Mail Archives: djgpp/1999/08/10/19:31:59
On Sat, 7 Aug 1999, OSO wrote:
> Basically, the docs say this is a suitable command for
> writing OS kernels or when a libc is not present in
> memory. But I have question about it.
Since nobody replied, and I don't have time to look into GCC sources
(which is IMHO what one should do when one has such questions), I'll
try to answer. But please take what's below with a grain of salt, since
I didn't really see what GCC does under -freestanding.
As far as I understand, this option simply restricts GCC to emitting code
that doesn't rely on any library facilities. For example, it doesn't use
built-in functions.
But this option has no effect on the machine instructions generated by
the compiler. The code still assumes flat model with no segmentation,
and by default it is a 32-bit code.
> Do Linux & DJGPP GCCs still generate code which is PMODE (and
> DPMI for DJGPP) dependent?
This is a misunderstanding. Code generated by GCC in its DJGPP port does
not depend on DPMI. It doesn't even depend on the CPU being in protected
mode. The DPMI dependence is in the library and in the startup code. If
your program doesn't have more than 64KB of code and data, you can toss
the startup code and have your program run in real mode (of course, you
will have to write your own library functions for I/O etc.)
The only thing that GCC assumes is that the memory address space is
flat, and that the CPU can execute 32-bit instruction. How this is set
up is left for the run-time environment and the startup code. In the
DJGPP case, these rely on DPMI. But we use DPMI and protected mode
simply because it is one way of running GCC-produced code on top of a
real-mode operating system such as DOS.
> Can this option force GCC into a Flat Real Mode (1MB Max Code, 4GB Max
> Data) compatible code generator?
As far as I understand, you can force GCC into any flat mode, provided
that you supply the startup code and a library that arranges for it. You
don't even need -freestanding to do that.
- Raw text -