SH7721 datasheet(227/1524 Pages) RENESAS

227 / 1524 page

Section 4 Memory Management Unit (MMU)

MMUS300S_000020020300

Rev. 3.00 Jan. 18, 2008 Page 165 of 1458

REJ09B0033-0300

Section 4 Memory Management Unit (MMU)

This LSI has an on-chip memory management unit (MMU) that supports a virtual memory system.

The on-chip translation look-aside buffer (TLB) 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 or 4 kbytes). The access rights to virtual address space can be set for each of the privileged

and user modes to provide memory protection.

4.1

Role of MMU

The MMU is a feature designed to make efficient use of physical memory. As shown in figure 4.1,

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 (figure 4.1 (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 (figure 4.1 (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 performed

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 (figure 4.1 (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 (figure 4.1 (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 occur 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

case, the MMU will generate an exception, change the physical memory mapping, and record the

new address translation information.

SH7721 Datasheet(PDF) 227 Page - Renesas Technology Corp