[U-Boot-Users] RAM vs. ROM version of PPCBoot My $0.02 worth

[Rod Boyce]

At the risk of prolonging a dieing thread I'm going to add my two cents work
as I think I have a slightly different perspective.
For the record I now 100% agree with Wolfgang it makes no sense to create a
new version of U-Boot linked at a different address.

BUT...

When I first started down this path I would have fought tooth and nail for
the other approach.  I come from an embedded 8/16-bit world where your code
runs from ROM of some description and RAM is a very limited commodity.  (For
instance some of the projects I worked on have only 64-bytes of RAM
including register space and stack.)  My first though when I started working
on PowerPC micro-controllers with a BDM was to use the BDM to initialise the
system and then load the code into RAM and debug from there.  What a luxury
having any kind of debugger that let you step through your code.  I created
a couple of test applications this way some of which are still used today in
our factory to debug the hardware.  I even created a RAM only version of
PPCBoot that would let me debug PPCBoot it became apparent very quickly that
this was causing me a great deal of grief while trying to get PPCBoot
working.  I created a ROM version of PPCBoot and all of a sudden everything
started to work.  There are a great many things you are going to have to
take out of PPCBoot to get it to work correctly in a RAM only mode.  Being
able to use RAM to debug from the outset looks like a good idea until you
get past taking your first baby steps with PowerPCs but having a minimal BDI
script and letting U-Boot do all the rest is a very good idea you will
discover this for yourself.  

I strongly suggest you listen to guys like Wolfgang he has been there and
done it more times than most of the rest of us I suspect.  Like me I suspect
by the time you have your board working correctly you are going to owe
Wolfgang a beer or to good luck.

Regards,
Rod Boyce


 -----Original Message-----
From: 	Chris Hallinan [mailto:clh at net1plus.com] 
Sent:	Tuesday, December 03, 2002 1:32 PM
To:	wd at denx.de
Cc:	u-boot-users at sourceforge.net
Subject:	RE: [U-Boot-Users] RAM vs. ROM version of PPCBoot 

Wolfgang Denk wrote:
>
> In message
> <NEBBKJPFOLJNBMFNKBIIKENKDCAA.clh at net1plus.com> you wrote:
> >
> > > Why RAM version?
> > > Mostly, the convenience is the main reason.
> >
> > I can't imaging debugging PPCBoot (or most other flash-resident)
> > code any other way.
>
> Oops? To me it makes absolutely no difference if the code
>  is  runing
> from  RAM  or  ROM.  Debugging  is exactly the same (OK,
> you might be
> limited regarding thenumber of breakpoints when running
> from ROM, but
> usually this is no real handicap).

I agree with your comments.  The main reason I use RAM version is
for load speed.

OK, OK, I just timed it.  To erase/program flash using BDI-2000, it
takes
erase: 3 seconds
prog:  6 seconds (with WORKSPACE enabled)

and to load into RAM:
load:  2 seconds (with dcache disabled)

So, you're right, the difference isn't really that much! :)

-Chris Hallinan
DS4.COM, Inc.





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