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Introduction to Systems Programming Lecture 8.3

Introduction to Systems Programming Lecture 8.3. Non-volatile Memory Flash. EEPROM. EEPROM – Electrically Erasable Programmable Read Only Memory. this is a historical term to differentiate from an older type of memory that used UV-light to for eraser

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Introduction to Systems Programming Lecture 8.3

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  1. Introduction to Systems Programming Lecture 8.3 Non-volatile MemoryFlash

  2. EEPROM • EEPROM – Electrically Erasable Programmable Read Only Memory. • this is a historical term to differentiate from an older type of memory that used UV-light to for eraser • “Flash” memory is the dominant type currently • NOR flash • NAND flash • Main feature: contents remain when power is off

  3. Examples of Flash Memory • Your computer's BIOS chip • USB sticks • Digital Camera memory • Cellphone memory • Solid-state disks (SSD) in laptops • Memory cards for video game consoles • Invented by Toshiba in 1980s

  4. Basics • Initially every bit is logical ‘1’ • Programming a bit – set it to ‘0’ by applying a voltage • Once a bit is set to ‘0’ – cannot be set back to ‘1’ by programming • Erasing – set bits back to ‘1’ by applying a higher voltage

  5. Limitations • Erasure can only be performed on a “block” – not on individual bits or bytes • Memory wear: a finite number of erase-write cycles. • Typical: guaranteed to withstand around 100,000 write-erase-cycles

  6. NOR Flash • Has Data and Address lines, like RAM • Byte addressable • Can store code – execute in place (XIP) • Typical (erase) block sizes 64 - 256 Kilobytes. • But: more expensive, uses more power • Typical usage: BIOS, Firmware

  7. NAND Flash • Not byte-addressable – can only be read in “pages” • Typical “page” size: 512-4096 bytes • Typical erase block size: 16KB – 512KB • Cheaper than NOR flash • Slower read time • Typical use: file systems (USB sticks, camera & cell phone memory)

  8. NAND vs NOR Flash Toshiba Sept 2007

  9. NAND vs. NOR Flash • NAND denser than NOR • preferred for data memory storage (flash drives) • NAND slower read time • NOR preferred as program memory storage for microcontrollers because of faster access time • NAND is always block read/block write; NOR allows read of individual memory locations • NAND programming/erase is faster than NOR programming/erase

  10. Flash File Systems • “Wear Leveling” – blocks are not rewritten in-place. • Instead, written elsewhere, file pointers updated • Avoid rewriting same bits too often • Error correcting – correct bad bits via ECC • one bit error in each 2048 bits (256 bytes) using 22 bits of ECC code • one bit error in each 4096 bits (512 bytes) using 24 bits of ECC code.

  11. Transfer Rates • NAND Flash: advertized raw read rate of 15,000KB/s = 14.65MB/s. • That’s about 70ns to read one byte • Slower than RAM – but much faster than disk • Write is much slower

  12. Flash as replacement to disks • Advantages of Flash as SSD, especially in laptop • Noise • Speed • Power consumption, Heat • Weight • Reliability (no moving parts) • Disadvantages (as of 2009) • Price per MB [Improving] • Capacity [Moore’s law, 2x every 1.5 years] • Finite write-erase cycle: [improving]

  13. Recent numbers (SanDisk) • SSD: 64GB, 128GB and 256GB • Mobile memory & USB: up to 16GB

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