Computer Architecture CDROM+Tape พันธุ์ปิติ เปี่ยมสง่า ภาควิชาวิศวกรรมคอมพิวเตอร์ มหาวิทยาลัยเกษตรศาสตร์ CS621 สถาปัตยกรรมคอมพิวเตอร์

I/O Systems Processor Cache Memory - I/O Bus Main Memory I/O interrupts Processor Cache Memory - I/O Bus Main Memory I/O Controller I/O Controller I/O Controller Graphics Disk Disk Network CS621 สถาปัตยกรรมคอมพิวเตอร์

Optical Storage CD-ROM Originally for audio 650Mbytes giving over 70 minutes audio Polycarbonate coated with highly reflective coat, usually aluminum Data stored as pits Read by reflecting laser Constant packing density Constant linear velocity CS621 สถาปัตยกรรมคอมพิวเตอร์

CD-ROM Drive Speeds Audio is single speed Constant linier velocity 1.2 ms-1 Track (spiral) is 5.27km long Gives 4391 seconds = 73.2 minutes Other speeds are quoted as multiples e.g. 24x The quoted figure is the maximum the drive can achieve CS621 สถาปัตยกรรมคอมพิวเตอร์

CD-ROM Format Mode 0=blank data field 00 FF x 10 00 Layered ECC Min Sec Sector Mode Data 12 byte Sync 4 byte Id 2048 byte 288 byte 2352 byte Mode 0=blank data field Mode 1=2048 byte data+error correction Mode 2=2336 byte data CS621 สถาปัตยกรรมคอมพิวเตอร์

Random Access on CD-ROM Difficult Move head to rough position Set correct speed Read address Adjust to required location (Yawn!) CS621 สถาปัตยกรรมคอมพิวเตอร์

CD-ROM for & against Large capacity (?) Easy to mass produce Removable Robust Expensive for small runs Slow Read only CS621 สถาปัตยกรรมคอมพิวเตอร์

Other Optical Storage CD-Writable CD-RW WORM Now affordable Compatible with CD-ROM drives CD-RW Erasable Getting cheaper Mostly CD-ROM drive compatible CS621 สถาปัตยกรรมคอมพิวเตอร์

DVD - what’s in a name? Digital Video Disk Digital Versatile Disk Used to indicate a player for movies Only plays video disks Digital Versatile Disk Used to indicate a computer drive Will read computer disks and play video disks Dogs Veritable Dinner Officially - nothing!!! CS621 สถาปัตยกรรมคอมพิวเตอร์

DVD - technology Multi-layer Very high capacity (4.7G per layer) Full length movie on single disk Using MPEG compression Finally standardized (honest!) Movies carry regional coding Players only play correct region films Can be “fixed” CS621 สถาปัตยกรรมคอมพิวเตอร์

DVD - Writable Loads of trouble with standards First generation DVD drives may not read first generation DVD-W disks First generation DVD drives may not read CD-RW disks Wait for it to settle down before buying! CS621 สถาปัตยกรรมคอมพิวเตอร์

Magnetic Tape Serial access Slow Very cheap Backup and archive CS621 สถาปัตยกรรมคอมพิวเตอร์

Digital Audio Tape (DAT) Uses rotating head (like video) High capacity on small tape 4Gbyte uncompressed 8Gbyte compressed Backup of PC/network servers CS621 สถาปัตยกรรมคอมพิวเตอร์

Tape vs. Disk • Longitudinal tape uses same technology as hard disk; tracks its density improvements Disk head flies above surface, tape head lies on surface Disk fixed, tape removable • Inherent cost-performance based on geometries: fixed rotating platters with gaps (random access, limited area, 1 media / reader) vs. removable long strips wound on spool (sequential access, "unlimited" length, multiple / reader) • New technology trend: Helical Scan (VCR, Camcoder, DAT) Spins head at angle to tape to improve density CS621 สถาปัตยกรรมคอมพิวเตอร์

Current Drawbacks to Tape Tape wear out: Helical 100s of passes to 1000s for longitudinal Head wear out: 2000 hours for helical Both must be accounted for in economic / reliability model Long rewind, eject, load, spin-up times; not inherent, just no need in marketplace (so far) Designed for archival CS621 สถาปัตยกรรมคอมพิวเตอร์

Summary Disk industry growing rapidly, improves: bandwidth 40%/yr , areal density 60%/year, $/MB faster? queue + controller + seek + rotate + transfer Advertised average seek time benchmark much greater than average seek time in practice Response time vs. Bandwidth tradeoffs Value of faster response time: 0.7sec off response saves 4.9 sec and 2.0 sec (70%) total time per transaction => greater productivity everyone gets more done with faster response, but novice with fast response = expert with slow Processor Interface: today peripheral processors, DMA, I/O bus, interrupts CS621 สถาปัตยกรรมคอมพิวเตอร์