Description: The "Undefined" instruction is, as the name suggests, not a documented instruction. The 8051 supports 255 instructions and OpCode 0xA5 is the single OpCode that is not used by any documented function. Since it is not documented nor defined it is not recommended that it be executed. However, based on my research, executing this undefined instruction takes 1 machine cycle and appears to have no effect on the system except that the Carry Bit always seems to be set.
Note: We received input from an hobbyprojects.com user that the undefined instruction really has a format of Undefined bit1,bit2 and effectively copies the value of bit2 to bit1. In this case, it would be a three-byte instruction. We haven't had an opportunity to verify or disprove this report, so we present it to the world as "additional information."
NOP, Instruction Set