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 TypeSizeAccess time Cost per MB CPU regs256 bytes1 nsForget it. Cache16 KB2 ns$ 20 RAM128 MB20 ns$ 1 disk100 GB100,000 ns$.001

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!