TNETX3150 datasheet(42/78 Pages) TI

42 / 78 page

TNETX15AE

ADDRESS-LOOKUP DEVICE

SPWS041A – AUGUST 1997 – REVISED OCTOBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

a graphical example of the lookup algorithm (continued)

Table

Figure 9. Address Distribution Table Hierarchy (Worst Case)

Another side effect of this algorithm is that the table order is already sorted (smallest to largest).

The choice of N (the number of bits to consider each cycle) must be an integer and is a balance of:

The size of each table = 2N words; larger means more wasted space per lightly used table, and smaller

means less wasted space per lightly used table.

The clocks required to process a whole address (in this case 48 IEEE addresses) = 48

÷ N rounded up to

the next integer; considering more bits at a time decodes 48 bits in less time (considering fewer at a time

takes more time). The upper limit is 10 or 11 clocks.

SRAM data storage

lookup table SRAM allocation

The TNETX15AE uses a 5-bit version of the lookup algorithm described in the previous section.

When a 5-bit version of the lookup algorithm is used, each address requires ten tables to store a 48-bit value.

Tables 0 through 9 are used to match the 48 bits involved in the lookup. Table 9 (shown in Figure 10) contains

the actual data describing an address.

The external SRAM must be a minimum of eight bits wide, so address descriptor information (see

findport and

indnodeage register definitions) can be stored in four locations as bytes. Starting at 16K, the external SRAM

must get wider so that a pointer of installed word width (when five bits are appended) gives a pointer with enough

bits to address any memory location. Both address descriptor blocks and pointer tables must be contiguous,

and both are allocated dynamically. The table is full when the next structure being allocated does not fit. Several

addresses may have to be deleted on a table-full condition to make room for a new address.

An allocated (but unused) table entry is set to 0 since zero cannot be a valid address for a table leaf.

TNETX3150 数据表(PDF) 42 Page - Texas Instruments