SH7763 Datasheet, PDF (203/2026 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTM RISC Engine Family SH-4A Series

Section 6 Memory Management Unit (MMU)
Section 6 Memory Management Unit (MMU)
This LSI supports an 8-bit address space identifier, a 32-bit virtual address space, and a 29-bit
physical address space. Address translation from virtual addresses to physical addresses is enabled
by the memory management unit (MMU) in this LSI. The MMU performs high-speed address
translation by caching user-created address translation table information in an address translation
buffer (translation lookaside buffer: TLB).
This LSI has four instruction TLB (ITLB) entries and 64 unified TLB (UTLB) entries. UTLB
copies are stored in the ITLB by hardware. A paging system is used for address translation, with
four page sizes (1, 4, and 64 Kbytes, and 1 Mbyte) supported. It is possible to set the virtual
address space access right and implement memory protection independently for privileged mode
and user mode.
6.1 Overview of MMU
The MMU was conceived as a means of making efficient use of physical memory. As shown in
(0) in figure 6.1, when a process is smaller in size than the physical memory, the entire process
can be mapped onto physical memory, but if the process increases in size to the point where it
does not fit into physical memory, it becomes necessary to divide the process into smaller parts,
and map the parts requiring execution onto physical memory as occasion arises ((1) in figure 6.1).
Having this mapping onto physical memory executed consciously by the process itself imposes a
heavy burden on the process. The virtual memory system was devised as a means of handling all
physical memory mapping to reduce this burden ((2) in figure 6.1). With a virtual memory system,
the size of the available virtual memory is much larger than the actual physical memory, and
processes are mapped onto this virtual memory. Thus processes only have to consider their
operation in virtual memory, and mapping from virtual memory to physical memory is handled by
the MMU. The MMU is normally managed by the OS, and physical memory switching is carried
out so as to enable the virtual memory required by a process to be mapped smoothly onto physical
memory. Physical memory switching is performed via secondary storage, etc.
The virtual memory system that came into being in this way works to best effect in a time sharing
system (TSS) that allows a number of processes to run simultaneously ((3) in figure 6.1). Running
a number of processes in a TSS did not increase efficiency since each process had to take account
of physical memory mapping. Efficiency is improved and the load on each process reduced by the
use of a virtual memory system ((4) in figure 6.1). In this virtual memory system, virtual memory
is allocated to each process. The task of the MMU is to map a number of virtual memory areas
onto physical memory in an efficient manner. It is also provided with memory protection functions
to prevent a process from inadvertently accessing another process's physical memory.
Rev. 1.00 Oct. 01, 2007 Page 137 of 1956
REJ09B0256-0100