Platforms - peripherals

[M.GRAYSON]

What is a peripheral?

Devices

-          Separate from the basic computer

-          Not the CPU, memory, power supply

Classified as

-          Input

-          Output

-          Storage

 

Connected via:

Ports:

-          Parallel

-          USB1.0 (1.5Mb/s)

-          USB1.1 (12 Mb/s)

-          USB2.0 (480 Mb/s)

-          USB3.0 (4 Gb/s)

-          Serial

 

Interface to system busses:

-          SCSI (Small Computer Systems Interface)

-          IDE (Integrated Drive Electronics, WD)

-          ATA

-          ATAPI; PATA

-          SATA

-          PCMCIA (Personal Computer Memory Card International Association)

 

Storage Devices

Primary memory

-          Data and programs must be copied to primary memory for CPU access

 

Secondary storage

-          Permanence of data

-          Direct access storage devices (DASDs)

-          Online storage

-          Offline storage – loaded when needed

 

Speed

-          Measured by access time and data transfer rate

 

Access time: average time it takes a computer to locate data and read it.

 

Data transfer rate: amount of data that moves per second.

 





Secondary storage devices



-          Hard drives

-          Floppy drives

-          CD-ROM

-          CD-RW

-          DVD-RAM

-          DVD-RW

-          Tape drives

-          Network drives

-          Direct access vs. sequential access

-          Rotation vs. linear

 

Magnetic Disks

Provide bulk of secondary storage of modern computers.

Disks can be removable.

Drive is attached to the computer via I/O (input/output) bus.

Buses vary, including:

-          EDIE

-          ATA

-          SATA

-          USB

-          Fibre channel

-          SCSI

Host controller in a computer uses a bus to talk to the disk controller built into drive or storage array.

 

Track – circle

Cylinder – same track on all platters

Block – small arc of a track

Sector – pie shaped part of a platter

Head – reads data off a disk

-          Number of bits on each track is the same, denser towards the centre.

CAV – Constant Angular Velocity

-          Spins the same speed for every track

-          Hard drives – typical 7200rpm, also slower and up to 15000rpm (or 120rps up to 250rps)

-          Floppy drives – 360rpm

Head crash results from disk head making contact with the disk surface

-          Parked heads



Locating a block of data

Average seek time: the time required to move from one track to another.

Latency: the time required for a disk to rotate to the beginning of the correct sector.

Transfer time (or transfer rate): the time required to transfer a block of data to the disk controller buffer.



 

Disk Access Times

Average seek time: the average time to move from one track to another

Average latency time: the average time to rotate to the beginning of the sector.

-          Average latency time = ½ *  1/rotational speed

Transfer time: 1/ (# of sectors * rotational speed)

Positioning time (random-access time): the time to move the disk arm to the desired cylinder (seek time) plus the time for desired sector to rotate under the disk head (rotational latency).

Total time to access a disk block: the average seek time + the average latency time + the average transfer time)

 

Disk Addressing

Disk drives are addresses as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.

The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially.

-          Sector 0 is the first sector of the first track on the outermost cylinder.

-          Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from the outermost to innermost.