I started investigating huge pointer support in 16-bit C compilers and
discovered there are at least two different implementations:
Borland C++ 3.1:
Huge pointers aren't changed when converting from a far pointer. They
are normalized as soon as some pointer arithmetic is performed on them.
Normalization means changing the segment and the offset in such a way
that it still points to the same linear address ( =segment*16+offset ),
but the offset is always between $0000 and $000F. So:
HugePointer(Ptr($1234,$5678)) = $1234:$5678
HugePointer(Ptr($1234,$5678))+1 = $179B:$0009
HugePointer(Ptr($1234,$5678))-1 = $179B:$0007
Microsoft C/C++ 7.0 and Open Watcom 1.9:
Huge pointers aren't changed when converting from a far pointer (same as
Borland). However, during pointer arithmetic, they aren't normalized
like in Borland, but the segment is changed only when the offset
overflows, i.e.:
HugePointer(Ptr($1234,$5678)) = $1234:$5678
HugePointer(Ptr($1234,$5678))+1 = $1234:$5679
HugePointer(Ptr($1234,$5678))-1 = $1234:$5677
HugePointer(Ptr($1234,$FFFF))+1 = $2234:$0000
HugePointer(Ptr($1234,$0000))-1 = $0234:$FFFF
Back in the old days, I never used huge pointers, because I wrote in
Turbo Pascal, which didn't have them, so I don't have an opinion which
way is better. So, which way should FPC for i8086 follow?
Nikolay
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