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CRC stands for cyclic redundancy check which is a hashing algorithm used for error checking information. Data is fed into the algorithm, and a semi-unique number called a checksum is the result. The checksum is strongly affected by the input data so even two very similar inputs will yield very different checksums. This is useful because ROM files are often corrupted, incorrectly dumped, or purposely modified and it's difficult to know if a ROM is good. By performing a CRC on a ROM, you will know if known instantly if have a genuine ROM.

The CRC algorithm used on this site is CRC32 which yields a 32-bit number represented as 8-digits of hexadecimal. Most CRCs have already been calculated by No-Intro. If you need to create a CRC yourself, make sure you perform the CRC on just the ROM's data, not the ROM with an included header.