c++ - Can we assume the last 2 bits of a memory address are 00 and reuse those bits? A windows 7 page fault blue screen -

my friend programming in c++ on 64-bit windows 7 pc , came crazy idea save little bit of memory: observed last 2 bits of memory addresses seemed 00, figured use bits other things , when memory address needed, use bit-mask set last 2 bits 0, either when writing or reading memory. reason why he's using last 2 bits needs work on 32-bit systems too. anyway, on windows 7 64-bit system got following blue screen error when running program:

page_fault_in_non_paged_area

could crazy memory savings idea causing this? i.e., can happen last 2 bits of memory address not 00, , he's accessing memory that's partly on 1 of memory pages, partly off page? in event, needs work on popular operating systems. question applies other operating systems well.

if (in windows 7 64 bit, @ least) scheme guaranteed work (if coded properly), else causing unusual blue screen crash?

your friend taking advantage of feature known tagged pointers. on windows, raymond chen has warning regarding on blog:

there no /8tb flag on 64-bit windows

a customer reported 64-bit application crashing on windows 8.1. traced problem fact user-mode address space 64-bit applications on windows 8.1 128tb, whereas 8tb on earlier versions of windows x64. ...

...

as how ended having dependency on address space being @ 8tb, didn't say, have guess: using unused bits tagging.

if going use tagged pointers, you need put tag bits in least significant bits, since bits control. example, if align objects on 16-byte boundaries, have 4 available bits tagging. if you're going use upper bits tagging, @ least verify upper bits available.

something more important watch out - memory pointer allocated os might aligned in way allows tagging, if intermediate memory manager sits between user's code , os (which case), manager allocates os memory internally , divides app use, pointers manager gives out app might not aligned in way allows tagging. cannot tag arbitrary memory pointer without knowing came or how aligned.