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address translation table in external memory is searched, and the corresponding physical address
and page management information are recorded in the TLB. After the return from the exception
handling routine, the instruction which caused the TLB miss exception is re-executed.
3.3.6
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 MMUCR.SV bit. 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 a particular virtual address 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 3.4.3, Address Translation Method).
3.3.7
Address Space Identifier (ASID)
In multiple virtual memory mode, the 8-bit address space identifier (ASID) is used to distinguish
between processes running simultaneously while sharing the virtual address space. Software can
set the 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 processes running
simultaneously while using the virtual memory space on an exclusive basis.
3.4
TLB Functions
3.4.1
Unified TLB (UTLB) Configuration
The unified TLB (UTLB) is so called because of its use for the following two purposes:
1. To translate a virtual address to a physical address in a data access
2. As a table of address translation information to be recorded in the instruction TLB in the event
of an ITLB miss
Information in the address translation table located in external memory is cached into the UTLB.
The address translation table contains virtual page numbers and address space identifiers, and
corresponding physical page numbers and page management information. Figure 3.7 shows the
overall configuration of the UTLB. The UTLB consists of 64 fully-associative type entries. Figure
3.8 shows the relationship between the address format and page size.
Rev. 4.0, 04/00, page 38 of 850
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