TNETX3150A datasheet(38/78 Pages) TI

38 / 78 page

TNETX15AE

ADDRESS-LOOKUP DEVICE

SPWS041A – AUGUST 1997 – REVISED OCTOBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

operating logic arbitration (continued)

INIT

LKUP

DEL

ADD

REG

FIND

AGE

Decreasing

Priority

Figure 4. Logic-Operation Priorities

One logic operation can interrupt a lower-priority logic operation to acquire the bus. For example, an LKUP

operation interrupts an ADD operation.

For the case of ADD and DEL (with the same priority), the arbiter grants the bus to the first logic operation that

requests it. It then grants an uninterruptable bus (unless by a LKUP) to that logic operation until that logic

operation is completed. If both ADD and DEL request the bus at the same time, the bus is granted to ADD. This

ensures that ADD is not interrupted by a DEL operation and vice versa. This hierarchy explains why a host

request for access to RAM completes with some variability — there may be higher-priority operations accessing

RAM at that instant.

lookup algorithm

The TNETX15AE device uses a table-based lookup algorithm to provide deterministic lookups with less than

248 memory words. The tables are hierarchical and are linked to the lower tables by threads. Each table can

thread to several different tables in the hierarchy. The lowest table in the hierarchy (leaf) does not point to

anything and contains information about the address to be matched.

Each level in the hierarchy is assigned to a specific range of bits in the address. The bits in the range are used

as an offset within the table at each level. If a thread exists at that offset, the TNETX15AE follows that thread.

The TNETX15AE matches an address when it finds a complete thread to a leaf. The thread structure is shown

in Figure 5.

The first level (root level) has only one table from which it can branch out to 2N possible tables. N is the number

of bits compared at a given time. Each additional table in the hierarchy branches down to 2N other possible

tables. The second level contains 2N table and 22N threads. The third level contains 22N tables and 23N threads,

and so on.

Because of this exponential growth, the threads and the amount of possible paths at each level soon overtake

the number of addresses required. If this growth were to go unchecked with an N of 5, the third level would

contain 1,024 tables and 32,768 threads. If only 1024 addresses are required, there are more tables allocated

than needed, and most contain NULL pointers.

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