SH7708 datasheet(69/633 Pages) RENESAS

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Section 3 Memory Management Unit (MMU)

3.1

Overview

3.1.1

Features

The SH7708 Series has an on-chip memory management unit (MMU) that implements address

translation. The SH7708 Series features a resident translation lookaside buffer (TLB) that caches

information for user-created address translation tables located in external memory. It enables high-

speed translation of virtual addresses into physical addresses. Address translation uses the paging

system and supports two page sizes (1 kbyte and 4 kbytes). The access right to virtual 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.

When address translation from virtual memory to physical memory is performed using the MMU,

it may happen that the relevant translation information is not recorded in the MMU, with the result

that one process may inadvertently access the virtual memory allocated to another process. In this

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