SH7721 datasheet(277/1524 Pages) RENESAS

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Section 6 X/Y Memory

Rev. 3.00 Jan. 18, 2008 Page 215 of 1458

REJ09B0033-0300

access via the L bus is performed in one cycle. Several cycles are necessary for accessing via the I

bus. According to the CPU operating mode, access from the CPU is as follows:

(1)

Privileged DSP mode (SR. MD = 1 and SR.DSP = 1)

The X/Y memory can be accessed by the DSP directly from space P2. The MMU can be used to

map the virtual addresses in spaces P0 and P3 to this memory.

(2)

User DSP mode (SR.MD = 0 and SR.DSP = 1)

The X/Y memory can be accessed by the DSP directly from space Uxy. The MMU can be used to

map the virtual addresses in space U0 to this memory.

6.2.3

Access from Bus Master Module

The X/Y memory is always accessed by bus master modules such as the DMAC and USB host via

the I bus, which is a physical address bus. Addresses in which the upper three bits are 0 in

addresses shown in table 6.1 must be used.

6.3

Usage Notes

6.3.1

Page Conflict

In the event of simultaneous accesses to the same page from different buses, the conflict on the

pages occurs. Although each access is completed correctly, this kind of conflict tends to lower

X/Y memory accessibility. Therefore it is advisable to provide software measures to prevent such

conflict as far as possible. For example, conflict will not arise if different memory or different

pages are accessed by each bus.

6.3.2

Bus Conflict

The I bus is shared by several bus master modules. When the X/Y memory is accessed via the I

bus, a conflict between the other I-bus master modules may occur on the I bus. This kind of

conflict tends to lower X/Y memory accessibility. Therefore it is advisable to provide software

measures to prevent such conflict as far as possible. For example, by accessing the X/Y memory

by the CPU not via the I bus but directly from space P2 or Uxy, conflict on the I bus can be

prevented.

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