What is not r0 in keil

Since you want to perform the XOR at runtime, you will have to use instructions to accomplish that. Unfortunately, the XRL instruction only operates on the A register, so you might have to do some rearranging. Assuming A is not available, but R3 is, you can do:

If A is available, you can use the CJNE accepting a memory operand knowing that registers are memory mapped:

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