
Chapter 3
Functional Operation
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Rev. 3.00
April 2003
AMD-8111 HyperTransport I/O Hub Data Sheet
AMD Preliminary Information
3.10.14.1
MAC Control Pause Frames
The format of a MAC Control Pause frame is shown in Table 33.
Table 33.
MAC Control Pause Frame Format
When a network station that supports IEEE 802.3x Flow Control receives a pause frame, it must
suspend transmissions after the end of any frame that was being transmitted when the pause frame
arrived. The length of time for which the station must suspend transmissions is given in the
request_operand field of the pause frame. This pause time is given in units of pause_quanta, where,
one pause_quantum is 512 bit times. The request_operand field is interpreted as Big-Endian data—
octet 17 is the most significant byte and octet 18 is the least significant byte.
3.10.14.2
Back Pressure
The network controller supports collision-based back pressure for congestion control when the device
is operating in half-duplex mode.
When the MAC begins receiving a frame that passes the address matching criteria and if back
pressure is enabled, the MAC intentionally causes a collision by transmitting a “phantom” frame.
This phantom frame consists of a normal 7-byte preamble and 1-byte SFD followed by 63 bytes of
alternating 1 and 0 bits. This frame should be interpreted as a runt frame with FCS error by any
standard receiver.
Back pressure does not affect the transmission of a frame. The MAC only forces a collision when it
begins to receive a new frame.
Back pressure is enabled when the BKPRS_EN bit in CMD3 is set, the device is operating in half-
duplex mode, and the receiver is congested. Receiver congestion is defined in the next section. The
BKPRS_EN bit allows the software to separate the decision about whether or not to apply back
pressure from the decision about when to apply back pressure. If the BKPRS_EN bit is set, back
pressure is applied when the device is operating in half-duplex mode and the receiver is congested. If
the BKPRS_EN bit is 0, back pressure is never applied.
The generation of a Back-Pressure collision causes the XmtBackPressure MIB Counter to increment.
Octet Numbers
1-6
7-12
13-14
15-16
17-18
Field Name
Value
Destination Address
Source Address
Length/Type
MAC Control Opcode
Request_operand
01-80-C2-00-00-01
Sender
’
s physical address
88-08
00-01
Pause time measured in pause_quanta, which are
equal to 512 bit times
Zeros
FCS
19-60
61-64
Pad
FCS