Intel NETWORK PROCESSOR IXP2800 Instrukcja Użytkownika Pobierz pdf (Strona 240)

100

101

240 Hardware Reference Manual

Intel

IXP2800 Network Processor

SHaC — Unit Expansion

Equation 7. (48-bit hash operation)

Equation 8. (64-bit hash operation)

Equation 9. (128-bit hash operation)

The division results in a quotient Q(x), a polynomial of order-46, order-62, or order-126, and a

remainder R(x), and a polynomial of order-47, order-63, or order-127. The operands are related by

the equation:

Equation 10.

The generator polynomial has the property of irreducibility. As a result, for a fixed multiplier M(x),

there is a unique remainder R(x) for every input A(x). The quotient Q(x), can then be discarded,

since input A(x) can be derived from its corresponding remainder R(x). A given bounded set of

input values A(x) — for example, 8K or 16K table entries — with bit weights of an arbitrary

density function can be mapped one-to-one into a set of remainders R(x) such that the bit weights

of the resulting Hashed Arguments (a subset of all values of R(x) polynomials) are all

approximately equal.

In other words, there is a high likelihood that the low-order set of bits from the Hash Arguments are

unique, so they can be used to build an index into the table. If the hash algorithm does not provide

a uniform hash distribution for a given set of data, the programmable hash multiplier (M(x)) may

be modified to provide better results.

x() 1 x

++++=

x() 1 x

++++=

128

x() 1 x

128

++++=

Ax()Mx() Qx()Gx() Rx()+=

1 2 ... 235 236 237 238 239 240 241 242 243 244 245 ... 429 430

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