Take the 2-minute tour ×  Why can't a 64-bit OS run on a 32-bit
CPU, but a 32-bit OS works on a 64 bit CPU?  share|improve this
question  migrated from stackoverflow.com Feb 4 '10 at 20:11
This question came from our site for professional and enthusiast
programmers.  Why can't you run software for CPU type "A" on a
CPU of type "B"? Because A != B. –  John Saunders Feb 4 '10 at
20:08 simple answer: Because it is a PITA. :) –  Tony Feb 5 '10
at 4:05 Of course you can, all you have to do is remove the
32-bit CPU and put a 64-bit CPU in. –  Hello71 Aug 19 '10 at
23:08  7 Answers 7  up vote 16 down vote accepted  I'm assuming
we're talking about standard PCs here.  Most 64-bit processors -
especially the ones from Intel and AMD - have the full 32-bit
instruction set as well as the 64-bit extensions, so they can
run 32-bit software.  32-bit processers don't have a 64-bit
instruction set though. Many were made even before the 64-bit
instruction set was defined, so it's not possible they could
implement it correctly