Messages in this thread | | | From | Segher Boessenkool <> | Subject | Re: [x86 setup 13/33] Header file to produce 16-bit code with gcc | Date | Tue, 10 Jul 2007 22:08:21 +0200 |
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>>>> gcc for i386 can be used with the assembly prefix ".code16gcc" to >>>> generate >>>> 16-bit (real-mode) code. This header file provides the assembly >>>> prefix. >>> This only works correctly with newer GCCs if you pass the >>> -fno-toplevel-reorder option (and it only works on older >>> GCC versions by accident). >> >> And on older ones (gcc-3.3-hammer / 3.4 / 4.0 ) -fno-unit-at-a-time >> >> Still it seems quite fragile to me agreed. > > The alternative, of course, is to compile to an .s file and insert > .code16gcc into the .s file. This makes the Makefile uglier, but > would > be more resilient against oddball gcc changes.
This would be even more fragile. The exact format of GCC's assembler code output isn't defined at all, so in principle this is a hopeless task. In practice just putting the .code16gcc directive on the first line would likely work though, GCC never generates a .code32 AFAIK, but it isn't guaranteed that this will work (or will keep working).
> I'm a bit surprised about the claim w.r.t. -fno-unit-at-a-time > (although > I guess that is the default and one would thus typically not see > this.)
-fno-unit-at-a-time is the default on three year old compilers, yes. Newer compilers have unit-at-a-time enabled by default.
> Got any pointers why that would cause a global asm() to be scrambled > around?
An asm() outside of a function is a top-level thing, just like functions and file-scope variable definitions, and with unit-at-a-time compilation all such blocks can be reordered (or even omitted, or inlined, or whatever). Note that even before unit-at-a-time was introduced there was no guarantee of emitting everything in the order it appears in the source file.
Segher
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