SH7727 Datasheet, PDF (169/1109 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperH RISC engine Family/SH7700 Series

Section 3 Memory Management Unit (MMU)
Section 3 Memory Management Unit (MMU)
3.1 Overview
3.1.1 Features
The SH7727 has an on-chip memory management unit (MMU) that implements address
translation. The SH7727 features a resident translation look-aside buffer (TLB) that caches
information for user-created address translation tables located in external memory. It enables high-
speed translation of logical addresses into physical addresses. Address translation uses the paging
system and supports two page sizes (1 kbyte and 4 kbytes). The access right to logical address
space can be set for privileged and user modes to provide memory protection.
3.1.2 Role of MMU
The MMU is a feature designed to make efficient use of physical memory. As shown in figure 3.1,
if a process is smaller in size than the physical memory, the entire process can be mapped onto
physical memory. However, if the process increases in size to the extent that it no longer fits into
physical memory, it becomes necessary to partition the process and to map those parts requiring
execution onto memory as occasion demands (1). Having the process itself consider this mapping
onto physical memory would impose a large burden on the process. To lighten this burden, the
idea of virtual memory was born as a means of performing en bloc mapping onto physical
memory (2). In a virtual memory system, substantially more virtual memory than physical
memory is provided, and the process is mapped onto this virtual memory. Thus a process only has
to consider operation in virtual memory. Mapping from virtual memory to physical memory is
handled by the MMU. The MMU is normally controlled by the operating system, switching
physical memory to allow the virtual memory required by a process to be mapped onto physical
memory in a smooth fashion. Switching of physical memory is carried out via secondary storage,
etc.
The virtual memory system that came into being in this way is particularly effective in a time-
sharing system (TSS) in which a number of processes are running simultaneously (3). If processes
running in a TSS had to take mapping onto virtual memory into consideration while running, it
would not be possible to increase efficiency. Virtual memory is thus used to reduce this load on
the individual processes and so improve efficiency (4). In the virtual memory system, virtual
memory is allocated to each process. The task of the MMU is to perform efficient mapping of
these virtual memory areas onto physical memory. It also has a memory protection feature that
prevents one process from inadvertently accessing another process’s physical memory.
Rev. 5.00 Dec 12, 2005 page 97 of 1034
REJ09B0254-0500