External Memory - Processor Architecture and Microprogramming - Lecture Slides, Slides of Microprocessor and Assembly Language Programming

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Chapter 6

External Memory

Types of External Memory

• Magnetic Disk —RAID —Removable
• Optical —CD-ROM —CD-Recordable (CD-R) —CD-R/W —DVD
• Magnetic Tape

Read and Write Mechanisms

• Recording and retrieval via conductive coil called a head
• May be single read/write head or separate ones
• During read/write, head is stationary, platter rotates
• Write — Current through coil produces magnetic field — Pulses sent to head — Magnetic pattern recorded on surface below
• Read (traditional) — Magnetic field moving relative to coil produces current — Coil is the same for read and write
• Read (contemporary) — Separate read head, close to write head — Partially shielded magneto resistive (MR) sensor — Electrical resistance depends on direction of magnetic field — High frequency operation - Higher storage density and speed

Inductive Write MR Read

Disk Data Layout

Disk Velocity

• Bit near centre of rotating disk passes fixed point slower than bit on outside of disk
• Increase spacing between bits in different tracks
• Rotate disk at constant angular velocity (CAV) —Gives pie shaped sectors and concentric tracks —Individual tracks and sectors addressable —Move head to given track and wait for given sector —Waste of space on outer tracks - Lower data density
• Can use zones to increase capacity —Each zone has fixed bits per track —More complex circuitry

Finding Sectors

• Must be able to identify start of track and sector
• Format disk —Additional information not available to user —Marks tracks and sectors

Gap1 Id Gap2 Data Gap3 Gap1 Id Gap2 Data Gap

Track

Sync Byte Head^ Sector^ CRC^

Sync Byte Data CRC

ST506 format (old!)

• Foreground reading —Find others

Fixed/Movable Head Disk

• Fixed head —One read write head per track —Heads mounted on fixed ridged arm
• Movable head —One read write head per side —Mounted on a movable arm

Removable or Not

• Removable disk —Can be removed from drive and replaced with another disk —Provides unlimited storage capacity —Easy data transfer between systems
• Nonremovable disk —Permanently mounted in the drive

Multiple Platters

Cylinders

Winchester Hard Disk (1)

• Developed by IBM in Winchester (USA)
• Sealed unit
• One or more platters (disks)
• Heads fly on boundary layer of air as disk spins
• Very small head to disk gap
• Getting more robust

Winchester Hard Disk (2)

• Universal
• Cheap
• Fastest external storage
• Getting larger all the time —Multiple Gigabyte now usual