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Main and Secondary Memory: Types and Examples

Learn about main and secondary memory in computer systems, including different types of secondary storage such as disk, flash memory, and tape. Explore examples of disk geometry and understand the access time of different storage media. Discover the speed and capacity of CDs and how fragmentation can impact performance.

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Main and Secondary Memory: Types and Examples

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  1. CS 101 – Sept. 28 • Main vs. secondary memory • Examples of secondary storage • Disk (direct access) • Various types • Disk geometry • Flash memory (random access) • Tape (sequential access)

  2. Memory system • After the CPU, next most important feature • CPU contains registers – only enough for immediate calculations • Main memory– running programs • Secondary memory– long-term storage • “open” vs. “save”

  3. Main memory • Runs on electricity: “volatile” but fast • Principle of random access • Examples: • RAM (vast majority of main memory is here) • Cache (small version of RAM, much faster) • BIOS (“Basic I/O System”)

  4. Secondary memory • Non-electric, “non-volatile” technologies • Magnetic • Optical • How to find information? It may be: • Random access (e.g. flash memory; stick) • Direct access (disk) • Sequential access (tape)

  5. Example comparison

  6. Disk examples • Hard drive • Can’t take out. Don’t confuse with RAM • Floppy disk • 8” , 5 ¼”, 3 ½”

  7. Disk examples • ZIP disk, “super” disk • CD (including: CD-ROM, CD-R, CD-RW) • “ROM” = read-only • “R” = recordable • “RW” = read/write • DVD

  8. Disk geometry • Tracks • Sectors • Platters

  9. Access time • Seek time – wait to find right track (~ 8 ms) • Head moves at 50 mph! • Latency – wait to rotate (~ 4 ms) • Based on 7200 RPM • Transfer – grab data from track

  10. Example: the CD • Each track has lands and pits. • 700 MB total, including overhead • About 25,000 tracks • About 50 sectors per track • One bit = about 1.6 microns long, 0.7 microns wide

  11. CD speed • Nominal speed is 80 mins (150 KB/sec) • Ex. 12x speed, ~ 7 mins (1.7 MB/sec) • By comparison, a floppy takes about 100 times slower: (1 MB/min) • Fragmentation can ruin time!

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