SMC Networks SMC91C95 - Manual (Page 31)

31
RECEIVE FRAME STATUS WORD
This word is written at the beginning of each receive frame in memory. It is not available as a register.
HIGH
BYTE
ALGN
ERR
BROD
CAST
BAD
CRC
ODD
FRM
TOOLN
G
TOO
SHORT
LOW
BYTE
HASH VALUE MULT
CAST
5 4 3 2 1 0
ALGNERR - Frame had alignment error.
BRODCAST - Receive frame was broadcast.
BADCRC - Frame had CRC error.
ODDFRM - This bit when set indicates that the
received frame had an odd number of bytes.
TOOLNG - The received frame is longer than
802.3 maximum size (1518 bytes on the cable).
TOOSHORT - The received frame is shorter than
802.3 minimum size (64 bytes on the cable).
HASH VALUE - Provides the hash value used to
index the Multicast Registers. Can be used by
receive routines to speed up the group address
search. The hash value consists of the six most
significant bits of the CRC calculated on the
Destination Address, and maps into the 64 bit
multicast table. Bits 5,4,3 of the hash value select
a byte of the multicast table, while bits 2,1,0
determine the bit within the byte selected.
Examples of the address mapping:
ADDRESS HASH VALUE 5-0 MULTICAST TABLE BIT
ED 00 00 00 00 00
0D 00 00 00 00 00
01 00 00 00 00 00
2F 00 00 00 00 00
000 000
010 000
100 111
111 111 MT-0 bit 0
MT-2 bit 0
MT-4 bit 7
MT-7 bit 7
MULTCAST - Receive frame was multicast. If
hash value corresponds to a multicast table bit that
is set, and the address was a multicast, the packet will pass address filtering regardless of
other filtering criteria.