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Distinguish different type of RAM, ROM......

Posted on 1997-09-25
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Dear sir,

  I would like to ask what is the differences among EEPROM,
EPROM, SRAM, Flash ROM and Flash RAM?

Thanks!

Benson
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Question by:benson031397
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mauder earned 10 total points
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eeprom means electrically erasable and programmable rom which means that a electrical current on one of the pins is enough to erase its contents.
eprom means erasable programmable rom and has to be erased with a
UV licht source (they can be recognised by the smal window which is covvered with a black square sticker to prevent losing the programmed data)
SRAM or static ram is a fast ram because the refresh is either very low or not nessesary at all(i'm not sure of this), which is why they were used as cache ram in "older" systems. DRAM or dynamic ram has a high refresh rate in order to maintain the integrity of the stored data and therefore is slower.
Flash ROM is a ROM module (used in modern pc's) which can be upgraded with software. When in use as a systembios you can update the bios program using a manufacturers flash file for the type of flashbios used (AWARD bios etc.)
About FLASH ram i'm not sure but the difference is that you don't need a special file to update the stored information.
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by:j2
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corrections:

SRAM or static ram is a fast ram because the refresh is either very low or not nessesary at all(i'm not sure of this),

SRAM requires NO refresh.
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About FLASH ram i'm not sure but the difference is that you don't need a special file to update the stored information.

Totally wrong, you require a special dataset to upgrade it, the only thing that differs FLASH memories from convention EEPROMS is the speed.. FLASH is superior in speed, and also requires much less circuitry to be reprogrammed.
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by:mauder
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Well before i'm totally wrong again, i found a site containing the information you want and much more. Check it out.


http://hpmag.cern.ch/computing/dict/index.html
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by:kapoor
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Any questions on hardware refer to this very good site http://sysdoc.pair.com/

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by:ixl
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My web site describes all of these terms in great detail:
http://www.pcguide.com/ref/ram
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by:tomii
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SRAM is faster not only because it has no refresh, but also because it has a driving capability.  All the traces on a board generate a certain amount of capacitance & resistance, & therefore require a certain amount of drive to get voltage changes from one point on the board to another.  If things were ideal, these changes would propogate at approximately the speed of light.  Unfortunatelly, there's no such thing as ideal, so we must deal with an RC time constant.  SRAMs have the capability to supply much more current than DRAMs, at the expense of space.  Your average DRAM cell may be only one transistor & capacitor, whereas your average SRAM cell may have as many as seven transistors.  Because the individual cells take more space, a SRAM is significantly (on the order of 5 to 10X) more expensive than DRAM.  On the other hand, SRAm is measured in latency times on the order of 5-15 nanoseconds (x10^-12), while drams may be measured on the order of 50-70ns.

Flash ROMs are essentially EEPROMs...  There is some special differences on some in which they may be byte selectable as to what is erased.  Usually, however, you will find that they are sector eraseable/writeable, meaning that say, 1/4 of the IC must be erased at a time, but not the whole thing.  I don't know the difference between FlashRAMs & FlashROMs...  I think they're two different names for the same thing.
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by:benson031397
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However, as I know, the information in SRAM can be kept under a battery supplt, anyone disagree?
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by:mnw21
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The information in SRAM or DRAM can be kept under battery support. It doesn't matter which one it is. It is just that those PCMCIA cards that have SRAM happen to have a little battery on them to be used to keep the data. However, DRAM is much more suitable for use with a battery than SRAM, because it uses much less power.

SRAM is much faster that DRAM, for the reasons above, but that is payed off by it taking much more space on the chip (thus cost), and more power.

RAM (DRAM or SRAM) can be written to and read from at will.
An EPROM can be written to, if it has been erased beforehand by ultraviolet light.
EEPROM is like an EPROM, except that the erasing can be done electrically, rather than by ultraviolet light.
Flash ROM is very much like EEPROM, except that it is much faster, and it is easier to program, and it can be erased in smaller chunks.

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by:benson031397
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No.  I mean when u switch off the computer, all the information in DRAM will go out.  However, the information in SRAM is still here beause of the battery backup.


Benson
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by:khemicals
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ROM read-only memory – On a motherboard, this stores the BIOS code, and is most often a 64 KB or 128 KB EEPROM.
Types include EPROM (erasable, programmable ROM), which uses UV light to erase and a ROM burner to program, and
EEPROM (electrically erasable, programmable ROM), known as Flash ROM, which can be written and erased while
in-circuit.  The ROM is usually socketed with 28 pins and has a reflective sticker on it to cover the EPROM's window.
Shining UV light through this window will erase the EPROM's memory so it can be reprogramed.

Flash memory:A special type of EEPROM that can be erased and
                reprogrammed in blocks instead of one byte at a time.
                Many modern PCs have their BIOS stored on a flash
                memory chip so that it can easily be updated if
                necessary. Such a BIOS is sometimes called a flash
                BIOS. Flash memory is also popular in modems because it enables the modem
                manufacturer to support new protocols as they become standardized.
SRAM:Short for static random access memory, and
                pronounced ess-ram. SRAM is a type of memory that
                is faster and more reliable than the more common
                DRAM (dynamic RAM). The term static is derived
                from the fact that it needs to be refreshed less often
                than dynamic RAM.

                While DRAM supports access times of about 60 nanoseconds, SRAM can
                give access times as low as 10 nanoseconds. In addition, its cycle time is much
                shorter than that of DRAM because it does not need to pause between
                accesses. Unfortunately, it is also much more expensive to produce than
                DRAM. Due to its high speed, SRAM is often used only as a memory cache.
flash RAM is a completely non-volatile random access memory.

What are the advantages of Flash Memory over EPROM?

Unlike an EPROM, a Flash Memory does not require an ultraviolet light source for erasure. Flash memory contents can be rewritten: in a test socket; in a PROM
programmer; on-board during subassembly test; in-system during final test; and in-system after sale. A traditional EPROM simply cannot be used in this fashion.

Specifically, the Intel 28F020 provides high access speeds, low power consumption and immunity to noise. The 28F020 provides access speeds of 70ns, meaning
zero wait states for most microprocessors. The device has a maximum standby current of 100 microamps when unselected.

Typically, the device can be completely erased in 2 minutes, and completely programmed in 4 minutes.

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