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AM79C976 Datasheet(PDF) 242 Page - Advanced Micro Devices |
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AM79C976 Datasheet(HTML) 242 Page - Advanced Micro Devices |
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242 / 309 page  242 Am79C976 8/01/00 P R E L I M INARY 7DEOH 7UDQVPLW'HVFULSWRU 6:67</( Offset Bit Name Description 031 OWN This bit indicates whether the descriptor entry is owned by the host (OWN = 0) or by the Am79C976 controller (OWN = 1). The host sets the OWN bit after filling the buffer pointed to by the descriptor entry. The Am79C976 controller clears the OWN bit after transmitting the contents of the buffer. Both the Am79C976 controller and the host must not alter a descriptor entry after it has relinquished ownership. 0 30 Reserved location . 0 29 ADD_FCS ADD_FCS dynamically controls the generation of FCS on a frame by frame basis. This bit should be set with the ENP bit. However, for backward compatibility, it is recommended that this bit be set for every descriptor of the intended frame. When ADD_FCS is set, the state of DXMTFCS is ignored and transmitter FCS generation is activated. When ADD_FCS is cleared to 0, FCS generation is controlled by DXMTFCS. When APAD_XMT (CSR4, bit 11) is set to 1, the setting of ADD_FCS has no effect. ADD_FCS is set by the host, and is not changed by the Am79C976 controller. This is a reserved bit in the C-LANCE (Am79C90) controller. 028 LTINT Last Transmit Interrupt. When enabled by the LTINTEN bit (CSR5, bit 14), LTINT is used to suppress interrupts after selected frames have been copied to the transmit FIFO. When LTINT is cleared to 0 and ENP is set to 1, the Am79C976 controller will not set TINT (CSR0, bit 9) after the corresponding frame has been copied to the transmit FIFO. TINT will only be set when the last descriptor of a frame has both LTINT and ENP set to 1. When LTINTEN is cleared to 0, the LTINT bit is ignored. 0 27-26 Reserved. 025 STP Start of Packet indicates that this is the first buffer to be used by the Am79C976 controller for this frame. It is used for data chaining buffers. The STP bit must be set in the first buffer of the frame, or the Am79C976 controller will skip over the descriptor and poll the next descriptor(s) until the OWN and STP bits are set. STP is set by the host and is not changed by the Am79C976 controller. 024 ENP End of Packet indicates that this is the last buffer to be used by the Am79C976 controller for this frame. It is used for data chaining buffers. If both STP and ENP are set, the frame fits into one buffer and there is no data chaining. ENP is set by the host and is not changed by the Am79C976 controller. 0 23 Reserved. 022 KILL This bit causes the transmission of the corresponding frame to be aborted. If the transmitter has not started sending the frame at the time that the descriptor processing logic encounters the KILL bit, no portion of the frame will be sent. If part of the frame has been sent, the frame will be truncated, and an FCS field containing the inverse of the correct CRC will be appended. 0 21-16 Reserved. 0 15-0 BCNT Buffer Byte Count is the usable length of the buffer pointed to by this descriptor, expressed as the two’s complement of the length of the buffer. This is the number of bytes from this buffer that will be transmitted by the Am79C976 controller. This field is written by the host and is not changed by the Am79C976 controller. There are no minimum buffer size restrictions. 4 31-18 Reserved. 4 17-16 TCC[1:0] VLAN Tag Control Command. This field contains a command that causes the transmitter to add, modify, or delete a VLAN tag or to transmit the frame unaltered. 00 = Transmit the data in the buffer unaltered 01 = Delete the VLAN tag (the 13th through 16th bytes of the frame) 10 = Insert a VLAN tag containing the TCI field from the descriptor 11 = Replace the TCI field of the frame with TCI data from the descriptor |
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