Motorola MC68824 User Manual page 147

I
passes them up to the upper bridge to be transmitted onto the upper network. Both upper and
lower bridges use the TBC individual address mask to consider only that part of the address that
is relevant to it.
BACKBONE BUS
UPPER BRIOGE UPPER BRIDGE
A B
LOWER BRIDGE LOWER BRIDGE
UPPER BRIDGE UPPER BRIDGE UPPER BRIDGE UPPER BRIDGE
LOWER BRIDGE LOWER BRIDGE LOWER BRIDGE
LOWER BRIDGE
C.2.
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Hierarchical Addressed Bridging Implementation
Each upper bridge in the above figure is responsible for passing information down from the
backbone. In the addressing scheme, the address field can be logically divided into regions. For
example, the two most significant bits of the address differentiate which region the frame goes
to; the next two significant bits of the address differentiate which segment the frame goes to;
and the four least significant bits differentiate which station the frame goes to. For simplicity in
the following example, we assume that only those eight bits form the address.
Bridges A and B recognize frames for regions 1 and 2 respectively. Bridges C, D, E, and F recognize
frames for their respective segments. As an example we'll follow a frame with a destination
address of 1010 0101 (region 1, segment 1, station 5).
The equation individual address (lA) AND destination address(DA) = IA AND this station (TS) is
true, so bridge A accepts the frame. If the example were using a group address, the equation
would be GA mask AND DA=DA.
MOTOROLA
C-2
MC68824 USER'S MANUAL