Renesas SH7781 - Manual (Page 181)

7. Memory Management Unit (MMU)
Rev.1.00 Jan. 10, 2008 Page 151 of 1658
REJ09B0261-0100
the return from the exception handling routine, the instruction which caused the TLB miss
exception is re-executed.
(4)
Single Virtual Memory Mode and Multiple Virtual Memory Mode
There are two virtual memory systems, single virtual memory and multiple virtual memory, either
of which can be selected with the SV bit in MMUCR. In the single virtual memory system, a
number of processes run simultaneously, using virtual address space on an exclusive basis, and the
physical address corresponding to a particular virtual address is uniquely determined. In the
multiple virtual memory system, a number of processes run while sharing the virtual address
space, and particular virtual addresses may be translated into different physical addresses
depending on the process. The only difference between the single virtual memory and multiple
virtual memory systems in terms of operation is in the TLB address comparison method (see
section 7.3.3, Address Translation Method).
(5) Address Space Identifier (ASID)
In multiple virtual memory mode, an 8-bit address space identifier (ASID) is used to distinguish
between multiple processes running simultaneously while sharing the virtual address space.
Software can set the 8-bit ASID of the currently executing process in PTEH in the MMU. The
TLB does not have to be purged when processes are switched by means of ASID.
In single virtual memory mode, ASID is used to provide memory protection for multiple processes
running simultaneously while using the virtual address space on an exclusive basis.
Note: Two or more entries with the same virtual page number (VPN) but different ASID must
not be set in the TLB simultaneously in single virtual memory mode.