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     ......
     With the MC68030 and earlier processors the data cache could only
     operate in writethrough mode. In writethrough mode main
     memory is updated whenever the data cache is written into.
     Copyback mode allows for a substantial performance improvement
     over writethrough mode.

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     Correct handling of copyback mode could require that some existing
     code (only special applications that make DMA calls directly or
     applications with self-modifying code) be modified as described
     above and then re-compiled or re-assembled. This is not required
     if only the HP supplied I/O library transfer procedures are used.
     Also there is a provided routine (ASM_COPY_OFF) that can be used
     to change the default caching mode from copyback to writethrough.
     This is an alternative when existing code cannot be updated.

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