
Chapter 3
Functional Operation
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Rev. 3.00
April 2003
AMD-8111 HyperTransport I/O Hub Data Sheet
AMD Preliminary Information
3.10.4.1.1 Framing
The MAC engine autonomously handles the construction of the transmit frame. Once the transmit
FIFO has been filled to the predetermined threshold and access to the channel is currently permitted,
the MAC engine commences the 7-byte preamble sequence (a repeating 10101010 sequence, where
first bit transmitted is a 1). The MAC engine subsequently appends the Start Frame Delimiter (SFD)
sequence (10101011) followed by the serialized data from the transmit FIFO. Once the data has been
transmitted, the MAC engine appends the FCS (most significant bit first) which was computed on the
entire frame, starting with the destination address. The user is responsible for the correct ordering and
content of the destination address, source address, length/type, and frame data fields of the frame.
The receiver section of the MAC engine detects the incoming preamble sequence when the RX_DV
signal is activated by the external PHY. The MAC discards the preamble and begins searching for the
SFD. (In the case of 100BASE-T4, for which there is no preamble to discard, the SFD is the first two
nibbles received.) Once the SFD is detected, all subsequent nibbles are treated as part of the frame. If
automatic pad stripping is enabled, the MAC engine inspects the length field so that it can strip
unnecessary pad characters and pass the remaining bytes through the receive FIFO to the host. If pad
stripping is performed, the MAC engine also strips the received FCS bytes, although normal FCS
computation and checking do occur. Note that apart from pad stripping, the frame is passed
unmodified to the host. If the length field has a value of 46 or greater, all frame bytes including FCS
are passed unmodified to the receive buffer, regardless of the actual frame length.
If the frame terminates or suffers a collision before 64 bytes of information (after SFD) have been
received, the MAC engine automatically deletes the frame from the receive FIFO, without host
intervention. The network controller has the ability to accept runt packets for diagnostic purposes and
proprietary networks.
3.10.4.2
Destination Address Handling
The first 6 bytes of information after SFD is interpreted as the destination address field. The MAC
engine provides facilities for physical (unicast), logical (multicast), and broadcast address reception.
3.10.4.2.1 Error Detection
The MAC engine provides several facilities which count and recover from errors on the medium. In
addition, it protects the network from gross errors due to inability of the host to keep pace with the
MAC engine activity.
On completion of transmission, the MAC engine updates various counters that are described in the
Statistics Counters section. The host CPU can read these counters at any time for network
management purposes.
The MAC engine also attempts to prevent the creation of any network error due to the inability of the
host to service the MAC engine. During transmission, if the host fails to keep the transmit FIFO filled
sufficiently, causing an underflow, the MAC engine ensures the message is sent with an invalid FCS,
which causes the receiver to reject the message.