
106
Functional Operation
Chapter 3
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
or to modify the TCI field of a VLAN tag. This feature allows VLAN software to control the VLAN
tag of a frame without modifying data in transmit buffers. The receiver can determine whether a
frame is untagged, priority-tagged, or VLAN-tagged, and it can copy the TCI field of the VLAN tag
into the Receive Descriptor and remove the tag from the frame.
The Tag Control Command (TCC) is a 2-bit field in the Transmit Descriptor that determines whether
the transmitter will insert, delete, or modify a VLAN tag or transmit the data from the transmit buffers
unaltered. The encoding of the TCC field is shown in Table 29 on page 106.
If the transmitter adds, deletes, or modifies a VLAN tag, it appends a valid FCS field to the frame,
regardless of the state of the Disable Transmit FCS (DXMTFCS) bit (CMD2, bit 8).
The receiver examines each incoming frame and writes the frame’s VLAN classification into the Tag
Type (TT) field of the Receive Descriptor. If the frame contains a VLAN tag, the receiver copies the
TCI field of tag into the TCI field of the Receive Descriptor. The encoding of the TT field is shown in
Table 30. If the VL_TAG_DEL bit in CMD3 is set, the controller deletes the 4-byte tag from the
frame as it copies the frame to host system memory.
3.10.9
Loopback Operation
Loopback is a mode of operation intended for system diagnostics. In this mode, the transmitter and
receiver are both operating at the same time so that the controller receives its own transmissions. The
controller provides two basic types of loopback. In internal loopback mode, the transmitted data is
looped back to the receiver inside the controller without actually transmitting any data to the external
network. The receiver moves the received data to the next receive buffer, where it can be examined by
Table 29.
VLAN Tag Control Command
TCC
(TMD2[17:16])
00
01
10
11
Action
Transmit data in buffer unaltered
Delete Tag Header
Insert Tag Header containing TCI
fi
eld from descriptor.
Replace TCI
fi
eld from buffer with TCI data from descriptor.
Table 30.
VLAN Tag Type
TT
(RMD1[19:18])
00
01
10
11
Description
Reserved
Frame is untagged
Frame is priority-tagged
Frame is VLAN-tagged