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by porlw 1072 days ago
I remember back in the day these were used by the A86 assembler to identify code assembled with the unregistered version of the software.
1 comments

Not exactly. It used the two different encodings possible for source/destination operand when both are registers.

For example, 'ADD AL,CL' can be encoded as (values in octal):

    000 310 : 3=reg/reg  1=source is CL  0=dest. is AL
    002 301 : 3=reg/reg  0=dest is AL    1=src. is CL
Other assemblers always use one of these consistently, A86 switches between them depending on some bits of the opcode and operands (but each instruction will always be encoded the same when it appears multiple times, there is no steganographic message embedded).