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>>> Hale Landis 04/14/02 23:45 PM >>>
...
> I'm still not sure what Pat means by "residue"
I imagine I meant different things in different contexts, sorry about that.
The "residue" that matters most is Microsoft's: the difference between the count of
bytes the host offered to copy and the count of bytes the host did copy.
To make that residue accurate, the host & the device have to negotiate a length of
bytes to copy across the bus.
[ If you deny this T10 Sam reality, then the rest of this email will not make sense,
so please don't quote it in reply. The contemporaneous message thread titled "on
residue - not cracking the Cdb" discusses how real this reality is or is not. ]
Now suppose we have a host that politely never does copy more bytes than the device
agrees to copy. Still the host & device may clock across the bus more bytes than the
host copies.
[ If you refuse to distinguish bytes clocked from bytes copied, then the rest of this
email will not make sense, so please don't quote it in reply. ]
The "bus residue" is then the count of these "extra" "pad" bytes.
I think "residue" is not a new term here? I hear the IgnoreWideResidue message of
parallel Scsi exists to say to be sure to discard the bus "residue".
Nonzero bus residue occurs in AtapiPio because that bus is 2 bytes wide. Therefore
any copy of an odd count of bytes necessarily involves clocking across an extra pad
byte - a residue of one, rather than zero.
For example:
BSY DRQ C/D I/O ByteCount = 0 1 d i x00:06
3 "word"s copied In
[ busResidue = 0 = 3 * 2 - x0006 ]
BSY DRQ C/D I/O ByteCount = 0 1 d i x00:05
3 "word"s copied In
[ busResidue = 1 = 3 * 2 - x0005 ]
Nonzero bus residue occurs in AtapiDa for the same reason ... but in Dma we can't see
whether the residue is 0, 1, or N * 2.
Changing to SwDma from Pio, we lose the ByteCount, so we can't know if the bus residue
is 0 or 1. We also lose the I/O bit. If the device asked to copy data In, then the
residue after copying N "word"s is N * 2.
I claim further that that MwDma in theory and UDma in practice are less determinate
still. I vote we defer discussing MwDma and UDma until we can agree how to express
this much of the reality of Pio & SwDma.
Pat LaVarre
