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Memory

CS423 Dick Steflik. Memory. 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

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Memory

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  1. CS423 Dick Steflik Memory

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. MMC • MultiMedia card • NAND technology developed by Siemans and SanDisk • been superseded by Secure Digital format

  8. 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

  9. 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.

  10. 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

  11. YAFFS • 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

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