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On Apr 10, 12:07pm, John H. Kim wrote:
> Subject: Re: Flash Cards
> On Mon, 10 Apr 1995 11:45:27 EST you wrote:
>
> >> > I am curious about how flashcards, that is pcmcia memory cards, work
> >> > on the ThinkPads.
>
> >I think the fact being lost here is that these are SRAM (Static RAM)
> >cards, there fore the added expense. SRAM is more expensive that DRAM
>
> No, it's flash RAM. It's non-volatile -- doesn't need any power to
> maintain its contents. That's why you can use it like a hard drive.
> SRAM can be accessed while power is being supplied to it (refreshed)
> so it doesn't impose wait states. DRAM is inaccessible when being
> refreshed and you wind up with wait states.
Static RAM does not have a refresh state.
A Dynamic RAM data element (bit) is implemented with a single current
transistor, which essentially acts as a capacitor. Since capacitors exhibit
current leakage, you have to periodically "refresh" the transistor by
essentially reading the element. The elements are arranged in rows, and you
have to read out an entire row into an internal buffer to read a single bit.
Thus, you are refreshing rows at a time.
Static RAM elements are implemented with four transistors forming a flip-flop
gate. This is why they have much lower densities than Dynamic RAM's. The
flip-flop will maintain its state until the power to the chip is shut off --
there is no refresh. You can also access individual bits without waiting for
the row strobe (RAS) to come by, which is one of the reasons SRAM is much
faster than DRAM.
FLASH memories are relatives of EEPROM's (electrically eraseable programmable
read-only memories). They are *very* slow compared with SRAM's and DRAM's,
although they are still orders of magnitude faster than hard drives. They also
suffer a relatively short read/write cycle life. In other words, after awhile
you're going to wear out the card. I'm not sure what the lifespan is, but it
is quite long. The PCMCIA bios also circulates data across the FLASH sectors
to distribute the read/write usage.
Robert
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| Robert George | Army Research Laboratory |
| robertg@assb01.arl.mil | AMSRL-SS-IC |
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