It's done this way to avoid using the same addresses as have been allocated
to PCI devices needs, as well as some of the unallocated-but-always-present
ISA devices, like the VGA aperture at 0xa0000 - 0xbffff.
For example, say you have the VGA framebuffer starting at 128Mb (0x08000000)
with 4Mb size, and a SCSI device is told to transfer data to memory at
0x08002000. Both the DMA window *and* the framebuffer will try to respond to
the addresses, most likely resulting in a hung bus (worst case) or data
corruption (best case).
--Jay++
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American Non Sequitur Society: we don't make sense, but we do like pizza...
Jay A Estabrook Alpha Motherboards - LINUX Project
Compaq Computer Corporation (508) 841-3241 or (DTN) 237-3241
334 South Street, Shrewsbury, MA 01545 Jay.Estabrook@digital.com
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