Re: Using memory above TOS (Jeremy Fitzhardinge)
21 May 1996 16:21:37 -0400

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From: (Jeremy Fitzhardinge)
Newsgroups: comp.compilers
Date: 21 May 1996 16:21:37 -0400
Organization: Softway Pty Ltd
References: 96-05-132
Keywords: architecture (Barton C. Massey) writes:
>For a language like Mercury, in which there's an explicit
>runtime system and signal handlers are not directly implemented
>by the user, optimizations which use memory above TOS may be

Trying to use memory below the stack on a SPARC completely
fails, because the stack is generally not used directly, but
kept cached in register windows. The kernel takes the liberty
of not saving register windows "below" the SP on context
switches. In other words, you can't really defer the
use of a "save" instruction if you need to store things
on the stack.

>P.S. -- Actually, now that I think about it, the main reason I
>avoided memory above TOS is that I wasn't sure that UNIX
>promised to keep pages above TOS backed up in VM;

More seriously, it might not exist at all. Unicies generally
allocate stack for your process automatically on stack growth,
but define "stack growth" as a memory access between the last
stack position and the current SP. If you generate a fault
just below the SP, you get a SIGSEGV.

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