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."
> 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?
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.
Bob
