Motorola MC68824 User Manual page 151

The first byte to be transmitted is byte 0 and the second byte to be transmitted is byte 1. The
TBe data buffer looks as follows:
MOTOROLA MODE INTEL MODE
15
15
RC
RC
RDI RDI
BYTE 0 BYTE 1 BYTE 1 BYTE 0
RD2 RD2
BYTE 0 BYTE 1 BYTE 1 BYTE 0
...
...
In order to perform source routing based reception, the TBe requires three parameters: SID, TID,
and SR-MASK. The initial values of these parameters are taken from the initialization table. They
may be chan.ged during operation by the SET VALUE command. They have the following format:
15
INIUABLE
+ IE
SR_SID
PRIVATE_AREA + IE BYTE 1 BYTE 0
INIUABLE
+ 20 SRJID
PRIVATCAREA
+ 20 BYTE 1 BYTE 0
INIUABLE
+22 SR_MASK
PRIVATE_AREA + 22
BYTE 1 BYTE 0
As proposed by IEEE 802.5, each RD has the following format:'
15 4 3 0
I
SEGMENT NUMBER
I
BRIDGE NUMBER
I
The segment number identifies a specific segment. The bridge number identifies a specific bridge
between that segment and another segment. Thus, two segments connected by three different
bridges would have three different bridge numbers creating three distinct routing designators.
Bridge numbers are local to segment pairs, meaning bridge numbers can be the same as long
as they are not connecting the same two segments.
Source 10 (SID) is the identification of the segment where the frame comes from. The target 10
(TID) is the identification of the segment where the frame is going to. The source routing mask
(SR MASK) is a two-octet parameter defining which bits in the RDs are the segment number
(denoted by a "1") and which bits are the bridge number (denoted by a "0"). The definition of
the RD currently calls for 12 bits for the segment number and four bits for the bridge number, or
a mask of FOFF. For broadcast frames with the RI bit set, the TBe will scan the RII field of ordered
RD pairs and accept frames based on the value of SR SID and SR TID. On non-broadcast frames,
accept/reject criteria are based on the values of SR SID and SR TID.
rill
C.3.3 Source Routing Operation
When a station wants to send a frame to another station on a different LAN, it first sends a
broadcast frame to all segments. (The host must prepare the frame, the TBe transmits it). In this
frame, the routing information indicator (RII bit) is set to one, the broadcast field is set to 1XX,
and the direction bit is set to zero. The source address is the individual address of the transmitter,
MC68824 USER'S MANUAL