[NTLK] Storage card specifications

Bradley Loeding bradley.loeding at gmail.com
Thu Apr 7 22:07:19 EDT 2011

There isn't really a "better". They really are just two different things.
Apples and oranges. Certain applications are better suited to one versus the
other. At the risk of too much info:

SRAM is a form of RAM, basically just a bunch of glorified flip-flops. Bits
are stored in the form of state. When the input signal is removed the state
remains until a new signal is given, or the power is removed. As such, it
can be written to as many times as you like. Also, bits can usually be
accessed and written directly via an external address bus.

Flash is a kind of ROM. A modified form of EEPROM to be more correct. Bits
are stored as trapped charge and can remain trapped for many years before
eventually leaking away. Depending on the kind of flash, charge is added or
removed from cells via quantum tunneling. This process is very slightly
destructive to the cell structure, so bit flipping can only be done a finite
number of times. This isn't so bad because failing cells can have their
state transferred to fresh cells and re-indexed. This is the basis for
bad-block-management, or wear-leveling.

However unlike RAM, Flash writes are done in large blocks like hard disks.
Since writes can only happen on a block-by-block basis, low-level random
access is not available. To get around this, blocks are read out to external
RAM where the bits can be freely modified, then written back in blocks at a
later time. See the Wikipedia article on "paging" if this is a bit

Long story short, all this dancing around to make Flash quack like a hard
drive duck must be managed by someone. Thus the difference between linear
Flash and ATA Flash. Linear Flash gains flexibly and efficiency by having
the computer manage the low-level complexities and wear-leveling in the form
of a software driver. ATA Flash gains ubiquity by doing the same all in
hardware as a transparent interface which sits on top of the Flash.

In conclusion, it all depends:
You may not want your Unix /tmp or /var mounted on Flash due to the lack of
write endurance. You may not want your wedding pictures in SRAM due to its
volatility. A real-time data logger may prefer using SRAM. Flash would do
very well as a hard drive replacement in a high vibration environment, etc.

I'm not sure if I really answered your question... but hopefully it clears
up the differences between the two forms of memory


On Thu, Apr 7, 2011 at 3:06 PM, Rusty Miles <Rusty.Miles at tn.gov> wrote:

> Other than the comparison points on the site mentioned (expense, write
> speed, capacity, batteries) are there any other advantages to SRAM over
> Flash?  Seems I remember seeing somewhere that one or the other (or maybe
> both) of these types of memory were only good for a finite number of
> operations (maybe that's true of all RAM).  In other words they eventually
> wear out, but maybe not quickly enough to be an issue.

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