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Maddy · Guest

Hi,

I cant help wonder why embedded processors have more Flash than RAM?
Right now, I am working with the AT91SAM7S, it has 256K Flash and only
64K RAM. Why is this?
I am assuming we cannot run out of Flash. So if I had a program greater
than 64K what am I suppposed to do?

Maddy

Meindert Sprang · Guest

"Maddy" <santhos@gmail.com> wrote in message
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Peter Dickerson · Guest

"Maddy" <santhos@gmail.com> wrote in message
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Andreas S. · Guest

Maddy wrote:

Phil W · Guest

Hi,

Obviously your concept of processors comes from PC type system where all
programs run in RAM.
However, in embedded microprocessors programs generally run in flash, eeprom
or rom, since there is no other storage medium that is non-volatile to hold
the program whilst the power is off.
In embedded systems there is generally no need to run programs in RAM since
they are usually dedicated to a single application and do not need the
ability to run different applications.
Have a look at the AVR. It does not even give you the option of running a
program from RAM, it's all assumed to be data.

Phil

"Maddy" <santhos@gmail.com> wrote in message
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enguinear · Guest

For an embedded computer just think of Flash as disk and there is
always more disk than RAM. Except you can access the flash much faster
than disk so you can execute code right out of the flash with out
loading it in RAM as you would from a disk. That actually cuts down
the amount of RAM needed. On the other hand Flash has limited
write/rewrite life so your dynamic storage for temporary feequently
changing data is why the RAM is there at all. Just write out to Flash
what you want to keep around between power cycles, without power
refreshes RAM forgets.

So Flash is disk, with limits and bennifits.

-dcb

On Thu, 3 Nov 2005 07:48:18 +1100, "Phil W" <electropw@hotmail.com>
wrote:

An Schwob in the USA · Guest

Well,
for one reason, every time your turn the power off to the RAM, the
contents is gone, a so called volatile memory. So, you need at least
all the program in the Flash to begin with, even if you want to execute
it out of RAM>
Secondly and much more important, you would not want to pay for the
chips if the Flash would be 64k and the SRAM 256k, it would probably be
double.
Last but not least, the combination 256k Flash and 64k RAM is a HUGE
RAM. As the SAM7S needs the SRAM to run fast code in ARM mode a part of
the SRAM is needed for that. If you look for example at the Philips
LPC2000 families of ARM microcontrollers, you will find the SRAM being
smaller, biggest reason is that these devices can run full speed in the
high performance ARM mode from Flash.
Discussions about this and other related subjects can be found in the
LPC2000 forum:
http://groups.yahoo.com/group/lpc2000/
or for the SAM7S here:
http://groups.yahoo.com/group/AT91SAM7/
An Schwob

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