Memory and File Systems
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Memory and File Systems






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Memory and File Systems Memory and File Systems Presentation Transcript

  • Memory CS423 Dick Steflik
  • DRAM
    • Dynamic Random Access Memory
      • each data bit is stored in a separate capacitive element in the Integrated Circuit
      • Because capacitors leak their charge the memory must be periodically refreshed, thus the name Dynamic or Volatile
      • requires one transistor and one capacitor per bit
        • simple, allowing high packaging densities
        • refresh rate dependent on implementation
          • 8 usec – 64 msec
      • includes DDR and SDRAM
  • SRAM
    • Static Random Access Memory
      • Static in the sense that it does not need refreshing like DRAM, but is still considered volatile (loses its memory when power is removed)‏
      • requires six transistors (multi ported memory may require 8, 10 or more transistors per bit)‏
      • lower packaging density than DRAM
      • easier to interface with than DRAM because of simplicity and no need to refresh
  • ROM
    • Read-only memory
      • mask ROM – non-volatile, permanently programed in the manufacturing process
      • PROM – Programmable Read-Only Memory
        • Programmable once then unalterable, non-volatile
        • essentially a fuse matrix in which programming is done by blowing the fuses
      • EPROM – Erasable Programmable Read-Only Memory
        • semi-permanent in that the memory can be erased (by exposing to ultrviolet light) and reprogrammed, considered to be non-volatile
      • EEPROM – Electronically Erasable Programmable Read-only
        • non-volatile, parallel and serial access.
        • serial access via SPI, I2C, 1-wire
  • Flash
    • Not an Acronym, just a name
      • stores one bit per cell, cells are floating gate MOSFETs
    • two types, NAND and NOR
      • NOR – used like traditional memory, execute-in-place memory (DRAM, SRAM)
      • NAND – accessed like a block device (disk), used for memory sticks, flash drived, MMC, CF
      • finite number of writes, unlimited reads
        • need wear leveling and bad block management
  • CF
    • Compact Flash
      • mostly NAND based, interface is smaller than, but electrically identical to, the ATA interface.
        • appears to the host device as if it were a hard disk. The CF device contains an ATA controller.
        • makes it easy to use CF to replace a small hard drive
        • used mostly in older digital cameras
        • convenient to use in ITX, mini-ITX systems that are IDE/ATA based
  • MMC
    • MultiMedia card
      • NAND technology developed by Siemans and SanDisk
      • been superseded by Secure Digital format
  • SD, SDHC
    • Secure Digital Card
      • NAND Flash
      • most commonly found in digital camera equipment
      • typically formatted as FAT, FAT32 by manufacturer but can be reformatted to any file system (ext2, jffs, cram,yaffs) for embedded systems use
      • MMC card can be used in SD slot but not vice-versa
  • JFFS
    • Journaling Flash File System
      • log-structured file system for use on NOR flash memory devices on the Linux operating system. It has been superseded by JFFS2
      • enforces wear leveling by treating the flash device as a circular log
        • At mount time, the file system driver must read the entire chain and then keep it in memory
        • The circular log design means all data in the filesystem is re-written, regardless of whether it is static or not. This generates many unnecessary erase cycles and reduces the life of the flash medium.
  • JFFS2
    • Journaling Flash File System (2)‏
      • includes support for NAND flash
      • better performance, JFFS treated the disk as a circular log. This generated a great deal of unnecessary I/O. The garbage collection algorithm in JFFS2 makes this mostly unnecessary.
      • supports compression
      • supports hard links
      • part of Linux kernel since 2.4.10
    • Yet Another Flash File System
      • designed specifically for NAND flash cards
      • log structured, used both with embedded OSs and systems with no OS. Simple OS interface
  • Compressed File Systems
    • FS decompresses data as it is retrieved and may or may not compress as data is put into storage
    • CramFS
    • e2compr
    • SquashFS
    • JFFS2
  • CramFS
    • read-only Linux file system
    • zlib-compressed one page at a time to allow random read access, files are compressed, meta-data is not
    • comes with a utility (mkcramfs) to pack files into new cramfs images.
    • often used for initrd images
  • e2compr
    • set of patches for ext2 file system kernel driver to make it work with compression
    • not a new file system, makes ext2 work with both uncompressed and compressed data
    • meta data us left uncompressed (for safety)‏
    • doesn't require a separate partition for compressed files
  • SquashFS
    • Compressed read-only file system
    • uses gzip compression (LZMA being worked on)‏
    • Live CDs
      • Ubuntu, Fedora, Gentoo
      • often used with UnionFS to provide read/write environment for Live CDs
        • SLAX, Debian Live, Mandiva
  • UnionFS
    • allows several file systems to be mounted as a single file system
    • allows files and directories of separate file systems, to be transparently overlaid, forming a single coherent file system.
    • Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.
    • In the case of a union of a read-only and a writable FS where an identical path is encountered the preference can be given to the writable path.
    • Example: Knoppix Live CD with a USB memorystick