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CS 220 / ARM / ARM1176JZ-S Technical Reference Mmanual.pdf
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Programmer’s Model

2.13Software considerations

When using the processor you must consider the following software issues:

Branch Target Address Cache flush

Waiting for DMA to complete.

2.13.1Branch Target Address Cache flush

When the processor switches from the Secure to the Non-secure state the Secure Monitor code is responsible for flushing the BTAC if necessary. See About program flow prediction on page 5-2 for more information.

2.13.2Waiting for DMA to complete

When it is necessary to wait for the generation of an interrupt by the DMA indicating the completion of a transfer between external memory and an Instruction TCM, the prioritization between core requests from a tight-loop and the DMA can mean the DMA is locked out from writing the TCM, so freezing the system. To avoid this, two mechanisms are recommended:

1.The use of the WFI operation in the wait-loop to freeze core execution while permitting the DMA to continue. Standby mode is not entered in this case as the DMA keeps on running and prevents this entry. See Standby mode on page 10-3 for more details.

2.Including at least five instructions, including NOP instructions, in the wait loop.

For details of the WFI operation see c7, Cache operations on page 3-69.

Note

In the ARM1176 instruction set, WFI is a valid instruction but is treated as a NOP.

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