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NAME

CachePreDMA
Take actions prior to hardware DMA (V37)

SYNOPSIS

paddress = CachePreDMA(vaddress, length, flags)
d0 a0 a1 d0

APTR CachePreDMA(APTR,LONG *,ULONG);

FUNCTION

Take all appropriate steps before Direct Memory Access (DMA). This function is primarily intended for writers of DMA device drivers. The action will depend on the CPU type installed, caching modes, and the state of any Memory Management Unit (MMU) activity.

This function supports advanced cache architectures that have "copyback" modes. With copyback, write data may be cached, but not actually flushed out to memory. If the CPU has unflushed data at the time of DMA, data may be lost.

As implemented

68000
Do nothing

68010
Do nothing

68020
Do nothing

68030
Do nothing

68040
Write any matching dirty cache lines back to memory. As a side effect of the 68040's design, matching data cache lines are also invalidated. Future CPUs may be different.

xxxxx
External cache boards, Virtual Memory Systems, or future hardware may patch this vector to best emulate the intended behavior. With a Bus-Snooping CPU, this function my end up doing nothing.

INPUTS

address
Base address to start the action.

length
Pointer to a longword with a length.

flags
Values: DMA_Continue - Indicates this call is to complete a prior request that was broken up.

DMA_ReadFromRAM
Indicates that this DMA is a

read from RAM to the DMA device (ie - a write to the hard drive) This flag is not required but if used must match in both the PreDMA and PostDMA calls. This flag *should* be used to help the system provide the best performance. This flag is safe in all versions of CachePreDMA()

RESULT

paddress
Physical address that corresponds to the input virtual address.

length
This length value will be updated to reflect the contiguous length of physical memory present at paddress. This may be smaller than the requested length. To get the mapping for the next chunk of memory, call the function again with a new address, length, and the DMA_Continue flag.

NOTES

Due to processor granularity, areas outside of the address range may be affected by the cache flushing actions. Care has been taken to ensure that no harm is done outside the range, and that activities on overlapping cache lines won't harm data.

SEE ALSO

"exec/execbase.i", CachePostDMA(), CacheClearU(), CacheClearE()