AM79C90 Advanced Micro Devices, AM79C90 Datasheet - Page 28

• Manufacturer Part Number: AM79C90
• Description: CMOS Local Area Network Controller for Ethernet (C-LANCE)
• Manufacturer: Advanced Micro Devices

Bit
12
11
10
09
08
07:00
Receive Message Descriptor 2 (RMD2)
15:12
11:00
28
15
AMD
HADR
Name
OFLO
BCNT
BUFF
CRC
ENP
STP
12 11
Description
OVERFLOW error indicates that the
receiver has lost all or part of the in-
coming packet due to an inability to
store the packet in a memory buffer
before the internal Receive FIFO
overflowed. OFLO is valid only when
ENP is not set.
CRC indicates that the receiver has
detected a CRC error on the incom-
ing packet. CRC is valid only when
ENP is set and OFLO is not.
BUFFER ERROR is set any time the
C-LANCE does not own the next
buffer while data chaining a received
packet. This can occur in either of
two ways: 1) the OWN bit of the next
buffer is zero, or 2) the Receive FIFO
overflow
C-LANCE
lookahead poll of the next receive
descriptor.
If a Buffer Error occurs, an Overflow
Error may also occur internally in the
Receive FIFO, but will not be re-
ported in the descriptor status entry
unless both BUFF and OFLO errors
occur at the same time.
START OF PACKET indicates that
this is the first buffer used by the
C-LANCE for this packet. It is used
for data chaining buffers.
END OF PACKET indicates that this
is the last buffer used by the
C-LANCE for this packet. It is used
for data chaining buffers. If both STP
and ENP are set, the packet fits into
one buffer and there is no data
chaining.
The HIGH ORDER 8 address bits of
the buffer pointed to by this descrip-
tor. This field is written by the host
and unchanged by the C-LANCE.
MUST BE ONES. This field is written
by the host and is not changed by the
C-LANCE.
BUFFER BYTE COUNT is the length
of the buffer pointed to by this de-
scriptor, expressed as a two’s com-
plement number. This field is written
by the host and is not changed by the
C-LANCE. Minimum buffer size is 64
bytes for the first buffer of packet.
occurred
has
Must be Ones
performed
before
17881B-27
P R E L I M I N A R Y
BCNT
0
the
Am79C90
a
Receive Message Descriptor 3 (RMD3)
15:12
11:00
Transmit Message Descriptor Entry
Transmit Message Descriptor 0 (TMD0)
Bit
15:00
Transmit Message Descriptor 1 (TMD1)
15
15
15
MCNT
Name
LADR
RES
12 11
8 7
RESERVED. Read as zeroes. Write
as zeroes.
MESSAGE BYTE COUNT is the
length in bytes of the received mes-
sage. MCNT is valid only when ERR
is clear and ENP is set. MCNT is writ-
ten by the chip and cleared by the
host.
Description
The LOW ORDER 16 address bits of
the buffer pointed to by this descrip-
tor. LADR is written by the host and is
not changed by the C-LANCE.
LADR
17881B-30
17881B-28
17881B-29
0
ONE
MORE
MCNT
RES
HADR
ENP
STP
DEF
ADD_FCS
ERR
OWN
0
0