Why is the 64bit version normally faster than the 32 bit?

[Greyguy]

If we look at KNodes/sec for SF7 it could be like 178 vs 138 (64bit vs 32bit).

[Michael Sherwin]

Because a 64 bit processor has 64 bit and 32 bit registers and can execute code written for either 64 bit processors or code written for 32 bit processors. A bitboard engine like SF uses 64 bit values. SF can be compiled to run on 64 bit hardware only or it can be compiled to run on both 32 bit and 64 bit hardware. When SF is running on 32 bit hardware or is running on 64 bit hardware but was compiled for 32 bit hardware a 64 bit bitboard operation has to be broken up into two 32 bit operations taking more time.

[Sven]

This is usually true for bitboard engines where processing of 64-bit numbers is being used heavily. A 32-bit version needs to emulate all operations on 64-bit numbers by twiddling with two 32-bit words instead of one 64-bit word.

[Kotlov]

Because 64 squares on a board ))