[SunHELP] Solaris 2.6 Memory Limitation

Greg sunhelp at sunhelp.org
Wed Feb 28 22:38:06 CST 2001

> It's true that 32bit systems can only address a maximum of 4GB of memory.

Not at all.  It's true that 32-bit mmu's can only support virtual address
spaces that each map 4GB of memory.  But the physical addressing of most
"32-bit" processors has been more than 32-bits for many years.

> That's not to say that a system running a 32bit o/s
> can physically contain only 4GB RAM. Solaris 2.6 is 32bit therefore it can
> only address up to 4GB.

Again, not true.  Each 32-bit virtual address space can address only
32-bits but since each process has its own address space you can
certainly use far more than 4GB of memory at any time simply by
running multiple processes.  So there's no excuse not to allow
for more than 4GB of physical memory.

Contrary to other responses, there isn't any overhead in accessing
physical memory beyond 4GB (where did that idea come from?), a
physical address is a physical address, they're all alike.  In general
a virtual page can correspond to any physical page, it's just a

Some stats of various SPARC microprocessors:

  Name		"bitness"	Virtual Address bits	Physical Address bits
  SuperSPARC	   32		32			36
  UltraSPARC I	   64		44 (yes, 44)		41
  UltraSPARC II	   64		44 (yes, 44)		41
 UltraSPARC III    64		64			43

Like most "64-bit" microprocessors, Itanium/MIPS/Alpha includes, the
UltraSPARC I and II don't have full 64-bit MMUs.  The UltraSPARC III
is the only one I know of that does.


More information about the SunHELP mailing list