SH7750_08 Datasheet, PDF (169/1162 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer

Section 3 Memory Management Unit (MMU)
3.3.5 Address Translation
When the MMU is used, the virtual address space is divided into units called pages, and
translation to physical addresses is carried out in these page units. The address translation table in
external memory contains the physical addresses corresponding to virtual addresses and additional
information such as memory protection codes. Fast address translation is achieved by caching the
contents of the address translation table located in external memory into the TLB. In the SH-4,
basically, the ITLB is used for instruction accesses and the UTLB for data accesses. In the event
of an access to an area other than the P4 area, the accessed virtual address is translated to a
physical address. If the virtual address belongs to the P1 or P2 area, the physical address is
uniquely determined without accessing the TLB. If the virtual address belongs to the P0, U0, or P3
area, the TLB is searched using the virtual address, and if the virtual address is recorded in the
TLB, a TLB hit is made and the corresponding physical address is read from the TLB. If the
accessed virtual address is not recorded in the TLB, a TLB miss exception is generated and
processing switches to the TLB miss exception routine. In the TLB miss exception routine, the
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.
Rev.7.00 Oct. 10, 2008 Page 85 of 1074
REJ09B0366-0700