Motorola MC68824 User Manual page 150

The format of the routing control octets is shown below:
D
c
B A
Broadcast
Broadcast
The coding is as follows:
OXX - Non-Broadcast
10X - All Routes Brodcast
11X - Limited Broadcast
Length
Where an X indicates don't care.
D Reserved
Non-broadcast frames always contain the specific route through the network which the frame
will take. All routes broadcast frames are transmitted on every route to the destination address.
Limited broadcast frames are only retransmitted by bridges who are in the special limited
broadcast mode. This restricts the number of routes a broadcast frame will take, thus reducing
the traffic on the network.
Length
The length of the RI field, including the control field and the route designator field is measured
in octets. The length field consists of five bits.
D - Direction Bit
The direction bit indicates to a bridge whether a frame is travelling from the originating
station to the target or vice versa. This bit allows the route designators to appear in the same
order regardless of the direction of transmission.
Reserved Bits
Reserved bits are set to zero when transmitting, and ignored when receiving.
When the recognize source routing (RSR) bit is set by the SET MODE 2 command, the TBC provides
source routing based frame reception in addition to normal address based frame reception. The
TBC does not by itself provide a bridge function but does receive and transmit frames containing
the source routing fields as any other MAC frames. Frames are copied based on recognition of
a segment pair as described in route designators. To implement a bridge, a MAC function (such
as the TBC) for each segment the bridge is attached to is required, as well as a host to provide
the additional bridge functionality.
C.3.2 Route Designators
Route designators (RDs) are pairs of octets which indicate the specific segments and bridges
through which the frame passes. Route designators contain bit strings with no arithmetic signif-
icance; i.e., concepts like addition, subtraction, greater than, or less than, do not apply to route
designators. Thus, the two bytes of an RD can not really be described as high and low, or most
significant and least significant. What is preserved in all 802 LANs is the byte transmission order.
The bit transmission order of each octet depends on the type of MAC. IEEE 802.4 transmits the
III
least significant bit first.
iii
Each route designator can be shown as follows:
RDl
RD2
Byte 0
Byte 1
Byte 0
Byte 1
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