1 Computers Internal and External Memory. 2 Characteristics of Computer Memory zLocation zCapacity zUnit of transfer zAccess Method zPerformance zPhysical.

1

Computers

Internal and External Memory

2

Characteristics of Computer Memory

LocationCapacityUnit of transferAccess MethodPerformancePhysical TypeOrganization

3

UNIVAC Console and CPU

5

Memory Hierarchy

CACHE

MAIN MEMORY

MAGNETIC DISK | DISK CACHE

MAGNETIC TAPE | OPTICAL DISK

SPEEDCOST

REGISTERS

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Capacity Units

BitByteKilobyte MegabyteGigabyteTerabyte

bB = 8 bKB = 1,000 BMB = 1,000,000 BGB = 1,000,000,000 BTB = 1,000,000,000,000

B

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Memory Access Method

Sequential - tapeDirect - floppy or hard diskRandom - internal memory

Dynamic (DRAM) simple, small, must be refreshed Static (SRAM)no refresh needed

Associative - some cache

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Random Access Memory

Chips and chip technology

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Flip Flop Circuit Diagram

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Memory Cells

0/1SELECTselect cell

CONTROLread or write

DATA IN / SENSEinput or output

SELECTselect cell

CONTROLread or write

1

11

Write to Memory

COLADDRESSBUFFER

MUX

REFRESH

ARRAY

DATAINPUT

BUFFER

DATAOUTPUTBUFFER

A0...

A10

D1..

D4

ROWADDRESSBUFFER

Read Enable

Col AddressRow AddressWrite EnableRow Address

ROWADDRESSBUFFER

COLADDRESSBUFFER

Col Address

MUX

Write Enable

DATAINPUT

BUFFER

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Pin Assignments

A0 … A10: address location (multiplexed)D1 … D4: data in or outVcc: power supplyVss: groundRAS: row address selectCAS: column address selectWE: write enableOS: output enable

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Cache Operation

ALU CNTL.....

BUS

MAIN MEMORY

CACHE

14

15

Cache

Main Memory

CPU

CACHE

Word

Block

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CacheSlot

NumberTag Block

0

1

2

3

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Random Access

Direct Access Storage Device (DASD) or disk drives Optical Magnetic

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Optical Disks (CD’s)

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How CD’s work

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Cylinder/Track/Block

Block (Sector)Track

Cylinder

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Disk Organization

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DASD Structure

Read-Write Heads

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Data Storage

FAT (File Access Tables), Directories and Catalogs

Update and DeleteFragmentation and reorganizationBlocks, Headers and Interblock Gaps

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DASD Access (PC)

MAIN MEMORY

DASD

BUFFER

CONTROLLER

CPU

CACHE

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27

DASD Access (Mainframe)

MAIN MEMORY

DASD

BUFFER

CONTROLLER

CPU

CHANNEL

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Data Structure

HEADER DATA CRCHEADER

BLOCK

• Header written when disk is formatted• Data copied into block• Cyclical Redundancy Check calculated

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Controller Operation(DASD retrieval)

CPU passes parameters to registers in the controller

The controller transfers data into the card buffer

The controller checks the CRC to assure the data was copied correctly

The controller (or CPU) transfers buffered data to memory one word at a time

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Time Units

MillisecondMicrosecon

dNanosecondPicosecond

ms = 1/1000 sµs = 1/1,000,000 sns = 1/1,000,000,000 sps = 1/1,000,000,000,000

s

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Representative Times

Network speed = megabits per second

Disk transfer = megabytes per second

Disk access = millisecondsMemory access = nanosecondsMachine cycle =

microseconds/nanoseconds

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Error Correction and Checking

Add bits to a block to use for error discovery Detection only Detection and retransmission Detection and recovery

Check Body Header

Block

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Error Detection Only (Asynchronous Transmission)

*******

Parity Bit

7 Data Bits

27 = 128 distinct characters

*

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Error Detection &Correction (Hamming Code: 4 bit word)

****

3 Error Checking Bits

4 Data Bits

***

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DATA

1

1

10

Error Detection &Correction (Hamming Code: 4 bit word)

1110

***

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PARITY (even)

1

1

10

1 0

0

Error Detection

1110

100

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PARITY (even)

0

1

10

1 0

0

Error Correction (4 bit word)

0110

100

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Error Correction & Detection

Error detection takes fewer bits than error correction

Longer packets take a smaller percent for correction but have more types of errors

Hamming’s scheme detects all errors at a high overhead cost; others may correct only single bit or double bit errors with shorter check fields

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CRC Error Checking

The transmitted messages are divided into predetermined blocks

The blocks are divided by a fixed divisor

The remainder is appended to the message

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IBM 1107 with tape drives

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Memory