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. > > 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. > 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. -- Michael Meissner, IBM 4 Technology Place Drive, MS 2203A, Westford, MA, 01886, USA meissner@xxxxxxxxxxxxxxxxxx
