The even numbers (68000, 68020, 68060) were reserved for major revisions to the 680x0 core. The odd numbers (68010, 68030, 68050) were minor upgrades from the previous chip. For minor enhancements and modifications to some user/superuser would have been a 68040 with some bugs fixed, which didn't really warrant a new name so it was sold as a 68040. (1995-11-29)
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