1Disk BasicsWARNING! This Material NOT in Text• “Disk Subsystem” – Interface to x86 – Responsible for Reading/Writing• Interface Circuit is “Disk Controller” - Partially inside Fixed Disk Housing and on Motherboard/CardDisk Layout• “Platter” – Circular Piece of Magnetic Material – Coated Aluminum• One Platter Has 2 “Sides”• Each Side is Divided into Concentric Rings Called “Tracks”• Each Track is Divided into Arcs Called “Sectors”• Each Sector Can Store Some Number of “Bytes” of Data• “Media Capacity” Determines Amount of Storage per Sector2Physical Disk Coordinates• A Platter can be “Double Sided” or “Single Sided”• A Single Fixed Disk can Contain Multiple Platters• Need Three-Dimensional Coordinate• A Stack of Tracks On Top of Each Other is a “Cylinder”• Each Side of a Platter Must Have a “Read/Write Head”Contains Induction Coil – Floats Over Surface(Except for Floppy Where it Actually Touches)• Position of Read/Write Head Given by “Cylinder” Number• Physical Location on Disk: <cylinder, head, sector>Multi-Platter Disk Layout3Disk Performance - Bandwidth• “Seek Time” – (Average) Time Required to Position R/W HeadOver a Track/Sector (5-15 ms)• “Rotational Latency” – After Seek Wait for Sector to be Under R/W HeadTypical Disks Rotate at 3600, 5400, 7200, 10800 RPMAverage Rotational Latency = ½ Rotation (4-8ms)• “Transfer Time” – Time for 1 Sector of Data to be Read/WrittenAfter Seek and Rotational Latency (25-50µs)• “Access Time” is Sum of Above – Dominated by Seek and Rotational LatencyDisk Performance - Capacity• “Linear Bit Density” – Number of Bits/cm that can be Stored Along a Track• Linear Bit Density (Typically 50,000-100,000 bits/cm) Depends on:Purity of Magnetizable Media Air Quality Data Encoding• “Winchester Disk” – Sealed at Factory to Preserve Air/Surface Quality-First “Winchester Disk” Was 30 MB Fixed and 30 MB Removable (IBM)-.30-30 Winchester• “Unformatted Capacity” – Disregards Preamble, ECC and Intersector Gap-Marketing Hype4Capacity Along a Sector• Preamble – Used to Synchronize R/W Head“formatting” a disk is “writing” all preambles on it• 4096 bits – Actual Data Storage Area (512 bytes is typical)• ECC – Error Correcting Code – Usually a Reed-Solomon CodeIncreasing Capacity - Cylinder ZonesLinear Bit Density – c=2 r012• “Zones” are Groups of Tracks• Each Zone has Same NumberSectors/Track• Attempts to Make Each SectorHave Same Linear Distance• Increase Overall Capacity• More Complex Seek ProcessEXAMPLEZONE 0: 4 Sectors/TrackZONE 1: 8 Sectors/TrackZONE 2: 16 Sectors/TrackTypically 10-40 Zones Per Disk Drive5Data Encoding in a Sector• MFM (Modified Frequency Modulation) Common technique used to encode the magnetic fluxes recorded on a drive into data. Still used on floppy drives and most original XT and AT systems. Notice that most drive types supported in CMOS have 17 sectors per track. This is the standard density for MFM encoding.• RLL (Run Length Limited)Encoding method that allows 50% more information to be recorded on a track than MFM. Actually accomplished by recording more fluxes for every byte, but packing them more tightly onto the surface. Often called 2,7 RLL because the recording scheme involves patterns with no more than 7 successive zeros and no less than two.• ARLL (Advanced Run Length Limited)More complex yet powerful derivatives of the RLL scheme. Include 1,7 and 3,9 encoding.Floppy DisksMost Common 5.25” and 3.5”Parameter Low Denstiy High Density Low Density High Density Size (inches) 5.25 5.25 3.5 3.5 Capacity (bytes) 360k 1.2M 720k 1.44M Tracks 40 80 80 80 Sectors/Track 9 15 9 18 Heads 2 2 2 2 RPM 300 360 300 300 Rate (kbps) 250 500 250 500 Enclosure flexible flexible rigid rigid6Disk Controllers• Each Disk Drive Has a Controller Associated With ItPurpose of Controller:1) Accept Commands from Software (READ, WRITE, FORMAT)2) Control Arm Motion3) Detect/Correct Errors4) Convert parallel data into Serial Bit-Stream (vice-versa)5) Remap bad Sectors to Spares (each track has spares)6) Caching Sectors• IDE – “Integrated Drive Electronics” – Most Controller Functionalityis Inside the Drive Enclosure• EIDE – “Extended IDE”• SCSI – “Small Computer System Interface” – Actually a BusStandard – Disk Media is SameDisk Controllers• ST506/412 Standard interface used on XT and AT drives and controllers. Originally developed by Seagate Technologies to support their ST506 (5 MB) and ST412 (10 MB) drives. The entire controller mechanism is located on a controller card and communications between the drive and controller flow over 2 ribbon cables - one for drive control and one for data.• ESDI (Enhanced Small Device Interface)Developed by Maxtor in the early 1980’s as an upgrade and improvement to the ST506 design. While the drive does not have an embedded controller, one of the most critical functions ,encoding-decoding, is performed on the drive. This allows for faster communications and higher drive capacities. Uses the same cabling as ST506 interface, but carries different signals on each line.• SCSI (Small Computer System Interface)Based on an original design by Shugart Associates, SCSI is not specifically a drive interface, but a method of allowing different devices to communicate with a PC. For hard drives the entire controller is built onto the drive PCB, allowing for very high speed transfers to and from the drive. Fully interpreted, parallel data is then transferred to and from the PC by way of a single cable through a bus interface that has configured the device as a hard drive.• IDE (Integrated Drive Electronics)A technology pioneered by Compaq and Conner that embedded a controller onto the hard disk PCB while maintaining compatibility with the register level commands sent by the computer’s INT 13 routines. IDE drives are configured and appear to the computer like standard ST506 drives.7Disk Controllers• ATA (AT Attachment)Implementation of the IDE design with a 16 bit AT style controller on board the drive.• XTA (XT Attachment)Rarely used implementation of IDE with an integrated 8 bit XT controller.• ATA-2Enhancement to the AT Attachment standard to provide for considerable performance improvement and more sophisticated drive identification.• EIDE (Enhanced IDE) and
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