Re: [ECOS] Re: RE : [ECOS] Using flash memory on AT91eb55

[hong zhang <henryzhang62 at yahoo dot com>]

Andrew,

How to identify my linux kernel image and ramdisk
image (rootfs) are ROM or RAM images?

----henry

--- Andrew Lunn <andrew@lunn.ch> wrote:

> On Fri, Dec 14, 2007 at 11:38:31AM +0100, Champion
> J?r?me wrote:
> > The evaluation board has 3 memory :
> > 
> >  * 256K byte of 16-bit SRAM (upgradeable to 1 MB)
> >  * 2M bytes of 16-bit Flash (of which 1 MB is
> available for user software)
> >  * 4M bytes of Serial Data Flash (upgradeable to
> 16 MB)
> > 
> > As I understand, the 256kb SRAM is used by the
> variables in my code.
> > 
> > 1 mb of flash is used by my code which take 900
> kb.
> > the other 1mb is used by the bootloader, which
> take only 200kb, so I think I can perhaps use the
> 800 other.
> > So I just have to make a "const char* myArray =
> 0x01050000" and the data will go there ? 
> > 
> > I can use the 4mb of serial flash, but I don't
> really know how to do it.
> > 
> > Why is it revelent if I work in the ram or the
> rom, the error happen at compilation time?
> 
> The compilation time error is just saying your don't
> have enough
> memory. Im assuming here it is saying you don't have
> enough RAM, but
> it could be saying the flash is too small.
> 
> The difference between a RAM image and a ROM image
> is to do with where
> the code lives when executed. With a ROM image, the
> executable code
> stays in ROM, and only your stacks, writable
> variables etc are in
> RAM. With a RAM image, everything is is RAM,
> including the code.
> 
> So a ROM image has the advantage you need less RAM.
> The disadvantage
> is that often flash is slower to read than RAM, so
> your program runs
> slower.
> 
> If you have a ROM image, the linker needs to decide
> for each variable
> where to put it. Does it put it in RAM or in FLASH.
> Anything which can
> be modified has to be in RAM. Anything which is
> constant can be put in
> FLASH. So if you declare your array as
> 
> const int foo[4] = { 1, 2, 3, 4};
> 
> the linker knows it can put it into FLASH. Without
> the const, it is
> possible your code does something like
> 
> foo[2] = 42;
> 
> and so it needs to be in RAM. 
> 
>     Andrew
> 
> -- 
> Before posting, please read the FAQ:
> http://ecos.sourceware.org/fom/ecos
> and search the list archive:
> http://ecos.sourceware.org/ml/ecos-discuss
> 
> 



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