Re: Ada equivalent for C-pointer?

[Nick Roberts <nick.roberts@acm.org>]

Stephen Leake <stephen_leake@acm.org> wrote:

> On older x86 systems, when 16 bit operating systems were used, there were
> two kinds of pointers; "near" pointers, 16 bits in size, and "far"
> pointers, 32 or 48 bits in size.
> 
> These days, you should interpret "far" as a noop, and just think of it as
> a pointer.

I want to be pedantic about this; please bear with me.

In what is called the 'flat' 32-bit memory model, a far pointer is the same
as near pointer (a 32-bit absolute offset, on its own). Since 32-bit Windows
(and indeed 32-bit Linux) only uses the flat memory model, this mode
appertains mandatorily.

However, in 32-bit segmented mode, a far pointer comprises a 32-bit offset
plus a 16-bit segment selector. These are stored in memory in a format which
is 64 bits in size (16 bits are wasted).

I understand there are some C compilers which support 32-bit segmented mode
compilation (e.g. WATCOM), but GCC never has and probably never will.

The distinction may seem pointless (sorry :-) to most people, but it matters
to me (because it will matter to AdaOS ;-)

To clarify, when we are talking about near and far pointers, we are
specifically talking about the 32-bit Intel Architecture (IA-32). I'm not
aware of any other contemporary architecture supporting a superimposed
segmentation scheme.

-- 
Nick Roberts