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Transmit Operation
Types of Packets Transmitted
The LANIC can transmit any size packet up to the maximum packet size. Short
packets are automatically padded to meet minimum packet length requirements.
Transmit Buffer Management
The host is responsible for setting aside transmit buffers in system memory. The
starting address and length of each buffer is sent to the LANIC via the XMIT
request. Each buffer is identified by a unique ID number. When the LANIC has
successfully transmitted a transmit buffer, or when it encounters an irrecovera ble
error, it returns the ID number and status to the host via the Response Queue
(RQ). The host must ensure that transmit buffers are frozen in system memory
from the time the XMIT request is entered into the CQ until the LANIC returns
the buffer ID in the RQ.
Queuing of Transmit Buffers
The transmit operation requires a number of steps and there in no way to predict
when a transmit buffer is actually sent. Transmit buffers queue up in the CQ
and in an internal transmit queue. The buffer at the head of the transmit queue
is processed when no receive packets are being processed. Even then, there may
be a delay due to the CSMA/CD protocol.
Transmit Operation Example
A transmit frame is assembled by the host in system memory. The host then adds
to the CQ an XMIT request containing the address and length of the transmit
frame and the ID number. The LANIC will process this command after it
completes operations in progress and gets the command from the CQ. The
LANIC then copies the contents of the frame buffer from system memory to its
local memory. The next phase is undertaken by the 82586 LCC chip. The 82586
handles the CSMA/CD protocol, serializes the data from the local memory
buffer, sends it out on the network, and generates the frame check sequence.
After the 82586 LCC is finished with the frame, LANIC firmware records the
frame completion status and ID number in the RQ.
Principles of Operation
3-5
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