
Chapter 3
Functional Operation
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Rev. 3.00
April 2003
AMD-8111 HyperTransport I/O Hub Data Sheet
AMD Preliminary Information
Transmit Start Point (XMTSP) in CTRL1 sets the point when the transmitter actually attempts to
transmit a frame onto the media. A minimum of XMTSP bytes must be written to the transmit FIFO
for the current frame before transmission of the current frame will begin. (When automatically
padded packets are being sent, it is conceivable that the XMTSP is not reached when all of the data
has been transferred to the FIFO. In this case, the transmission begins when all of the frame data has
been placed into the transmit FIFO.) The default value of XMTSP is 01b, meaning there has to be 64
bytes in the transmit FIFO to start a transmission.
In order to ensure that collisions occurring within 512 bit times from the start of transmission
(including preamble) are automatically retried with no host intervention, the transmit FIFO ensures
that data contained within the FIFO is not overwritten until at least 64 bytes (512 bits) of preamble
plus address, length, and data fields have been transmitted onto the network without encountering a
collision. If the REX_UFLO bit is set, the transmit data is not overwritten until the frame has been
either transmitted or discarded.
In a (nonstandard) system that allows frames that are larger than the transmit FIFO size, the XMTSP
bit should not be set to 01b (full frame), and the REX_UFLO bit should not be set.
3.10.5.1.1 Automatic Pad Generation
Transmit frames can be automatically padded to extend them to 64 data bytes (excluding preamble).
This allows the minimum frame size of 64 bytes (512 bits) for IEEE 802.3/Ethernet to be ensured
with no software intervention from the host/controlling process. Setting the APAD_XMT bit in
CMD2 enables the automatic padding feature. The pad is placed between the LLC data field and FCS
field in the IEEE 802.3 frame. FCS is always added if the frame is padded, regardless of the state of
DXMTFCS (CMD2, bit 8) or ADD_FCS (in the transmit descriptor). The transmit frame is padded
by bytes with the value of 00H. The default value of APAD_XMT is 0 after H_RESET, which
disables automatic pad generation.
If automatic pad generation is disabled, the software is responsible for insuring that the minimum
frame size requirement is met. The hardware can reliably transmit frames ranging in size from 16 to
65536 octets.
It is the responsibility of upper layer software to correctly define the actual length/type field contained
in the message to correspond to the total number of LLC Data bytes encapsulated in the frame
(length/type field as defined in the IEEE 802.3 standard). The length value contained in the message
is not used by the network controller to compute the actual number of pad bytes to be inserted. The
controller appends pad bytes dependent on the actual number of bits transmitted onto the network.
Once the last data byte of the frame has completed, prior to appending the FCS, the controller checks
to ensure that 544 bits have been transmitted. If not, pad bytes are added to extend the frame size to
this value, and the FCS is then added. See Figure 18.