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Chapter 5 Internal Memory

Chapter 5 Internal Memory. Semiconductor Memory Types. Semiconductor Memory. 16Mbit DRAM. Static RAM (SRAM). Desired for main memory Used in cache Basically an array of flip-flops Simple to interface and control Fast Relatively low density - complex Relatively expensive.

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Chapter 5 Internal Memory

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  1. Chapter 5Internal Memory

  2. Semiconductor Memory Types

  3. Semiconductor Memory 16Mbit DRAM

  4. Static RAM (SRAM) • Desired for main memory • Used in cache • Basically an array of flip-flops • Simple to interface and control • Fast • Relatively low density - complex • Relatively expensive

  5. Static RAM model

  6. Dynamic RAM (DRAM) • Used in main memory • Bits stored as charge in capacitors Essentially analog device Charges leak • Need refreshing even when powered Need refresh circuits • Higher density (more bits per chip) • Slower than Static RAM • Less expensive

  7. Dynamic RAM model

  8. Read Only Memory (ROM) • Permanent storage • Nonvolatile • Microprogramming (see later) • Library subroutines • Systems programs (BIOS) • Function tables

  9. Types of ROM • ROM: Written during manufacture • Very expensive for small runs • PROM: Programmable (once) • Needs special equipment to program • Read “mostly” • EPROM: Erasable Programmable • Erased by UV • EEPROM: Electrically Erasable • Takes much longer to write than read • Flash memory • Erase whole memory (block) electrically

  10. EPROM

  11. Memory Organization • A 16Mbit chip can be organised as 1M of 16 bit words • A bit per chip system has 16 lots of 1Mbit chip with bit 1 of each word in chip 1 and so on • A 16Mbit chip can be organised as a 2048 x 2048 x 4bit array • Reduces number of address pins • Multiplex row address and column address • 11 pins to address (211=2048) • Adding one more pin doubles range of values so x4 capacity

  12. Typical 16 Mb DRAM (4M x 4)

  13. Semiconductor Memory 16Mbit DRAM

  14. Refreshing • Refresh circuit is included on the chip • Count through rows • Read & Write back • Chip must be disabled during refresh • Takes time • Slows down apparent performance

  15. 256kByte Module Organisation

  16. 1MByte Module Organisation

  17. Error Detection/Correction Errors: • Hard Failure • Permanent defect • Soft Error • Random, non-destructive • No permanent damage to memory • Coding (example Hamming code) can be used for - Error detection - Error correcting

  18. Error Correcting Code Function

  19. Hamming Code Visualizing: With even parity: Word: Identifying error: With Error:

  20. Hamming Code To store an M bit word with detection/correction takes M+K bit words If K =1, we can detect single bit errors but not correct them If 2K - 1 >= M + K , we can detect and correct single bit errors, i.e. detect an error and identify which bit it is. Example: for M = 8: for K = 3: 23 – 1 < 8 + 3 for K = 4: 24 – 1 > 8 + 4 Therefore, choose K =4

  21. Increased word length for error correcting

  22. Layout of Data and Check Bits C1 is a parity check on every data bit whose position is xxx1 C2 is a parity check on every data bit whose position is xx1x C4 is a parity check on every data bit whose position is x1xx C8 is a parity check on every data bit whose position is 1xxx Why? Because we want the syndrome, the Hamming test word, to yield the address of the error.

  23. Improvements in memory RAM – continually gets denser. DRAM – Several improvements: SDRAM – synchronous DRAM DDR-SDRAM - doubles transfer speed RDRAM – asynchronous one transfer per clock cycle

  24. Comparison of improved DRAM Conventional DRAM – 40 to 100 MB/S transfer rate?

  25. Synchronous DRAM (SDRAM) • Access is synchronized with an external clock • Address is presented to RAM • RAM finds data (CPU waits in conventional DRAM) • Since SDRAM moves data in time with system clock, CPU knows when data will be ready • CPU does not have to wait, it can do something else • Burst mode allows SDRAM to set up stream of data and fire it out in block • DDR-SDRAM sends data twice per clock cycle (leading & trailing edge)

  26. SDRAM Read Timing

  27. SDRAM

  28. DDR SDRAM • SDRAM can only send data once per clock • Double-data-rate SDRAM can send data twice per clock cycle • Rising edge and falling edge

  29. RAMBUS • Adopted by Intel for Pentium & Itanium • Main competitor to SDRAM • Separate bus (hence the name RAMBUS) • Bus addresses up to 320 RDRAM chips at 1.6Gbps • Asynchronous block protocol • Precise control signal timing • 480ns access time • Then 1.6 Gbps

  30. RAMBUS Diagram

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