cvs commit: src/sys/i386/i386 pmap.c

Nate Lawson nate at
Sat Apr 29 17:37:48 UTC 2006

Peter Wemm wrote:
> Stephan realized that the kernel already allocates one PTE per virtual 
> page.  Although it normally holds physical addresses plus attributes, 
> as long as we don't set PG_V, then there are 31 other bits that we 
> could use for data storage.  We could put virtual addresses in there so 
> long as we didn't set PG_V, and abuse that to have a singly linked 
> freelist threaded through the PTEs.  It turned out even easier though.  
> As long as the virtual addresses are page aligned, we neatly avoid all 
> the PG_* mode bits as well. 

... because you can shift by the page size, saving 12 bits.  But are 
there any problems with 2 MB pages?

Great ideas, both you and ups at .  You know, you can store 2 pointers 
there with XOR.  ;-)


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