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There are basically two types of caching used with the 680x0 series:
write-through cache (68020, 68030)
copyback cache (68040)
Although you can use just a write-through cache with the 68040, most
programs are significantly sped up by enabling the copyback mode.
In simple terms, the write-though cache keeps frequently read data cached
in the processor, while writes go straight "through" to RAM. The copyback
mode caches both reads AND writes, only flushing written data in the cache
to RAM when needed. Large reads and writes to/from RAM will be slower with
a copyback cache, but most real-world applications will be significantly
faster with copyback mode enabled.