Re: [ECOS] Why virtual address and Physical address are different in the memory layout of IXPD425 board ?

[Sujoy Ray <raysujoy at yahoo dot com>]

Andrew,

Thanks for your suggestion. It really helped me to get
the concept. Apart from what you told, doese this
mapping has any relationship with LINUX booting ?

Regards,

Sujoy




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

> On Sun, Aug 08, 2004 at 11:14:03PM -0700, Sujoy Ray
> wrote:
> > 
> > Hello:
> > 
> > I was going through the memory laout of an IXPD425
> > board provided in the following link:-
> > 
> >
>
http://sources.redhat.com/ecos/docs-latest/redboot/ixdp425.html
> > 
> > 
> > It has following lines.
> > 
> > 
> > Virtual Address   Physical Address  XCB 
> Description
> > 0x00000000       0x00000000         010  SDRAM
> > (cached)
> > 0x10000000       0x00000000         010  SDRAM
> (alias)
> > 0x20000000       0x00000000        000 SDRAM
> > (uncached)
> > 
> > Now, I have following questions.
> > 
> > 1. Why in the third line (SDRAM uncached), the
> virtual
> > address and physical address are different? I know
> > that virtual address can be different from the
> > physical address. But normally it is not in the
> > embedded scenario. 
> 
> Probably the fundemental concept you are missing is
> the difference
> between cachaed an uncached memory. Device drivers,
> particularly PCI
> windows, need to access the ram in uncached mode.
> Normal programs make
> use of the cache because of the extra performance it
> gives. Any easy
> way to acheive this is to have the same memory in
> the address space
> twice, once caches and one uncached. The code can
> then decide which
> one to access.
> 
>  
> > 2. Who is going to use this address? I have seen
> that
> > all the addresses are assigned in the .ld file and
> > those are static allocation. If this is true, then
> how
> > this virtual address will be used.
> 
> The specific device drivers will know about this
> second mapping. It
> will probably be hard coded somewhere.
> 
>         Andrew
> 


__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 

-- 
Before posting, please read the FAQ: http://ecos.sourceware.org/fom/ecos
and search the list archive: http://ecos.sourceware.org/ml/ecos-discuss