Earlier it used to reassign informations among computing machine systems.

Hard Disk Drives ( HDD ) - HDD is fixed, magnetic memory device. It is critical in the computing machine system because Operating System and user 's information is saved on the HDD. Storage capacity of HDD is vendor specific.

CDROM and DVDROM - Are optical memory units used for reassigning informations and plans among the computing machines. Cadmiums are available in the size 650MB, DVDs are available up to 4 GB in a size.

Tape drives - Are normally used for informations backup intent.

3.6 Cache Memory

The cache is little sum of memory with high velocity. Cache memory supports informations and codification ( debuts ) that are used by CPU frequently. Use of cache memory reduced the waiting clip of CPU that makes CPU more effectual. SRAM is used to make cache memory. SRAM does non required refresh as it does non utilize capacitances as DRAM, it is 10 times faster than DRAM. Cache is of two types Level-1 that is placed ion the CPU. Level 2 cache is optional some processors have this type of memory that is some clip placed inside the CPU or placed near by CPU. To put to death following direction, CPU foremost look into cache memory for the same direction if that direction is non available so it goes farther.

Note: Dram is made up of capacitances which need changeless refreshing to continue informations. SRAM uses flip-flop circuit that does non necessitate refresh.

3.6.1 Cache line replacing Algorithms

While a new line is loaded in the cache, one of the bing lines must be replaced. Direct mapped cache, same block from line is removed take topographic point for new one. In associatory cache, it can seek for another topographic point.

Four of the most common line replacing algorithms are as follows:

LRU ( Lease Recently Used ) - the cache line that was last referenced in the most distance yesteryear is replaced.

FIFO ( First In - First Out ) - the cache line from the set that was loaded in the most distant yesteryear is replaced.

LFU ( Least Frequently Used ) - the cache line that has been referenced the times is replaced.

Random - a indiscriminately selected line from cache is replaced.

Note: The most normally used algorithm is LRU.

Cache Terminology

Cache usage few nomenclature for operation are as follows:

Hit - A cache entree finds data the cache memory

Miss - A cache entree does non happen informations or direction, coercing entree to following memory down in memory hierarchy.

Miss ratio - Percentage of girls to informations compared to all entrees.

Hit entree clip - Number of redstem storksbills to return a cache hit

Miss punishment - Number of redstem storksbills to treat a cache girl.

Virtual Memory

Operating system enables a procedure through which infinite of difficult disc thrust can be simulate to RAM. The HDD infinite used as RAM is known a Virtual Memory.

As cost per byte of HDD is less, so practical memory is good to run many big plans that need more memory. Access clip of RAM is in nanoseconds but practical memory gives entree clip in msecs. It is much slower than RAM. Figure 5.1 shows operation of practical memory.

Virtual memory uses paging and cleavage nomenclatures. Footings used by paging are as follows:

Virtual address- Address used by application plan from practical memory. It is composed of page figure and page beginning.

Physical reference - Address of physical memory.

Page - practical infinite divided into fixed-size pages.

Frame - Physical memory divided into fixed frames size.

MMU - Memory Management Unit ( MMU ) used to map practical reference to physical reference.

Page table - Each procedure has its ain page tabular array. Each page table entry contains the frame figure of the corresponding page in chief memory

Figure 3.4 Virtual memory operations

3.8 Chapter Review Questions

Which of the followers is primary volatile memory?

( A )

Random-access memory

( C )

Read-only memory

( B )

HDD

( D )

FDD

Autonomic nervous system: A

Which among the following are magnetic storage devices?

( A )

HDD

( C )

FDD

( B )

CDROM

( D )

DVDROM

Autonomic nervous system: A and C

Maximal storage capacity FDD is -- -- -- -

( A )

2GB

( C )

2.44 MB

( B )

1.44MB

( D )

None of these

Autonomic nervous system: Bacillus

A ­­­____ is used as cache memory in the computing machine system.

( A )

SRAM

( C )

Dram

( B )

SDRAM

( D )

Read-only memory

Autonomic nervous system: A

Which of the followers is primary non-volatile memory?

( A )

Cache

( C )

Read-only memory

( B )

Virtual

( D )

None of these

Autonomic nervous system: Degree centigrade

Which of the following ROMs ' content can be deleted utilizing UV?

( A )

Promenade

( C )

Erasable programmable read-only memory

( B )

Read-only memory

( D )

EEPROM

Autonomic nervous system: Degree centigrade

Virtual memory is construct of -- -- -- -- -- -- -- -- -- --

( A )

Using HDD infinite as RAM

( C )

Using FDD infinite as a Random-access memory

( B )

Logical based

( D )

Protocol based

Autonomic nervous system: A

8. In the memory hierarchy which memory is at first degree?

( A )

Register

( C )

L1 cache

( B )

L2 cache

( D )

None of these

Autonomic nervous system: A

9. Which memory takes 2nd topographic point in the memory hierarchy?

( A )

L1 cache

( C )

L2 cache

( B )

Read-only memory

( D )

HDD

Autonomic nervous system: Bacillus

10. MMU stands for -- -- -- -- -- -- -- -- -- --

( A )

Memory Mixture unit

( C )

Memory Management Unit

( B )

Memory measuring Unit of measurement

( D )

None of these

Autonomic nervous system: Degree centigrade

3.8.1 Answers

1. Angstrom

2. A and C

3. Bacillus

4. Angstrom

5. C

6. C

7. Angstrom

8. Angstrom

9. Bacillus

10. C

Drumhead

In the chapter, Memory Organisation, you learnt about:

Different memory units and its types.

Memory hierarchy based on memory features.

Cache and its replacing algorithm.

Virtual memory and paging.