[ECOS] ARM vectors.S question

Grant Edwards grante@visi.com
Wed Oct 27 07:01:00 GMT 1999

On Wed, Oct 27, 1999 at 06:21:41AM -0600, Gary Thomas wrote:

> > Perhaps my prejudices from doing 15 years of embedded systems are
> > showing, but I don't see how any system could be laid out this way.
> > Perhaps I don't understand how the power-up reset works in the CPU?
> No, we would never assume that RAM survives a reset :-)
> Most hardware that has RAM at 0x0 and the ROM somewhere else has shadow
> magic that makes the ROM appear at 0x0, either for a fixed number of cycles
> after reset or until a control latch is reset.

Ah yes, about 20 years ago I had an S-100 bus Z-80 bus system that did
the shadow rom for X cycles after reset trick.  I had forgotten about that.

> Firstly, you should consider working from the public CVS repository.  This
> has the latest stuff in it, including much work that lets items such as
> this be specified in platform specific files, not directly in "vectors.S".

OK, I'll take a look at the CVS tree -- having to modify what were
supposed to be platform-independant files made me think that I was
headed in the wrong direction.  Are there semi-stable CVS snapshots,
or is a guy pretty much rolling the dice when using sources from the
CVS tree?

> Are you using the CVS version, or some other source release?

I'm using the 1.2.1 official release.

> I'll try and provide help/guidance with whatever changes you need.
> Note that there are other places than "vectors.S" that expect low memory
> to be writeable so vectors can be updated.  Unless you can take the
> "remap" route, there will be other considerations.

Thanks.  That's a very useful tip.  I should be able to go either way:
remapping ROM/RAM in startup code or having ROM vectors go through
RAM.  Initially I was thinking of sticking with the ROM vectors, but
it sounds like remapping might be a better idea.

> > I must say that the ARM load/store instructions are about the coolest
> > I've seen since the PDP-11.
> I shudder to think what you've been working with...

Mostly the usual crop of embedded architectures (8086, 8051, 68HC11,
68332) and the TI TMS32C320 DSP for a while [it was wierd having
sizeof (everyting) == 1].

Grant Edwards

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