AM79C976 Advanced Micro Devices, AM79C976 Datasheet - Page 235

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

8/01/00
Offset
Offset
0Ch
4
4
4
4
8
0
4
8
8
8
19-18
17-16
31-16
15-0
31-0
31:0
31-0
15-0
Bit
Bit
20
31
30
29
USER SPACE
RFRTAG[31:0]
RBADR[31:0]
TCI[15:0]
TT[1:0]
Name
BCNT
MCNT
FRAM
Name
OWN
BAM
ERR
Broadcast Address Match is set by the Am79C976 controller when it accepts the
received frame, because the frame’s destination address is of the type ’Broadcast.’
BAM is valid only when ENP is set. BAM is set by the Am79C976 controller and
cleared by the host.
This bit does not exist when the Am79C976 controller is programmed to use 16-bit
software structures for the descriptor ring entries (BCR20, bits 7-0, SWSTYLE is
cleared to 0).
VLAN Tag Type. Indicates what type of VLAN tag, if any, is included in the received
frame.
00 = Reserved
01 = Frame is Untagged
10 = Frame is Priority-tagged
11 = Frame is VLAN-tagged
Reserved.
Buffer Byte Count is the length of the buffer pointed to by this descriptor, expressed
as the two’s complement of the length of the buffer. This field is written by the host
and unchanged by the Am79C976 controller.
Receive Buffer Address. This field contains the address of the receive buffer that is
associated with this descriptor.
User Space. Reserved for user defined data.
Receive Frame Tag. Indicates the Receive Frame Tag applied from the EADI
interface. This field is user defined and has a default value of all zeros. When
RXFRTG (CSR7, bit 14) is set to 0, RFRTAG will be read as all zeros. See the section
on Receive Frame Tagging for details.
VLAN Tag Control Information copied from the received frame.
Message Byte Count is the number of bytes of the received message written to the
receive buffer. This is the actual frame length (including FCS) unless stripping is
enabled and the length field is < 46 bytes. In this case, MCNT is 14 + length_field.
MCNT can take values in the range 15 to 59 and values greater than or equal to 64.
MCNT is expressed as an unsigned binary integer. MCNT is valid only when ERR is
clear and ENP is set. MCNT is written by the Am79C976 controller and cleared by
the host.
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 it has emptied
the buffer pointed to by the descriptor entry. The Am79C976 controller clears the
OWN bit after filling the buffer that the descriptor points to. Both the Am79C976
controller and the host must not alter a descriptor entry after it has relinquished
ownership.
Error Summary. ERR is the OR of FRAM, OFLO, and CRC. ERR is set by the
Am79C976 controller and cleared by the host.
Framing error indicates that the incoming frame contains a non-integer multiple of
eight bits and there was an FCS error. If there was no FCS error on the incoming
frame, then FRAM will not be set even if there was a non-integer multiple of eight bits
in the frame. FRAM is not valid in internal loopback mode. FRAM is valid only when
ENP is set and OFLO is not. FRAM is set by the Am79C976 controller and cleared
by the host.
P R E L I M I N A R Y
Am79C976
Description
Description
235