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Memory and Storage

Memory and Storage. What we’ll cover for this lecture topic:. Primary Organization Types Attributes Auxiliary (Secondary) Types Technology. Computers basically have two-level storage :. Primary storage: MEMORY (chips) Secondary/Auxiliary storage: STORAGE ( disks… ). Processor.

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Memory and Storage

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  1. Memory and Storage What we’ll cover for this lecture topic: • Primary • Organization • Types • Attributes • Auxiliary (Secondary) • Types • Technology

  2. Computers basically have two-level storage: • Primarystorage:MEMORY(chips) • Secondary/Auxiliarystorage:STORAGE (disks…) Processor CU ALU Outputs Inputs Memory RAM ROM Storage

  3. Primary Memory • Functional unit for storing & retrieving all 5 kinds of information -- used by the processor. • “electronic storage == no moving parts” ( fast!!!!) • For every transformation: • The processor must fetch a basic instruction, one at a time, and copy it to its own special storage registers before it can do any operation/transformation on data. • The processor also has to copy the data into its own special storage registers ( more about the actual process later). • The processor is also electronic … and very FAST !!! • We don’t want to slow down the processor. Therefore we use electronic storage (rather than electromechanical).

  4. Digression: a basic machine language statement: • OP CODE: part of machine code (a bunch of bits) that tells the CPU which operation to perform (add, multiply, compare, ...) • OPERAND: also part of the machine code (another bunch of bits) that tells the CPU at which memory address the data is stored that is should operate on. You will see a lot on about this soon when we study the architecture of the CPU and how it operates on data (e.g. transforms data).

  5. # 0000 0000 1 1 1 1 0 0 0 0 # 0000 0001 0 1 1 0 0 1 0 1 # 0000 0010 0 1 0 1 0 0 1 1 # 0000 0011 1 0 0 1 1 0 0 1 # 0000 0100 1 1 0 0 1 1 0 0 ... • MEMORY: Holds INSTRUCTIONS & DATA that we are interested in processing NOW. Arrangement (similar to P.O. boxes): • Each is called a memory cellor a memory location. • Each cell is identified by auniquebinaryaddress. • Each cell contains a certain number of bits (usually eight). • Similar to what in a spreadsheet? • Why is uniqueness important?

  6. memory CPU # 0000 0011 Register A _ _ _ _ _ _ _ _ 0 1 0 1 0 0 1 0 Memory addressing • Just as Excel uses a cell reference in a formula to locate a cell and to read the datastored in that cell, a program uses a memory address. • So a memory cell address is similar to a spreadsheet’s absolute cell reference (e.g. $B$24). • The cell contents can change, but the cell address or reference to the cell is always the same. • A program can write toand read froma memory cell through their address. LOAD #0000 0011 A ADD A 1 0 1 0 1 0 0 1 0 1 1 STORE A #0000 0011

  7. What happens to data when we write or read? • What happens when you type (write) something into a spreadsheet cell that already contained some data? • Writing destroys what was stored there. • What happens if an Excel formula reads a data value from another cell: • Reading does not destroy what is stored there. It just “hand-over” a copy. • “32-bit memory-addressing” means that the computer uses memory addresses that are 32 bits long: • the “address bus” are the wires that carry bits between the CPU and the memory • a 32-bit bus carries 32 bits at a time 232 4 GB (billion) different addressable memory cells.

  8. One memory cell: #0000 0001 0 1 0 0 0 0 0 1 • UNLIKE a spreadsheet cell, a single memory cell can hold much less: 8 bits • Cannot address bits individually (grouped by 8)

  9. # 0000 0001 • Interesting digression 0 1 0 0 0 0 0 1 • How many memory cells are required to hold one ASCII character? • How many cells required to hold a bit pattern that can represent a single pixel with a “24-bit color”? • How many cells required to hold an standard precision integer (which requires 32 bits)? • How many cells required to hold an high precision real number (which requires 64 bits)? • And…….what is the term for 8 bits (which happens to fit perfectly into one memory cell)?

  10. So, each memorycell (location) can hold 8 bits, or one byte. • Those 8 bits, in turn, could represent, at a given time: • Part of an integer or realnumber • Integers: typically 32 bits; Real: either 32 or 64 bits • The bit pattern for one alphanumeric character (one character byte) • Part of a bit pattern for a sound sample. • The bit pattern for one pixel with “8-bit color.” • 1/3rd of a bit pattern for one pixel with “24-bit color.” • A part of an instruction.

  11. What did we say keeps track of whatkind of information a bit pattern (in memory) is representing at a given moment? the program (e.g. software) Logical structure of1 memory cell: 8 bits Physical structureof1 memory cell: 8 capacitors • Total primary memory and auxiliary storage in one’s computer is measured in bytes. • Kilobyte (KB or K): ~ 1 thousand 210 • Megabyte (MB): ~ 1 million 220 • Gigabyte (GB): ~ 1 billion 230 • Terabyte (TB): ~ 1 trillion240

  12. Types of primary memory • RAM: Random Access Memory • RAM is fast. • Virtually no speed penalty for accessing bits, no matter which cellthey’re in: “Random access” • RAM is writable (by the user). (launch; load; edit…) • RAM is volatile. • Stores what’s being used now: Mostly determined by whom? Often called “user memory” -- • Typically 256 or 512 MB (and up) for personal computers. • Advice: Buy as much as you can afford. • Affects overall speed of system (page faults, caching)

  13. ROM (read-only memory) • Contents pre-written and permanent: non-volatile. • Typically small: <1 MB • Stores bootstrap program. • Stores BIOS (very basic parts of the operating system that rarely change are hardwired in ROM) • Often called: Firmware. Why do you suppose? • Just for culture: • Other flavors of nonvolatile primary memory: PROM; EPROM; EEPROM • Flash Drives • CMOS (battery-powered; see text for more info) “Virtual Memory” • Textbook has a good explanation of this process so I won’t cover it much here. Just understand the basic idea behind it. use disk space to simulate RAM

  14. Processor CU ALU Outputs Inputs Memory RAM ROM Storage

  15. Want more memory than is affordable in electronic memory. Cheaper: shares one set of electronics for R and W (the head). But muchslower: at least by 10,000 times. Physically moves medium past R/W mechanism. Need non-volatile storage that is recordable. Auxiliary Storage Primary memory:like your closet • Access is frequent and fast; but less is taken at a time (4 - 8 bytes). Secondarystorage:like your attic • Access is less frequent and much slower; but a whole block of data is taken at a time. (256 - 1024 bytes)

  16. Most common form of storage. Magnetized spots on a surface, which may be on a tape, which itself moves past a R/W head (like audio tape) Device: tape drive or unit Medium: tape (reel or cartridge) on a disk, whose R/W head moves over it as it spins. A disk may be floppy or rigid A disk may be fixed permanently in the drive or removable. Device: disk drive Medium: disk MAGNETIC Storage

  17. North-South (clockwise) South-North (counter-clockwise) How a magnetic recording works A moving electrical charge (current) creates a magneticfield... The resulting magnetic field can be in one of two different orientations (polarities): depending on the direction that the current flows “INDUCTION”

  18. So: we can send a current through a wire (the R/W mechanism) to magnetize a spot on an iron surface nearby. The spot (with magnetic field around it) will have one of two orientations. This way the head records (writes)physical representations of 0s and 1s on the iron surface.

  19. A moving magneticfield creates anelectric field... So: if there’s a wire coil nearby, the current is sent through it in one of two different directions The READ headinterprets the direction of the incoming current as a 0 or a 1.

  20. Ribbon coated with iron oxide Reel-to-reel (old mainframes) Cassettes, cartridges Sequential Access storage device: to reach data written at the endneed to go through all other data Generally used for: short-term backup transport Tape head 0 1 1 1 0 1 0 0 0 TAPE Magnetic tape

  21. Surface divided into wedges: sectors. Bits are stored within a sector along concentric circles called tracks. (Floppy disk: ~80 tracks) Track 0 Magnetic disks Sec 6 Track 1 Track 2 Sec 5 Sec 0 Sec 4 Sec 1 Sec 2 Sec 3 • All disk drives are “DASDs”: • Direct Access Storage Devices • File is accesseddirectly by giving the address of each block on the disk that contains the file. • Similar to random access except it has speed penalties. Why do you think that’s so? • DIGRESSION: Speed…. SEEK time, SPIN rate

  22. Floppy disks (diskette) and drives Transportable storage. Good to back-up a few files. Digression:Would you recommend backing up data from an entire hard drive on standard floppy disks? Why or why not? What could you backup? Flexible plastic mylar …; removable media Slow spin rates (360 to 400 rpm) Slow seek time (~300 ms) R/W heads touch disk surface Cheap, abundant, portable

  23. Have gotten smaller, cheaper, higher density outdated: 8” (20 KB); old: 5.25” (140 KB – 1.2 MB); low density 3.5”: 720 KB high density 3.5”: 1.44 MB most popular SuperDisk 3.5”: 120, 144, and 200 MB Write protect window… what for? Any special care for disks? Do floppy disks ever fail?

  24. Hard disks/disk drives (fixed) • Rigid aluminum or glass; various sizes; >1 platter • Much faster spin rates (currently up to 10,000 rpm) • Much faster access speeds (seek time = 5 – 15 ms) • Much faster transfer rates (15 MB/sec and up) • Fixed means: media NOT removable from the hardware • Internal and external • Capacities: • for older PC systems: 20 MB...80 MB...200 MB • Today: 20-500 GB and up

  25. Digression: What’s a head crash? • Very fast spin rates. • R/W head floats abovehard disk surface on tiny cushion of air……..

  26. Digression: What’s a head crash? • Very fast spin rates. • R/W head floats abovehard disk surface on tiny cushion of air. • High data density: smaller recorded bits. • Challenge: decrease head’s “flying height” to increase signal’s amplitude (so it’s detectable) • R/W head can crash into contaminant on disk surface, then onto disk! So: disks permanentlysealed in case, fixed in drive: “Winchester” technology

  27. Removable disk storage • Great forbackups • Verytransportable • Internal & external • many flavors • watch for compatibility issues. • A few choices(actual numbers just for culture--- try to get a sense of how much more they store than a standard removable floppy disk).Examples: ZIP: 250 MB Zip Drive - w/5 disks~$ 75 JAZ: 2 GB Iomega JAZ External ~$300 Syquest: 1.5 GB ~$300 (been around longer) Flash Drives

  28. Use lasers to record (write), and also to read. Record: high-powered laser burns pits into reflective coating. Read: weaker laser reads pits; differences in reflectance of pits & lands interpreted as 0 and 1. (See textbook figures) For PCs: ~650 MB per CD-ROM (275,000 pgs of text) Similar to audio CDs, but more error-correction. Internal & external drives. QUOTABLE OPTICAL Storage

  29. CD-ROM [standard; early generations] Many flavors … READ ONLY: • Rigid platter; various sizes, densities. • Single SPIRAL track (starts near center  outward) • Usually 650 MB, about 4 ¾” • Standard transfer rate was 150 Kb/sec but soon there came: 2x 4x … 24x … 40x … ? • Biggest Source of ERROR: dust • Disk cost: depends on what’s on it! • manufacturer • Read only; who writes? • Good for?Games, Software, Reference materials, Library materials, for being AOLnoyed, …

  30. RECORDABLES: Recordable by: USER (various technologies) readable by most standard, same-sized drives and players. WRITE ONCE: CD-R Good for? (used where data won’t change) • Multimedia presentations; archives; greatest hits; photos; video clips… • Hold 650 MB of data, or ~74 min. of audio • Can copy files to it (like removable magnetic media) but a little more complicated (software comes with drive) Unalterable once written

  31. WRITE MANY: CD-RW Recordable/writable many times (when used with appropriate software) RW Drives generally can record on both CD-R or CD-RW disks. Getting very popular Often used for backups somewhat more expensive 700-MB CDs that can be read by most CD-ROM drives compatibility issues

  32. Newer optical disc technology: DVD • “Digital video (versatile) disc” • Similar to DVD-video that you hook up to your TV…. • READ ONLY: • DVD-ROM: instead, holds computerdata & is hooked to your computer.. • RECORDABLE VERSIONS: • DVD-R (write once) and…..

  33. DVD-RAM, DVD-RW, DVD+RW (write many) • A “standards” problem: different formats--which drives can read and write to which formats….confusing for customers. • …. “There are several types of DVD drives, and at least four versions of media, none of which is completely compatible with the other….” from keynote addressed delivered at Intel Developer Forum week of 9 Sept 2002. • New drives can read and write to both DVD+RW and DVD-RW. • Generally “downward” compatible. • Becoming popular for backups as prices come down. • About 8X the storage capacity of a CD

  34. So what? DVD: Can store a whopping 2.6 to 8.75 GB! For huge graphics files, video, etc For desktop or notebooks: • Getting popular: DVD+-RW/CD-RW combo drive • Again, be aware of writability and compatibility issues. http://www.computershopper.com http://www.cnet.com …(lots!) http://www.adaptec.com

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