Is there a difference in speed between mov bx,ax and mov bh,ah?

Question

Is there a difference in speed between mov bx,ax and mov bh,ah in a 16-bit MS-DOS application for an 8086 architecture?

Answer 1

If in doubt, profile. But it's very unlikely in this case. From what I gleaned from the Intel 64 and IA32 Architectures Optimization Reference Manual it appears to be that they both perform identically but usually slower than their 32-bit or 64-bit counterparts.

Answer 2

You don't specify the architecture, but at least in 8086, 286, 386 and 486 they execute in the same number of clocks: 2 clocks in 8088/8086, 286 and 386 for either instruction, 1 clock in 486 for either instruction. See link.