On 8 Nov 2001, at 9:17, David Jonathan Justman wrote:

> A 16 bit program is one in which the basic unit of information
> transfer is 16 bits.  In a 32 bit program, it's 32 bits.  In
> theory, if the programs were otherwise identical, the 32 bit
> program should run almost twice as fast if the hardware and
> software are both optimized for 32 bits, since the 32 bit program
> can transfer data twice as fast, but it doesn't really work that
> way for many reasons.

  A 16-bit program runs in either "real mode" or "16-bit protected
mode", and so could potentially run on an 80286 CPU (IBM "AT" class).
The offset registers used for local memory references are only 16
bits, so each memory "segment" can be no more than 64K in size.  A
large program may need to have many segments, and may spend a lot of
its time switching from segment to segment.  Windows version 3.x was
written to this standard, as were virtually all applications to run
on that platform.
  Data registers on the 80286 were also limited to 16 bits.  The
80386 extended data registers (optionally) to 32 bits, and introduced
"32-bit protected mode" using 32-bit offset registers.  Many late
Windows 3.x applications used the 32-bit data registers to speed
operations (and thus required a 386 or better CPU), but still ran in
"16-bit protected mode" and so are not generally considered fully "32-
bit programs".
  Microsoft had meanwhile defined an interface for 32-bit programs to
talk to a 32-bit version of Windows:  NT.  A module called WOW
(Windows On Windows) allowed most 16-bit protected-mode Windows
applications to run on NT, and a module named Win32s allowed a fair
number of 32-bit Windows programs to be run on 3.1.  In each of these
cases, it was clear what sort of programs were considered "native"
and which were tolerated (or not...) guests.

  Win95 retained the ability to run 16-bit applications better than
NT did -- and in fact, retained some 16-bit code within Windows 95
itself -- but definitely crossed over to where 32-bit applications
were the norm and 16-bit supported only for backward compatibility.
And each version of WIndows since has pushed further towards the 32-
bit side.

David Gillett

                Curious about the people moderating your
                   messages? Visit our staff web site:
                    http://freepctech.com/staff.shtml