secondary storage

5/28/2018 Secondary Storage

1/214/3/2014Department of Information

Technology 1

Secondary Storage

CS1251Computer Organization

Carl Hamacher


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Virtual Memory (VM)

Processor address space greater than physical main

memory (RAM)

32-bit address = 4G bytes

Secondary storage (magnetic disk)

Virtual (logical) addresses translated into physical

addresses

Memory Management Unit (MMU)


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VM Organization

Processor

Cache

Main Memory

Disk Storage

MMU

Virtual Address

Physical Address

Physical Address

DMA Transfer

Data

Data



Cache vs. Virtual Memory

Cache Memory

Bridges speed gap between processor and main memory

Implemented in hardware

Virtual Memory Bridges size and speed gaps between main memory and

secondary storage

Implemented (in part) by software (operating system)



VM Address Translation

Pages

Fixed-length units

Blocks of words that occupy contiguous

locations in main memory

Commonly 2K to 16K bytes

Too smallmore (slow) accesses

Too largeunnecessary info in main memory




Virtual Page Number

High-order bits

Which page

Offset Low-order bits

Word within page

Control Bits

Validity

Modification

Restrictions (read/write)




Page Table

Info about main memory location of each page

Kept in main memory

MMU cache for most recently accessed pages Page Frame

Area of main memory that can hold one page

Page Table Base Register

Starting address of page table



Memory Management

Operating System

System Space

Supervisor State

Privileged Instructions Application Programs

User Space

User State

Protection

No program should be allowed to destroy either data or

instructions of another program



Magnetic Hard Disks

Phase Encoding (Manchester Encoding)

Change in magnetization guaranteed at midpoint of each

bit

Provides clocking info for synchronization Other self-clocking schemes

More efficient

Better storage density

More complex control circuitry



Hard Disk Components

Disk

Assembly of disk platters

Disk Drive

Electromechanical system Spins disk

Moves read/write heads

Disk Controller

Electronic circuitry



Hard Disk Organization

Organization of data

Concentric tracks

Divided into sectors

Divided into logical partitions Formatting

Divides disk into tracks and sectors

Excludes defective sectors

About 15% overhead

Sector headers

Error-correction codes

Intersector gaps



Access Time

Seek Time

Time required to move the read/write head to the proper

track

Latency Time Rotational delay

Time until head is over correct track

Times measured in ms



Disk Controller

Data Buffer

Difference in transfer rates between disk and bus

Also serves as a disk cache

Read/Write Requests Main memory addressdisk address

Word count

Booting

OS loaded into main memory

Nonvolatile ROM stores monitor program

Boot block with loader program



RAID

Redundant Array of Inexpensive Disks

RAID 0

Performance enhancement

Single large file stored in several separate disk units Disks deliver data in parallel

RAID 1

Relaibility

Storing identical copies of data on two disks

Mirror



RAID

RAID 2, RAID 3, RAID 4

Reliability

Various parity checking schemes

RAID 5 Reliability

Parity-based error-recovery scheme

Distributed across disks

RAID 10

Performance and reliability

Hybrid of RAID 0 and RAID 1



Optical Disks



CD vs. DVD

Compact Disk (CD)

CD-ROM

CD-R

CD-RW Digital Versatile Disk (DVD)

Shorter wavelength laser

Smaller focus, smaller pits, closer tracks

Capacity

CD = 800 MB

DVD = 4.7 GB



Magnetic Tape Systems

Off-line storage of large amounts of data

Back-up and archival storage

Data organized into records and files

Highest capacity

Slowest

Cheapest



Memory Hierarchy

Processor

Registers

Primary Cache

Secondary Cache

Main Memory

Secondary Memory

Increasing

Size

Increasing

Speed

Increasing

Cost


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