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AM79C976 Datasheet(PDF) 79 Page - Advanced Micro Devices |
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AM79C976 Datasheet(HTML) 79 Page - Advanced Micro Devices |
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79 / 309 page  8/01/00 Am79C976 79 P R E L I M INARY )LJXUH 9/$17DJJHG)UDPH)RUPDW The Am79C976 device includes several features that can simplify the processing of IEEE 802.3ac VLAN- tagged frames. 9/$1 )UDPH 6L]H 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 whe n to i n c r eme n t th e Xm tO v e r s i z eP k t s , XmtPkts1024to1518Octets, XmtExcessiveDefer, RcvPkts1024to1518Octets, and RcvOversizePkts MIB counters. $GPLW2QO\9/$1)UDPHV2SWLRQ 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. 9/$1 7DJVLQ'HVFULSWRUV 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. PREAMBLE SFD DESTINATION ADDRESS SOURCE ADDRESS LENGTH/TYPE = 8100h TAG CONTROL INFORMATION MAC CLIENT LENGTH/TYPE MAC CLIENT DATA FRAME CHECK SEQUENCE 7 OCTETS 1 OCTET 6 OCTETS 6 OCTETS 2 OCTETS 2 OCTETS 2 OCTETS 42-1500 OCTETS 4 OCTETS VLAN ID CANONICAL FMT INDICATOR USER PRIORITY 0 11 13 15 |
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