extended segments on 80386

[Paul Edwards]

Would it be possible for GCC to generate code
that reserves ESI and EDI as "extended segment"
registers to hold a source and destination
"extended segment" of any operation.

This will be the upper 32-bits of a 64-bit address.

When run on a normal 80386, such code will work
fine, and ESI and EDI will always be 0, so that source
and destination are always in the first 4 GiB.

When run on segmentation-aware hardware, the
API to obtain memory, e.g. INT 21H AH=48H (or
something much better, preferably), cx will be
set to 1 to indicate that a "far pointer" is being
requested, and if such memory is available,
EDI will be set to the segment, ie EDI = 2 will
mean the 8 GiB location. The 32-bit offset will
be returned via a normal register, e.g. EAX.

The application will set EDI to 0 before calling
the INT 21H.

Only segment-aware applications will set cx to 1
in the INT 21H request.

The exact same executable will thus work fine, within
the 4 GiB address space, just doing unnecessary
stores and loads of ESI and EDI (always 0), but on
segment-aware hardware, that exact same application
will have access to 16 EiB of memory, although with
size_t restricted to 4 GiB (although you could break
the size_t restriction by using a huge pointer instead
of a far pointer).

If ESI and EDI are not the most appropriate registers,
something else could be chosen.

Will that work?

Thanks. Paul.