Re: > 1GB on alpha. Patch to 1TB?

[Jay.Estabrook@digital.com]

>>> Thomas Bogendoerfer said:
> 
> what I'm wondering for quite a while, why does the DMA window start at 1GB
> (2GB with Richard's patch) and not starting from address 0 (so we could make
> it up to 4GB). Is this a hardware restriction (couldn't find anything in the 
> LCA documention) ? 

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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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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