Memory Unit

MEMORY UNIT COURSE TEACHER

S. A. AHSAN RAJON Computer Science and Engineering Discipline, Khulna University, Khulna. E-Mail: [email protected] Web: www.rajon.forumerzone.com

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FLASHBACK

Units of Measurement Everything in a computer is stored as combination of 1s and 0s.  A bit can be a 1 or a zero  1 byte = 8 bits  1 kilobyte = 1 024 bytes  1 megabyte = 1 048 576 bytes  1 gigabyte = 1 073 741 824 bytes

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Standard Computer 

Hard disk drive

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 persistent



memory

RAM  temporary

memory

Central processing unit  BIOS 

 Basic

Input Output System--chip used to boot computer.

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Motherboard  integrates

everything together.  Based on Von Neumann architecture.

Standard Computer Von Neumann Architecture  Single processing unit  Separate storage unit to hold instructions and data

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BIOS Chip containing basic software that is used to boot up the computer.  Prepares the machine so that other software programs stored on the hard drive and elsewhere can load and assume control of the PC.  Software can be interacted with via the BIOS screen.

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MAIN BOARD Motherboard  Ties all the different components together.  Generally offers networking hardware, some basic graphics and sound, etc.

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MEMORY UNIT

MEMORY Memory may be defined as the devices which is used to store data and information.  Memory components of a computer system are divided into three groups-

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 Main

Memory or Primary Memory  Secondary Memory or Auxiliary Memory or Back-up Memory  Internal Processor Memory

MEMORY 

Main Memory or Primary memory

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 Characterized

by the fact that, locations in main memory can be directly accessed by the CPU instruction set.  Used for program and data storage during computer OPERATION 

Secondary Memory or Auxiliary Memory or BackUp memory  Much

larger in size but much slower than memory.  Used for storing system program and large data files which are continually not required by CPU 

Internal Processor Memory  Usually

comprise of small set of high speed registers used as working registers for temporary storage of instructions and data.

MEMORY: DO WE NEED ?? To store program and data during execution  To Store program for repetitive use  To Store data for future or periodical use.  To Store results for execution.

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MAIN MEMORY   FUNDAMENTALS OF COMPUTER SCIENCE

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BIT? BYTE? MEGABYTE ? GIGABYTE ? TERABYTE ?  NIBBLE: 4 bits  WORD:  Unit of bits that a computer can process at a time.  Set of bits processed by microprocessor as a unit.  32-bit computer ?? Word-length = 32 ???

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A 1KB computer storage has exactly 1024 storage locations typically presented as 0000 to 1023. A computer with 64K storage has 64536 storage locations in memory. 1 MB is roughly 1 Million (Exactly, 220)bytes of storage. Roughly 1GB has 1 Billion bytes of storage. Roughly 1TB has 1 Trillion bytes of storage.

MAIN MEMORY

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Two kinds of main memory are commonly used in modern computers.  Semiconductor

Memory  Magnetic Memory

TERMINOLOGIES 

Memory Access

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 

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Instructions or data are read from or written into the memory one word at a time. Reading from the memory or writing data into memory is usually called MEMORY ACCESS.

ACCESS TIME 

The performance of a memory device is primarily determined by the rate at which information i.e. one word from the memory is read/written. Known as Access Time.

MEMORY CAPACITY

NUMBER OF BITS NEEDED TO ADDRESS

1KB

OR

210 bytes

10

64KB

OR

216 bytes

16

1MB

OR

220 bytes

20

TYPES OF MEMORY 

VOLATILE MEMORY

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 If

stored information is destroyed when power goes off.

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NON-VOLATILE MEMORY  Stored

information is preserved when power goes off or power failure. – Most Semi-conductor Memory (RAM)  Volatile – Magnetic Memory (disk)  Non–Volatile

MEMORY UNIT 

SERIAL ACCESS MEMORY

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 storage

locations can be accessed in only in certain pre-defined sequence

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RANDOM ACCESS MEMORY (RAM)  If

memory locations can be accessed in any order and access time is independent of the location being accessed, the memory is called Random Access Memory. E.g. ferrite core and semiconductor memories.

 Categorized

as –

Static RAM Dynamic RAM

STATIC RAM 

Static RAM (SRAM)

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 Made

up of flip flop  Stores bit as voltage High  1, Low  0  Designed using Bipolar Transistors  Need no refreshing  Require greater power supply  Four times larger in area  Contain less data

DYNAMIC RAM 

Dynamic RAM (DRAM)

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 Made

up of MOS transistors  Stores Bit as charge in capacitors; presence of charge  1, else  0  High density and low power consumptions  Charge is leaked; require refreshing; read data from the capacitor and write it back periodically.  Smaller area

ROM 

READ ONLY MEMORY

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 There

are some random access memories in which data are permanently recorded during fabrication itself.  The memory only allows information stored into it to be read and would not permit any writing or modification by users.  All microcomputers have at least one ROM unit that contains a small program called BOOTSTRAP LOADER.  This program is automatically copied into primary storage when a computer is powered up and being to execute.  This program is responsible for reading in a copy of the operating system from a secondary storage device.

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Here is a schematic

PROM 

PROGRAMMABLE READ ONLY MEMORY

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 Nichrome

or Polysilicon wires arranged in a

matrix.  These wires can be viewed functionally as diodes or fuses.  The memory can be programmed by the special PROM programmer that selectively burns the fuses according their bit pattern to be stored.  This is called “burning the PROM” and the info stored is permanent.

EPROM

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ERASABLE PROGRAMMABLE READ ONLY MEMORY  Stores

bit by charging floating of FET.  Applies high voltages to charge the gate  All the info erased by exposing the chip to Ultra-violate Light (UV Ray) through Quartz window.  The chip can be reprogrammed.  Useful for labs / Experiments because can be reused again.

E-EPROM 

ELECTRONICALLY ERASABLE PROGRAMMABLE READ ONLY MEMORY

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 Used

vastly in remote control applications.  Erased using computers  10ms to erase a total chip  Any specific part may be altered by which is the most important point.  Information can be altered using electrical signals at register levels rather than erasing all the info.  Software updates may be done for the ‘EEPROM using devices’ from remote link via telephone lines.  In Chip erase mode, entire chip can be erased in 10 ms to 15 ms whereas it takes 15 to 20 mins to erase EPROM.

MAGNETIC CORE STORAGE

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A ring of iron oxide (ferrite), magnetized by a current through a wire, the direction of which determines the direction of magnetism (polarity) in the core. When current is removed, the core remains magnetized.

1

0

MAGNETIC CORE STORAGE If a core is magnetized in one direction and a current ispassed through the core from the other direction, the core changes its state.  In doing so it produces s detectable ‘kick’.

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SEMICONDUCTOR STORAGE 

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Virtually all computers made today have semiconductor elements in their memory. Typical semiconductor memory consists of a rectangular array of memory cell. Basic semiconductor memory cell is transistor or a circuit capable of storing charge and is used to store 1 bit of information. Metal Oxide Semiconductor (MOS) is used for making computer’s main memory chip. Chip is a term used to refer to a semiconductor memory device having a silicon base. Electronic circuit which consists of a collection of different components like transistors, resistors, capacitors is known as integrated circuit (IC)and that is fabricated on a single chip from a memory device.

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COMPARISON: MOS vs. CORE 

Core memory has to be assembled manually whereas MOS memory can be produced mechanically. Hence MOS memory is cheaper than core memory.

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MOS memory is available on a miniaturized chip and is therefore much smaller in size than core memory

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Core memory has a destructive read operation, while MOS memory has a non-destructive read operation. This enhances the speed of MOS (Chip) Memory.

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COMPARISON: MOS vs. CORE 

MOS memory is volatile, whereas core memory is non-volatile.

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Core memory is available only in large blocks whereas MOS memory is available in wide variety of sizes up to 65 Kbytes.

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Core memory has limited range of cycle and access time whereas MOS memory has wide range of cycle and access time and faster than core memory.

SECONDARY MEMORY 

Much larger in capacity

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 Magnetic

Tape  Magnetic Disk  Floppy Disk  Optical Disk 

Three common characteristics  Non-Volatile

Storage  Cost-Efficiency  Optical Disk

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PRIMARY VS SECONDARY MEMORY PRIMARY MEMORY

SECONDARY MEMORY

COST

Expensive

Less expensive

CAPACITY

Limited

Nearly Limited

ACCESS TIME

Billionth of second

Millionth of second

PROCESSING

Directly accessible to the processing unit

Data must be routed through the primary storage

STORAGE MEDIA

Semiconductor magnetic core

Magnetic Tape/ Disk

LOCATION

Within CPU

Outside but connected to CPU

SECONDARY STORAGE

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There are two basic type of storage  Sequential Access Storage To read one particular record in the file all records preceding it must be read E.g. magnetic tape  Direct Access Storage Media where an individual record can be located and read immediately without reading any other record. Directly retrieved by selecting and using any of the locations on storage media. Each storage position must– Have unique address – Can be individually accessed in approximately the same length of time.

Storage Hierarchy Ladder

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Storage Hierarchy Ladder

REGISTERS CACHE MAIN MEMORY MAGNETIC DISK MAGNETIC TAPE | OPTICAL DISK

SELF STUDY 

SELF STUDY

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 Magnetic

Tape  Magnetic Disk  Optical Disk  Magnetic Bubble

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THANK YOU !!!