Re: [f-cpu] new cjump instruction

["Christophe Avoinne" <christophe.avoinne@laposte.net>]

----- Original Message -----
From: "Yann Guidon" <whygee@f-cpu.org>
To: <f-cpu@seul.org>
Sent: Saturday, April 12, 2003 3:27 AM
Subject: Re: [f-cpu] new cjump instruction


> >I think that using partial addresses (as opposed to displacements)
> >in jumps or memory accesses is bullsh^H^H^H^H^H^Hnonsense.
> >
> >

Not totally a nonsense but something too much restrictive indeed.

The idea to have a short absolute jumper in the same page (supposedly a
minimal size for a page is 4096-byte long) that we are running would prevent
from any TLB exception to occur. So it could be used fastly in small loops
that are within a page.

Nevertheless, we need to care about page boundary, not about range boundary,
so your compiler needs to be aware of it and probably to rearrange code in
another suitable order; which is quite restrictive and something
definitively not wishable for a compiler.

For shared libraries, the page boundary should not be a problem because it
is usually only the index page of the address that changes, not the offset
due to the virtual address mapping used for sharing code in several
different addresses.

But for object linking, it might become an obstacle if object codes are not
mergeable at least on page boundary.

You can call it "bullshit" because it will be unusable if we don't respect
the page boundary.




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