Lecture6

Main memory

External Storage

CS 21 - Computer Organization and Assembly Language Programming Lecture 6 Memory: Main Memory and Storage

University of the Philippines - Diliman College of Engineering Department of Computer Science

Optical Disks

Main memory

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External Storage

Optical Disks

Main memory

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Main memory

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Optical Disks

Main memory

External Storage

Optical Disks

Main memory

memory that is between the cache and the External Memory outside the CPU, but still in the mainboard/motherboard usually comes in modular packages implemented using semiconductor technology

Main memory

External Storage

Semiconductor Memory

Types of Semiconductor Memory: RAM ROM EPROM Flash memory

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Main memory

External Storage

RAM: Random Access Memory Characteristics of Random Access Memory: almost exclusively the choice for Main memory in modern computer systems �Randomly� Accessible(what does this mean?)

Read-write type Data is volatile Could either be Static or Dynamic

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Main memory

External Storage

ROM: Read Only Memory Characteristics of Read Only Memory: like RAM, ”Randomly” Accessible Read-Only type Data is non-volatile

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External Storage

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EPROM: Erasable Programmable Read Only Memory Characteristics of Erasable Programmable Read Only Memory: like RAM, �Randomly� Accessible Read and write capability, but could only be written on in special conditions Writing could be facilitated via UV light, or via electrical means Data is non-volatile

Main memory

External Storage

Flash Memory Characteristics of Flash Memory: like RAM, �Randomly� Accessible Read and write capability usually uses USB interface increase in capacity makes it a challenger for External Memories

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Main memory

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External Storage

lowest in the memory hierarchy largest in capacity, lowest in speed most common are magnetic and optical disks

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Main memory

External Storage

Optical Disks

Magnetic Disks commonly known as Hard Disk Drives data is stored in platters attached to a central spindle platters are coated with magnetizable material head or heads modify the orientation of magnetized particles on the platters

Main memory

Magnetic Disks

External Storage

Optical Disks

Main memory

External Storage

Magnetic Disks

data on a platter is organized into concentric circles called tracks one track is divided into sectors, where data is stored

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Main memory

External Storage

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Magnetic Disks

Disk access time depends on three factors: Seek time: the time it takes for the disk arm to move the heads to the right position/cylinder. (5 15 msec) Rotational Latency: the time it takes for the platter to rotate the starting sector under the head. (typical hard disks rotate around 5400rpm) Transfer time: the time during which the sectors, in one track, are read by the head. (5 20 MB/sec)

Main memory

External Storage

Optical Disks

Magnetic Disks

Most of the data in the world exists in magnetic disks! How about crashes?

Main memory

External Storage

Optical Disks

Magnetic Disks

We cope with crashes, technical glitches, and limited disk lifespan with

Main memory

External Storage

Optical Disks

Magnetic Disks

We cope with crashes, technical glitches, and limited disk lifespan with DATA REDUNDANCY

Main memory

External Storage

Data Redundancy

Individual users: backup with optical media.

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Main memory

External Storage

Data Redundancy

Individual users: backup with optical media.

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Main memory

External Storage

Data Redundancy

Individual users: use cloud computing/storage.

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Main memory

External Storage

Data Redundancy

Individual users: use cloud computing/storage.

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Main memory

External Storage

Data Redundancy: RAID

Businesses/Corporate Entities: RAID.

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Main memory

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Data Redundancy: RAID

Businesses/Corporate Entities: RAID. RAID stands for Redundant Array of Inexpensive Disks several disks appear as a single disk to the user there are several kinds of RAID, identified by number

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Main memory

External Storage

Data Redundancy: RAID 0 RAID 0.

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Main memory

External Storage

Data Redundancy: RAID 0 RAID 0. data is striped across several disks multiple disks appear as one, capacities add up dude, where’s the redundancy?

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Main memory

External Storage

Data Redundancy: RAID 1

RAID 1.

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Main memory

External Storage

Data Redundancy: RAID 1

RAID 1. shadowing or mirroring of data failure of single disk transparent to users

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Main memory

External Storage

Optical Disks most common form of secondary storage in use today used in data back-ups very cheap key enabler: LASER technology in most cases: Read-only, could only be written on once

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Main memory

External Storage

Optical Disks

Optical Disks data is WRITTEN by a powerful laser creating microscopic indentations(pits) in CD data is READ by shining a another laser in the disk and sensing the reflected light pits and lands placed on the disk in a spiral when unwound, spiral would reach 5.6 km!

Main memory

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Optical Disks How about DVDs? the physical disc is actually the same key difference is in laser frequence used(0.65nm from 0.78nm) smaller pits (0.4 microns to 0.8 microns) tighter spiral (0.74 microns between track compared to 1.6 microns per track)

Main memory

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How about DVDs? result? more data! (4.7GB from 650MB) Dibidi! Dibidi!

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What is the main problem with optical disks?

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What is the main problem with optical disks? too accessible = prone to damage

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What is the main problem with optical disks? too accessible = prone to damage Solution? Error Correction Codes/ECC

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Optical Disks Error Correction Codes key concept: in a word or byte, we can deduce a damaged bit using the remaining undamaged ones enables SOME tolerance for scratched or damaged disc surfaces knowing this, how should you clean your CDs and DVDs?