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CRCs are among the best checksums available to detect and/or correct errors in communications transmissions. Unfortunately, the modulo-2 arithmetic used to compute CRCs doesn't map easily into software. This article shows how to implement an efficient CRC in C.

I'm going to complete my discussion of checksums by showing you how to implement CRCs in software. I'll start with a naïve implementation and gradually improve the efficiency of the code as I go along. However, I'm going to keep the discussion at the level of the C language, so further steps could be taken to improve the efficiency of the final code simply by moving into the assembly language of your particular processor.

For most software engineers, the overwhelmingly confusing thing about CRCs is their implementation. Knowing that all CRC algorithms are simply long division algorithms in disguise doesn't help. Modulo-2 binary division doesn't map particularly well to the instruction sets of off-the-shelf processors. For one thing, generally no registers are available to hold the very long bit sequence that is the numerator. For another, modulo-2 binary division is not the same as ordinary division. So even if your processor has a division instruction, you won't be able to use it.

Modulo-2 binary division

Before writing even one line of code, let's first examine the mechanics of modulo-2 binary division. We'll use the example in Figure 1 to guide us. The number to be divided is the message augmented with zeros at the end. The number of zero bits added to the message is the same as the width of the checksum (what I call c); in this case four bits were added. The divisor is a c+1-bit number known as the generator polynomial.

Modulo-2 Binary Division Example

Figure 1. An example of modulo-2 binary division

The modulo-2 division process is defined as follows:

  • Call the uppermost c+1 bits of the message the remainder
  • Beginning with the most significant bit in the original message and for each bit position that follows, look at the c+1 bit remainder:
    • If the most significant bit of the remainder is a one, the divisor is said to divide into it. If that happens (just as in any other long division) it is necessary to indicate a successful division in the appropriate bit position in the quotient and to compute the new remainder. In the case of modulo-2 binary division, we simply:
      • Set the appropriate bit in the quotient to a one, and
      • XOR the remainder with the divisor and store the result back into the remainder
    • Otherwise (if the first bit is not a one):
      • Set the appropriate bit in the quotient to a zero, and
      • XOR the remainder with zero (no effect)
    • Left-shift the remainder, shifting in the next bit of the message. The bit that's shifted out will always be a zero, so no information is lost.

The final value of the remainder is the CRC of the given message.

What's most important to notice at this point is that we never use any of the information in the quotient, either during or after computing the CRC. So we won't actually need to track the quotient in our software implementation. Also note here that the result of each XOR with the generator polynomial is a remainder that has zero in its most significant bit. So we never lose any information when the next message bit is shifted into the remainder.

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