Is there a pattern to x86 op codes? (other than direction and size bits)

问题

The op code portion of many important x86 instructions like MOV and ADD has the last two bits standardized as a direction bit and a data size bit:

Is there any pattern to the part of the op code shown in gray here? For example, for ADD some of the instructions use the code 000000 and others have code 100000.

来源：https://stackoverflow.com/questions/41720529/is-there-a-pattern-to-x86-op-codes-other-than-direction-and-size-bits