AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 79

• Manufacturer Part Number: AM79C976
• Description: PCnet-PRO 10/100 Mbps PCI Ethernet Controller
• Manufacturer: Advanced Micro Devices

The Am79C976 device includes several features that
can simplify the processing of IEEE 802.3ac VLAN-
tagged frames.
While the maximum frame size for IEEE 802.3 frames
without VLAN tags is 1518 bytes, the maximum frame
size for VLAN-tagged frames is 1522 bytes. The VLAN
frame size bit (VSIZE, CMD3, bit 20) determines the
maximum frame size. When VSIZE is set to 1 the max-
imum frame size is 1522 bytes. Otherwise, the maxi-
mum frame size is 1518 bytes.
The maximum frame size is used for determining
w h e n t o i n c r e m e n t t h e X m t O v e r s i z e P k t s ,
XmtPkts1024to1518Octets, XmtExcessiveDefer,
RcvPkts1024to1518Octets, and RcvOversizePkts
MIB counters.
The Admit Only VLAN (VLONLY) bit in the Command1
Register can be programmed to reject any frame that is
not VLAN-tagged. When VLONLY is set, untagged or
priority-tagged frames will be flushed from the receive
FIFO and will not be copied into system memory. Only
frames with a Length/Type field equal to 8100h and a
non-zero VLAN ID field will be received. The VLAN ID
field consists of bits [11:0] of the 15th and 16th bytes of
the frame.
8/01/00
42-1500 OCTETS
7 OCTETS
6 OCTETS
6 OCTETS
2 OCTETS
2 OCTETS
2 OCTETS
4 OCTETS
1 OCTET
TAG CONTROL INFORMATION
MAC CLIENT LENGTH/TYPE
FRAME CHECK SEQUENCE
DESTINATION ADDRESS
LENGTH/TYPE = 8100h
SOURCE ADDRESS
MAC CLIENT DATA
PREAMBLE
SFD
P R E L I M I N A R Y
Am79C976
When the SWSTYLE field in CSR58 contains the value
4 or 5, VLAN tag information can be passed between
the host CPU and the network medium through Trans-
mit or Receive Descriptors. The transmitter can be pro-
grammed to insert or delete a VLAN tag 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 de-
termine 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
The Tag Control Command (TCC) is a 2-bit field in the
Transmit Descriptor that determines whether the trans-
mitter will insert, delete, or modify a VLAN tag or trans-
mit the data from the transmit buffers unaltered. The
encoding of the TCC field is shown in Table 9.
If the transmitter adds, deletes, or modifies a VLAN tag,
it will append a valid FCS field to the frame, regardless
of the state of the Disable Transmit FCS (DXMTFCS)
bit in CSR15.
When SWSTYLE is 4 or 5, the receiver examines each
incoming frame and writes the frame’s VLAN classifica-
tion into the Tag Type (TT) field of the Receive Descrip-
tor. If the frame contains a VLAN tag, the receiver will
copy the TCI field of tag into the TCI field of the Receive
Descriptor. The encoding of the TT field is shown in
Table 10.
15
13
11
CANONICAL FMT INDICATOR
USER PRIORITY
VLAN ID
0
79