Re: [ECOS] ecos start in ram

[Grant Edwards <grante at visi dot com>]

On Fri, Feb 16, 2001 at 02:32:23PM +0000, Bart Veer wrote:
> 
>     James>         now I am porting eCos to my test board (based
>     James> ARM7TDMI), the ROM address is 0x0 and the RAM address is
>     James> 0xc000000, but in ecos the RAM address is 0x0, so how to
>     James> config memory layout to run Redboot and eCos? I also think
>     James> the vectors.S file should be changed, will you tell any?

For a while I ran eCos with ROM at 0x0.  It's a pain.  You'll
have to modify vectors.S to move the "fixed vectors" stuff
somewhere else (the vsr_table is the part that seemed to
matter).  Then you'll have to modify the routines that write to
the vsr_table -- last time I looked, some of them assumed it
was at a particular hard-wired address.

You'll also have to set up your ROM so that the interrupt
vectors (except for reset) point to code that does indirect
jumps via a jump table in RAM.  NB: This table is different
than the vsr_table in the fixed vectors sections.  Then you
need to modify the startup code in vectors.S so that it fills
in this table instead of of locations at 0x00-0x20.

The really painful part is that you have modified
"architecture" HAL files that aren't part of your platform HAL.
Updating to newer versions of eCos requires re-doing the same
changes again to the architecture HAL files.

> IIRC on some ARM processors you normally need RAM at location
> 0x0, because that is where interrupt vectors etc. are held. You
> also need ROM at location 0x0 because bootstraps happen from
> that address. In the absence of an MMU, the normal way to
> resolve this is to have a memory remap facility: on power up
> there is ROM at location 0x0, so bootstrap can proceed; early
> on during the bootstrap, the remap switch is toggled and the
> memory map changes; the ROM moves somewhere else in memory, and
> there is now RAM at location 0x0 so that the interrupt vectors
> etc. can be updated.

Re-mapping is _far_ easier than what I previously described.
You can still run with your code in ROM if you want to, but
it's easier debugging if code is in RAM.

> So for a typical eCos application, the RAM might be at
> 0xC0000000 immediately after power-up but it will be remapped
> to 0x0 early on. Hence the eCos memory map will have RAM at
> 0x0.
> 
> The exact details are of course hardware-specific, and will
> have to be addressed by your platform port.

If you don't have a way to remap memory, make the hardware guys
add a way.  It only takes a few gates and a flip-flop.  They've
probably got more than enough extra capacity in that PLD.

-- 
Grant Edwards
grante@visi.com