3123_Exam 2 Study Guide

3123_Exam 2 Study Guide - The von Neumann Architecture CPU...

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1 IT 3123 Hardware and Software Concepts Review November 9 Copyright © 2005 by Bob Brown Notice: This session is being recorded. The von Neumann Architecture CPU I-O Accumulator PC IR MAR MDR Arithmetic- Logic Unit Control Unit Command Main Memory Main Memory Main memory is short-term storage • Holds programs and data for running processes • Does not (usually) hold data for processes that are not running • Contents are lost when power is turned off. • Is not generally thought of as “storage.” Peripherals • Devices that are separate from the basic computer • Not the CPU, memory, power supply • Classified as input, output, and storage • Connect via • Ports • parallel, USB, serial • Interface to system bus • SCSI, IDE, PCMCIA Storage Speed • Measured by access time and data transfer rate • Access time: average time it takes a computer to locate data and read it • millisecond = one-thousandth of a second • Data transfer rate: amount of data that moves per second Hierarchy of Storage Device Access Times Transfer Rate CPU Registers ½ nanosecond Cache memory 10-30 ns Main memory 50-100 ns Hard disk 10-50 ms. 600k-6m/sec Floppy disk 100 ms. 100-200k/sec CD-ROM 100-600 ms 500k-4m/ sec Tape ½ sec up 11m /sec
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2 Sequential and Direct Access Sequential access: You must pass over all prior data to reach a given piece of data; tape devices (music and computer) are sequential access. Di t Atb l k f d t Direct access: Access to any block of data on the medium takes “about as long” as access to any other; disks, CD-ROM, and DVD are direct access. • Compare “direct access” to “random access.” Magnetic (Hard) Disks A “sector” is a physical portion of a track on the disk. A “block” is the data written in a sector. Often used interchangeably. Disk Access Time Average Seek Time: average time to move from one track to another Average. Latency Time: average time to rotate to the beginning of the sector ½ * 1/ t ti l d = ½ * 1/rotational speed Transfer Time: 1/(# of sectors * rotational speed) Total Time to access a disk block: Seek Time + Latency Time + Transfer Time About “Head Crashes” • Disk heads “fly” a few thousandths of an inch from the disk surface on an air bearing created by the spinning disk. • The disk is moving very fast. • If the head touches the disk, it can abrade the magnetic material; this is a “ head crash. • The magnetic material is destroyed, and the debris causes other head crashes. Redundant Disks • If redundant (duplicative) data are stored on two or more disks, the failure of any single disk will not cause data loss. • RAID = redundant array of inexpensive di k disks. • “Levels” of RAID • RAID 1: Mirrored disk volumes • RAID 5: Bits of each data word duplicated across three or more drives.
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3123_Exam 2 Study Guide - The von Neumann Architecture CPU...

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