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Computers

Computers. Internal and External Memory. Characteristics of Computer Memory. Location Capacity Unit of transfer Access Method Performance Physical Type Organization. UNIVAC Console and CPU. Memory Hierarchy. COST. SPEED. REGISTERS. CACHE. MAIN MEMORY. MAGNETIC DISK | DISK CACHE.

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Computers

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  1. Computers Internal and External Memory

  2. Characteristics of Computer Memory • Location • Capacity • Unit of transfer • Access Method • Performance • Physical Type • Organization

  3. UNIVAC Console and CPU

  4. Memory Hierarchy COST SPEED REGISTERS CACHE MAIN MEMORY MAGNETIC DISK | DISK CACHE MAGNETIC TAPE | OPTICAL DISK

  5. Bit Byte Kilobyte Megabyte Gigabyte Terabyte b B = 8 b KB = 1,000 B MB = 1,000,000 B GB = 1,000,000,000 B TB = 1,000,000,000,000 B Capacity Units

  6. Memory Access Method • Sequential - tape • Direct - floppy or hard disk • Random - internal memory • Dynamic (DRAM) simple, small, must be refreshed • Static (SRAM) no refresh needed • Associative - some cache

  7. Random Access Memory • Chips and chip technology

  8. Flip Flop Circuit Diagram

  9. CONTROL read or write DATA IN / SENSE input or output SELECT select cell Memory Cells CONTROL read or write SELECT select cell 1 0/1

  10. Write Enable Row Address Row Address Col Address ROW ADDRESS BUFFER MUX COL ADDRESS BUFFER Write to Memory Read Enable Write Enable ARRAY Col Address A0 . . . A10 ROW ADDRESS BUFFER DATA INPUT BUFFER DATA INPUT BUFFER MUX D1 . . D4 COL ADDRESS BUFFER DATA OUTPUT BUFFER REFRESH

  11. Pin Assignments • A0 … A10: address location (multiplexed) • D1 … D4: data in or out • Vcc: power supply • Vss: ground • RAS: row address select • CAS: column address select • WE: write enable • OS: output enable

  12. CACHE Cache Operation ALU CNTL ..... BUS MAIN MEMORY

  13. Cache CPU Word CACHE Main Memory Block

  14. Cache

  15. Random Access • Direct Access Storage Device (DASD) or disk drives • Optical • Magnetic

  16. Optical Disks (CD’s)

  17. How CD’s work

  18. Cylinder/Track/Block Track Block (Sector) Cylinder

  19. Disk Organization

  20. DASD Structure Read-Write Heads

  21. Data Storage • FAT (File Access Tables), Directories and Catalogs • Update and Delete • Fragmentation and reorganization • Blocks, Headers and Interblock Gaps

  22. DASD Access (PC) MAIN MEMORY CPU BUFFER CACHE DASD CONTROLLER

  23. DASD Access (Mainframe) MAIN MEMORY CPU BUFFER CHANNEL DASD CONTROLLER

  24. Data Structure BLOCK HEADER HEADER DATA CRC • Header written when disk is formatted • Data copied into block • Cyclical Redundancy Check calculated

  25. Controller Operation(DASD retrieval) • CPU passes parameters to registers in the controller • The controller transfers data into the card buffer • The controller checks the CRC to assure the data was copied correctly • The controller (or CPU) transfers buffered data to memory one word at a time

  26. Millisecond Microsecond Nanosecond Picosecond ms = 1/1000 s µs = 1/1,000,000 s ns = 1/1,000,000,000 s ps = 1/1,000,000,000,000 s Time Units

  27. Representative Times • Network speed = megabits per second • Disk transfer = megabytes per second • Disk access = milliseconds • Memory access = nanoseconds • Machine cycle = microseconds/nanoseconds

  28. Error Correction and Checking Block Add bits to a block to use for error discovery • Detection only • Detection and retransmission • Detection and recovery Check Body Header

  29. Error Detection Only (Asynchronous Transmission) * Parity Bit * * * * * * * 7 Data Bits 27 = 128 distinct characters

  30. Error Detection &Correction (Hamming Code: 4 bit word) * * * 3 Error Checking Bits * * * * 4 Data Bits

  31. Error Detection &Correction (Hamming Code: 4 bit word) DATA * * * 1 1 1 1 1 1 0 0

  32. Error Detection PARITY (even) 100 1 1 0 1 1 1 1 1 0 0 0

  33. Error Correction (4 bit word) PARITY (even) 100 1 1 0 1 0 0 1 1 0 0 0

  34. Error Correction & Detection • Error detection takes fewer bits than error correction • Longer packets take a smaller percent for correction but have more types of errors • Hamming’s scheme detects all errors at a high overhead cost; others may correct only single bit or double bit errors with shorter check fields

  35. CRC Error Checking • The transmitted messages are divided into predetermined blocks • The blocks are divided by a fixed divisor • The remainder is appended to the message

  36. IBM 1107 with tape drives

  37. Memory

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