The function of this group of instructions is to subtract 1 from the content of the register pointed to, and send the result back to the original register. If the content of the original register is 00H, it will be FFH after subtracting 1. The operation result does not affect any flag bit. This group of instructions has direct, register, and inter-register addressing modes. When the direct address is an I/O port latch, the "read-modify-write" operation is similar to the add 1 instruction.
DEC A; (A) -1 → (A) The content in accumulator A is subtracted by 1, and the result is sent back to accumulator A.
DEC data; (data) -1 → (data) The content in the direct address unit is reduced by 1, and the result is sent back to the direct address unit
DEC @Ri; ((Ri)) -1 → ((Ri)) The content in the address unit pointed to by register Ri is reduced by 1, and the result is sent back to the original address unit
DEC Rn; (Rn) -1 → (Rn) The content in register Rn is reduced by 1, and the result is sent back to register Rn
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