Download Memory and File Systems

Memory
CS423
Dick Steflik
DRAM

Dynamic Random Access Memory

each data bit is stored in a separate capacitive
element in the Integrated Circuit
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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

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simple, allowing high packaging densities
refresh rate dependent on implementation
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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
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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
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
essentially a fuse matrix in which programming is done by
blowing the fuses
EPROM – Erasable Programmable Read-Only Memory
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
Programmable once then unalterable, non-volatile
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
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Not an Acronym, just a name

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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)
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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.

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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 viceversa
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
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
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e2compr
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SquashFS

JFFS2
CramFS
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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
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
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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
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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
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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