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Re: 32-bit libraries in 64-bit build.


On Wed, 2009-05-20 at 13:55 -0400, Michael Meissner wrote:
> On Wed, May 20, 2009 at 10:03:08AM -0700, Bob Plantz wrote:
> > On Wed, 2009-05-20 at 12:31 -0400, Michael Meissner wrote:
> > 
> > > structures as well.  64 and 32-bit programs use a completely different ABI, and
> > > the stack is set up differently.
> > 
> > The main difference being that the first six arguments are passed in
> > registers instead of on the stack. It's very "RISC-like."
> 
> Also the alignment of the stack, whether or not the implementation can assume
> the use of the SSE2 registers, size of long, how 64-bit values are returned,
> what the stack unwind tables look like, etc.  Lots of differences.

Yes, I should have said "one difference being..."

> 
> > >   Finally, some 32-bit instruction encodings
> > > are not available when the computer is in 64-bit mode, and others behave
> > > differently when the 64-bit registers are enabled.
> > > 
> > 
> > Isn't this true only at the individual instruction level?
> 
> If the 32-bit library was compiled with encodings that conflict with the 64-bit
> instructions, then instructions will do funky things if you somehow manage to
> bolt a 32-bit library into a 64-bit program, because the library will be called
> in 64-bit mode.

I'm not aware of any 32-bit instructions that behave differently in
64-bit mode. Except, of course, instructions that manipulate the stack
(push, pop, call, ret, etc.) do so in 32-bit or 64-bit increments,
respectively. This is the machine code 0x55 is pushl %ebp in 32-bit mode
and it is pushq %rbp in 64-bit mode.

Do you have other examples?

BTW, I'm not trying to be argumentative here. I've written an
introductory computer architecture textbook based on x86-64 and am eager
to avoid saying stupid things in my book. :-) The more I can learn, the
better.

> 
> > For example, you can do
> >     movb    %ah, %al    # 32-bit, no REX prefix byte
> >     movb    %al, %r8b   # 64-bit, requires REX prefix byte
> > 
> > but you cannot do
> >     movb    %ah, %r8b   # mixed mode, conflict
> > 
> > The first sequence of instructions will run on my machine under 64-bit
> > Ubuntu.
> >
> > An aside: The gnu assembler, gas, gives an error message about the REX
> > prefix being required for the second case. It took me a while to figure
> > it out. The error message should really say something about this being
> > an attempt to mix 32-bit and 64-bit modes.
> 
> Well as they say, patches are welcome.
> 

Mea culpa. I should learn how to do this. I thought I would have more
time to contribute when I retired, but I was surprised to learn how many
things there are to do in life besides "working." I don't know how I
ever found enough time to work!

Bob




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