Computer Architecture

By: Vivek SthanamKamyar ArbabifardPrudhvi Potu

2.3.2 Magnetic Disks2.3.3 IDE2.3.4 SCSI

Secondary Storage devices

Two Major types of storage devices:

Direct access storage devices (DASDs)

Magnetic Disks:

hard disks (high capacity, low cost per bit)

Floppy disks ( low capacity, slow, cheap)

Optical Disks:

CD-ROM = ( Compact disc, read only memory)

Serial Devices

Magnetic tapes ( very fast sequential access)

2.3.2 Magnetic Disks

Magnetic disks: long-term, nonvolatile storage for files, and a level of the memory hierarchy below main memory.

Characteristics (as of 2000)

Seagate Cheetah

IBM Travel star

IBM GB Microdrive

Disk diameter (inches) 3.5 2.5 1.0

Formatted data capacity 73.4 GB 32.0 GB 1.0 GB

Cylinders 14,100 21,664 7,167

Disks 12 4 1

Recording surfaces (heads) 24 8 2

Bytes per sector 512-4,096 512 512

Avg. sectors/track (512B) 424 360 140

Max. areal density (Gb/in2) 6.0 14.0 15.2

Rotation speed (RPM) 10,033 5,411 3,600

Avg. random seek (r/w) (ms) 5.6/6.2 12.0 12.0

Min. seek (r/w) (ms) 0.6/0.9 2.5 1.0

Max. seek (r/w) (ms) 14.0/15.0 23.0 19.0

Data transfer rate (MB/sec) 27-40 11-21 2.6-4.2

Magnetic Disks

Magnetic disks: long-term, nonvolatile storage for files, and a level of the memory hierarchy below main memory.

Average seek time is defined to be:

Where n is the number of all possible seeks.

Average rotation time is defined to be halfway round the disk;

n

TimeSeekTimeSeekAverage

n

i

i 1

___

Magnetic Disks Cont.

Transfer time is the time it takes to transfer a block of bits, typically a sector, under read/write head, which is a function of the block size, disk size, rotation speed, recording density of the track, and the speed of the electronics connecting the disk to the computer.

Read ahead is used to amortize the long access by reading more than what is simply requested. A buffer of a few MB is used to store the pre fetched data that assumes possible spatial locality.

Areal density is a measure of recording density in terms of bits per square inch: trackaon

Inch

Bitssurfacediskaon

Inch

TracksDensityAreal ______

Disk History

1989:63 Mbit/sq. in60,000 MBytes

1997:1450 Mbit/sq. in2300 MBytes

1997:3090 Mbit/sq. in8100 MBytes

Actuator

Platters (12)

{Arm

Head

Spindle

Disk Device Terminology

Platter

OuterTrack

InnerTrackSector

Actuator

HeadArm

° Several platters, with information recorded magnetically on both surfaces (usually)

° Bits recorded in tracks, which in turn divided into sectors (e.g., 512 Bytes)

° Actuator moves head (end of arm,1/surface) over track (“seek”), select surface, wait for sector rotate under head, then read or write

° “Cylinder”: all tracks under heads

Disk Device Performance

° Disk Latency = Seek Time + Rotation Time + Transfer Time + Controller Overhead

° Seek Time? depends no. tracks move arm, seek speed of disk

° Rotation Time? depends on speed disk rotates, how far sector is from head

° Transfer Time? depends on data rate (bandwidth) of disk (bit density), size of request

Platter

Arm

Actuator

Spindle

Disk Device Performance° Average distance sector from head?

° 1/2 time of a rotation

• 7200 Revolutions Per Minute 120 Rev/sec

• 1 revolution = 1/120 sec 8.33 milliseconds

• 1/2 rotation (revolution) 4.16 ms

° Average no. tracks move arm?

• Sum all possible seek distances

from all possible tracks / # possible

- Assumes average seek distance is random

• Disk industry standard benchmark

Disk Performance Model /Trends

° Capacity

+ 100%/year (2X / 1.0 yrs)

° Transfer rate (BW)

+ 40%/year (2X / 2.0 yrs)

° Rotation + Seek time

– 8%/ year (1/2 in 10 yrs)

° MB/$

> 100%/year (2X / <1.5 yrs)

Fewer chips + areal density

Floppy Disks Advantages: • Small• Easy to carry• Cheap• Useful for transferring small files • Can be used many times • Security tab to stop data from being overwritten

Disadvantages: • Easy to be damaged • Small storage capacity • Many new computers don’t have floppy disk drives • Can transport a virus from one machine to another • Slow to access and retrieve data when compared to a hard disk • Data can be erased if the disk comes into contact with a magnetic field 

Magnetic Tape

Advantages:• Relatively cheap per megabyte of storage 

• Can store large amounts of data - over 100 Gb 

• Can be set up to do the back-up overnight or over the week 

Disadvantages: 

• Needs serial access, so can be quite slow to access data 

• Need a special piece of equipment to record and read the data on the tape 

Hard Disk

Advantages: • Stores data for operating systems, software and working data

• Good for any application requiring very fast access

• Good for online and real-time processes

• Can handle major procedures such as payroll processing

Disadvantages: • Generally not portable

2.3.3 IDE - Definition

• Integrated Drive Electronics or IBM Disc Electronics, IDE is more commonly known as ATA or Parallel ATA (PATA).

• It is a standard electronic interface used between a computer motherboard's data paths or bus and the computer's disk storage devices.

• The IDE interface is based on the IBM PC Industry Standard Architecture (ISA) 16-bit bus standard.

Why IDE?

• Before IDE, controllers and hard drives were separate.

• For example, a controller from one manufacturer might not work with a hard drive from another manufacturer. The distance between the controller and the hard drive could result in poor signal quality and affect performance.

• This caused much frustration for computer users.

• Thus, IDE was created as a way to standardize the use of hard drives in computers.

• The basic concept of IDE is to combine the hard drive and controller.

• The controller is a small circuit board with chips that provide guidance as to exactly how the hard drive stores and accesses data.

Current And Past Configuration

• ATA-1:

• This is first standard which was developed by Compaq, Control Data Corp. and Western Digital, by using an 8- or 16-bit interface.

• It instituted the use of a master/slave configuration, and defined multi-Word Direct Memory Access (DMA)mode and Programmed I/O (PIO) modes 0-2.

• ATA-2:

• It is known as Enhanced IDE (EIDE).

• It has outlined PIO mode 3 and PIO mode 4.

Current And Past Configuration

ATA-3:

It improved the reliability of high-speed transfers.

Also, added Self-Monitoring Analysis and Reporting Technology (SMART).

ATA/ATAPI-4:

It increased data transfer rate support to 33 megabytes per second (MBps) known as Ultra DMS/33.

It has the AT Attachment Packet Interface (ATAPI) feature.

AT/ATAPI-5:

Supports data transfer rates of up to 66 MBps.

ATA/ATAPI-6: 

It Supports Ultra DMA/100, which lets drives theoretically reach 100 MBps.

Also includes Automatic Acoustic Management, which allows drives using this feature to automatically adjust access speed and reduce running noise.

IDE Interface Components

Characteristics of IDE

Speed: The fastest IDE devices are able to transfer data at 133 Megabytes per

second (Mbps).

Jumpers: IDE devices have jumpers, the computer uses them to differentiate

between multiple devices sharing a cable.

Cable Size: IDE device use a 40-pin connection.

Cable Count: IDE devices require fewer cable count than other devices.

2.3.4 SCSI Howard Shugart (Inventor of the Floppy Drive) Shugart Associates Systems Interface - SASI (1979) ANSI modified the SASI and standardized it - SCSI (1986)

SCSI: Small Computer System Interface SCSI is pronounced scuzzy

Higher version Fast SCSI Ultra SCSI Ultra2 SCSI Ultra3 SCSI Ultra4 SCSI Ultra5 SCSI

Characteristics IDE-Like Organization

Sylander Track Sector

Different Interface Higher Transfer Rates

Standard disk in many high-end workstations Useful for RAID configurations

More than just hard-disk interface Available in 8 and 16 bit (wide) versions

More Than Hard-disk Interface It is a bus connecting to:

A SCSI Controller (Usually plug-in card) Up to 7 SCSI devices or 15 in wide versions

SCSI Hard Disks CD-ROM CD Recorders Scanner Tape Units

SCSI Devices Unique ID from 0 to 7 or 0 to 15 in wide versions Two connectors

Input Output Cables connect the output of one to the input of the other one in series

(chain) Last device needs termination to prevent reflection

SCSI Cable Most common cable for 8-bit SCSI

50 wires 25 of them are ground paired to provide excellent noise immunity 8 for data 1 for parity 9 for control other for power and reserved for future

The 16-bit devices need a second cable The cables may several meter long

Operation SCSI controllers and peripherals can operate either as initiator or as targets

Usually the controller act as initiator issuing commands Other peripherals act as targets Commands are blocks of up to 16 byte telling the target what to do Commands and responses occur in phases

using various control signals to: delineate phases arbitrate bus access between multiple devices The arbitration is important because SCSI allows all devices to run

at once potentially improving performance

SCSI Commands SCSI commands are sent in a command descriptor blocks (CDP) CDP consists of

1 byte of operation code (opcode) 5 or more byte containing command specific parameters

After receiving the CDP by the device, it will return a status code byte and other information

In the published SCSI standards command are designated as: mandatory (required for all devices) optional vendor-specific

Due to different device types an opcode may produce different, but usually comparable effects

Ex. opcode 0x01 recalibrates the disk drive by seeking back to physical sector 0, but rewinds the medium in a tape drive

IDE Vs. SCSIIDE SCSI

Cost Overall, IDE is a much cheaper solution.

When compared with IDE, SCSI is often more expensive to implement and support.

Expansion

IDE/EIDE allows 2 two devices per channel. Most computers have 2 channels.

SCSI is capable of supporting up to 7 or 15 devices.

Ease IDE is commonly a much easier product to setup than SCSI.

Configuring SCSI can be more difficult for most users when compared to IDE.

Faster Today, the latest IDE and SCSI drives running at the same RPM are very close.

All the fastest drives are almost always available for SCSI first and in many cases 10,000+ RPM hard drives are only available as SCSI drives.

Resources

All motherboards today have an ATA/IDE interface and unless additional drives are needed no additional resources need to be taken.

Unlike IDE, SCSI requires an interface expansion card in most cases (unless the motherboard already has it). Adding any new hardware means more system resources are going to be required.

Credits Kamyar Arbabifard

Section 2.3.4 Additional References

SCSI Commands Reference Manual“ Seagate

Vivek Sthanam Section 2.3.2

Additional References Kanellos, Michael (24 August 2006). "A divide over the future of hard drives". CNETNews.com. Retrieved 24

June 2010. "IBM OEM MR Head | Technology | The era of giant magnetoresistive heads". Hitachigst.com. 27 August 2001.

Retrieved 4 September 2010

Prudhvi Potu Section 2.3.3

Additional References Frawley, Lucas. "Parallel vs. Serial ATA". What Is? The Information for Your Computer Questions.

Directron.com. Retrieved 23 January 2012 "Serial ATA: A Comparison with Ultra ATA Technology". Seagate Technology. Archived from the original on

2012-01-05. Retrieved 23 January 2012.