The DEC instruction decrements the specified operand by 1. An original value of 00h underflows to 0FFh. No flags are affected by this instruction.
NOTE : When this instruction is used to modify an output port, the value used as the port data is read from the output data latch, not the pins of the port.
DEC @Ri DEC REG DEC Direct
DEC @R1 ;Makes R1=R1-1 DEC A ;Makes A=A-1 DEC 35h DEC R7