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MMC MultiMedia card  NAND technology developed by Siemans and SanDisk  been superseded by Secure Digital format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

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

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