At 10:34 PM -0800 12/12/05, NODEraser wrote:
Why does AppleTalk via Printer/Serial slow down the computer so much,
while AppleTalk via Ethernet doesn't?
LocalTalk 101
LocalTalk (AppleTalk via the Serial Port) was designed around a
particular integrated circuit, the Zilog 8530. This chip implemented
some of the protocol in hardware. This was at a time when an
Ethernet NIC was a full size ISA (IBM pc) card filled with parts.
While the chip did a lot the CPU still had to transfer each byte.
The earliest Macs couldn't transfer this data (either sending or
receiving) in the background (interrupts) so when a packet came in
the software took over the processor.
One of the features that chip provides is address detection. The CPU
only needs to read data when the packet is addressed to the machine
in question. Otherwise a busy network would jam up all the machines
on it.
Now with the advent of the Q840AV and Q660AV models the data transfer
was handled by 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. Most subsequent Macs
(Power Macs) use DMA.
LocalTalk transfers at 230,400 bits per second and the maximum
LocalTalk packet size (not counting overhead) is 625 bytes. So a
full size packet takes about 22mS to transfer. Transfer 10 packets
per second and you are using 22% of the processor for LocalTalk. And
that is independent of how fast the processor is.
Ethernet on the other hand has a memory buffer that handles all the
transfers so the CPU only has to direct the transfers.
The 8530 provided a means to put basic networking into all Macs at a
low cost. It also does does regular serial transfers for the printer
and modem. And to top it off it is a dual serial device in one chip.
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.
--
Clark Martin
Redwood City, CA, USA
Macintosh / Internet Consulting
"I'm a designated driver on the Information Super Highway"
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