assembly - Difference between MOV r/m8,r8 and MOV r8,r/m8

Question

By looking at intel volume of instructions, I found this:

1) 88 /r MOV r/m8,r8
2) 8A /r MOV r8,r/m8

When I write a line like this in NASM, and assemble it with the listing option:

mov al, bl

I get this in the listing:

88D8 mov al, bl

So obviously NASM chosed the first instruction of the two above, but isn't the second instruction an option two? if so, on what basis did NASM chosed the first?

Answer 1

Interesting side effect of having different encodings: From the A86 manual.

6. A86 takes advantage of situations in which more than one set of opcodes can be generated for the same instruction. (For example, MOV AX,BX can be generated using either an 89 or 8B opcode, by reversing fields in the following effective address byte. Both forms are absolutely identical in functionality and execution speed.) A86 adopts an unusual mix of choices in such situations. This creates a code-generation "footprint" that occupies no space in your program file, but will enable me to tell, and to demonstrate in a court of law, if a non-trivial object file has been produced by A86. The specification for this "footprint" is sufficiently obscure and complicated that it would be impossible to duplicate by accident. I claim exclusive rights to the particular "footprint" I have chosen, and prohibit anyone from duplicating it. This has at least two specific implications:

   a. Any assembler that duplicates the "footprint" is mine. If it is not identified as mine and issued under these terms, then those who sell or distribute the assembler will be subject to prosecution.

   b. Any program marked with the "footprint" has been produced by my assembler. It is subject to condition 5 above.