[geeks] Flash drive questions

[Phil Stracchino]

Charles Shannon Hendrix wrote:
> I thought magnetic memory was going to make flash a dinosaur any day
> now, and that flash had upper limits that made it ridiculous to keep
> using?  Reference: IEEE Computer and IEEE (standard) mags for the
> last couple of years.

Freescale just started volume production of MRAM.  We discussed it
briefly a few weeks back.

"Freescales 4Mbit MRAM has an access time of 35ns for both the read and
the write making it competitive with mid-range SRAM. It is positioned,
and priced, to be a replacement for battery-backed SRAM.
It is not fast enough to compete with high-speed SRAM which has sub-10ns
access times. "It can't replace Level One cache", said Tehrani."

http://www.mram-info.com/

It may not be competitive with system SRAM, but that's surely far more
than competitive with flash (except on cost, so far).  And it's denser:
 you can put four MRAM cells in the space occupied by one SRAM cell.


Incidentally, I also note the following, from
http://en.wikipedia.org/wiki/Flash_memory:

"The read-only mode of NOR memories is similar to reading from a common
memory, provided address and data bus is mapped correctly, so NOR flash
memory is much like any address-mapped memory. NOR flash memories can be
used as execute-in-place memory, meaning it behaves as a ROM memory
mapped to a certain address. NOR flash memories have no intrinsic bad
block management, so when a flash block is worn out, either the software
using it has to handle this, or the device breaks.

[...]

"NAND flash memories cannot provide execute-in-place due to their
different construction principles."


So, you can either have write-remapping in your flash RAM to spread
utilization, or execute code from it, but not both from the same type of
flash RAM.


Also from http://en.wikipedia.org/wiki/MRAM:

"It is also worth comparing MRAM with another common memory system,
Flash RAM. Like MRAM, Flash does not lose its memory when power is
removed, which makes it very common as a "hard disk replacement" in
small devices such as the iPod or digital cameras. When used for
reading, Flash and MRAM are very similar in power requirements. However,
Flash is re-written using a large pulse of voltage (about 10 V) that is
stored up over time in a charge pump, which is both power-hungry and
time consuming. Additionally the current pulse physically degrades the
Flash cells, which means Flash can only be written to some fixed number
of times before it must be replaced.

"In contrast, MRAM requires only slightly more power to write than read,
and no change in the voltage, eliminating the need for a charge pump.
This leads to much faster operation, lower power consumption, and no
effective "lifetime". These advantages are so overwhelming that it is
expected Flash will be the first memory type to eventually be replaced
by MRAM."

The overall summary from that article:

"MRAM has similar speeds to SRAM, similar density but much lower power
consumption than DRAM, and is much faster and suffers no degradation
over time in comparison to Flash memory. It is this combination of
features that some suggest make it the "universal memory", able to
replace SRAM, DRAM and EEPROM and Flash. This also explains the huge
amount of research being carried out into developing it."



-- 
 Phil Stracchino                     Landline: 603-886-3518
 phil.stracchino at speakeasy.net         Mobile: 603-216-7037
 Renaissance Man, Unix generalist, Perl hacker, Free Stater