Date: Mon, 31 Jul 95 23:19 MDT
From: mat AT ardi DOT com (Mat Hostetter)
To: nverever <nverever AT ee>
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Re: Memory protection problems
Newsgroups: comp.os.msdos.djgpp
References: <3vcetp$ion AT sifon DOT cc DOT mcgill DOT ca>

>>>>> "nverever" == nverever  <nverever AT ee> writes:

    nverever> Ok, I'm writting a tight inline assembler function that
    nverever> takes 3 parameters so far.  The thing is, that one of
    nverever> those parameters is a pointer in memory that is
    nverever> allocated when the program is loaded first, but stays
    nverever> constant after that. I was thinking that I could save
    nverever> the time of passing the parameter if I wrote an
    nverever> initialisation routine that would modify an inline
    nverever> instruction such as "movl $0, %edx" and replace the
    nverever> constant $0 with whatever the pointer is. Sounds
    nverever> great,but I hit a problem I could of guessed should be
    nverever> there, I get an exception fault when I try to modify the
    nverever> opcode.

But if you do "movl $0,%edx", the assembler will generate only a 1
byte constant for the zero!  You'd need something like "movl
$0x12345678,%edx".

    nverever> I figure this is probably because the selector
    nverever> in cs is read-only. If that is indeed the problem, I
    nverever> know that it's possible to change the selector to
    nverever> read/write and then back to read only with some dpmi
    nverever> functions, but I don't have a dpmi server on my system
    nverever> and I assume that most people don't either. Help on this
    nverever> solution if it could work would be appreciated.

I don't think you need to do this.  You're modifying the memory via
%ds, which is read/write.  I'm pretty sure I've done exactly what you
are saying (I stuck constants for %ds into an interrupt handler movw).

    nverever> Also, I heard that it's possible to make an array of
    nverever> bytes and make the program execute those
    nverever> instructions. If this is really so I could simply copy
    nverever> the function into an array, modify the desired
    nverever> instructions and call that new function. The only
    nverever> problem, is that I don't know how to call that new
    nverever> function if this can be done at all since it's in the
    nverever> data selector, not the code selector.

Addresses for %ds and %cs are equivalent, since they overlap.  The
68040 emulator our commercial Mac emulator uses dynamically generates
code in the data segment and runs it.  I wrote that emulator, and it
works fine under djgpp (much better under V2 beta).

You can call such code simply by making a C function pointer that
points to it and call it.  If you do it this way make sure you save
and restore registers preserved by the C calling convention.

-Mat