At 12:43 AM -0800 12/13/2005, Clark Martin wrote:
DMA (Direct Memory Access). A separate controller chip that handles
data transfers to/from memory. It is also used by other functions
such as SCSI. Now here I'm not sure what happens but even though
DMA should be taking a lot of the load off the CPU LocalTalk
transfers still slow things down. I suspect that the software is
still taking control of the CPU during the packet transfer but using
DMA to for the transfer.
Correct. The DMA packet is only 8 to 64 bytes. So there are
multiple interrupts per packet ... and since the classic OS was only
preemptive at fork level, this ment a LOT of context switching for
sub-packet building. bog bog bog lag lag lag.
The capabilities of this chip determined the AppleTalk Protocol even
AT over Ethernet (EtherTalk). The limitations of AT caused by this
are one of the reasons that Apple is abandoning it, in particular
the limitation on packet size of 625 bytes.
ASICs are cheap. So it was political, not hardware that caused the change.
Apple gutted their networking group a decade ago. Remember - Open
Transport was actually licensed Mentat/TCP wedged into the OS. The
rest of the world was going to IP. And the lack of interoperability
at the higher protocol levels was keeping Apple out of much of the
business world. Like DECnet and SNA, AppleTalk needed to die. sigh.
As much as I hate that Tiger has dumped AFP/AT, it had to happen.
And I'll bet printing support over AT isn't far behind... Sometimes
folx like us need to be dragged kicking and screaming into the
future. :\
- Dan.
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