8051 DEC Instruction

Decrement

Instruction	DEC byte
Function	Decrement
Description	DEC byte decrements the variable indicated by 1. An original value of 00H underflows to 0FFH. No flags are affected. Four operand addressing modes are allowed: accumulator, register, direct, or register-indirect. Note: When this instruction is used to modify an output port, the value used as the original port data will be read from the output data latch, not the input pins.
Example	Register 0 contains 7FH (01111111B) . Internal RAM locations 7EH and 7FH contain 00H and 40H, respectively. The following instruction sequence, DEC @R0 DEC R0 DEC @R0 leaves register 0 set to 7EH and internal RAM locations 7EH and 7FH set to 0FFH and 3FH.
Variants	DEC A DEC Rn DEC direct address DEC @Ri

Instruction	DEC A
Bytes	1
Cycles	1
Encoding	0 0 0 1 0 1 0 0
Operation	A = A - 1
Flags Affected	C AC F0 RS1 RS0 OV P
Example	DEC A
Bytes: Number of bytes required to encode the instruction. Cycles: Number of instruction cycles required to execute the instruction. Note that there are 12 oscillator cycles to one instruction cycle on a standard 8051. Encoding: Lists the byte encoding for the instruction. Operation: Lists, step-by-step, the operations performed by the instruction. Flags Affected: are highlighted in Bold

Instruction	DEC Rn
Bytes	1
Cycles	1
Encoding	0 0 0 1 1 n n n
Operation	Rn = Rn - 1
Flags Affected	C AC F0 RS1 RS0 OV P
Example	DEC R4
Bytes: Number of bytes required to encode the instruction. Cycles: Number of instruction cycles required to execute the instruction. Note that there are 12 oscillator cycles to one instruction cycle on a standard 8051. Encoding: Lists the byte encoding for the instruction. Operation: Lists, step-by-step, the operations performed by the instruction. Flags Affected: are highlighted in Bold

Instruction	DEC direct address
Bytes	2
Cycles	1
Encoding	0 0 0 1 0 1 0 1 A7...A0
Operation	(direct) = (direct) - 1
Flags Affected	C AC F0 RS1 RS0 OV P
Example	DEC 34h
Bytes: Number of bytes required to encode the instruction. Cycles: Number of instruction cycles required to execute the instruction. Note that there are 12 oscillator cycles to one instruction cycle on a standard 8051. Encoding: Lists the byte encoding for the instruction. Operation: Lists, step-by-step, the operations performed by the instruction. Flags Affected: are highlighted in Bold

Instruction	DEC @Ri
Bytes	1
Cycles	1
Encoding	0 0 0 1 0 1 1 i
Operation	(Ri) = (Ri) - 1
Flags Affected	C AC F0 RS1 RS0 OV P
Example	DEC @R0
Bytes: Number of bytes required to encode the instruction. Cycles: Number of instruction cycles required to execute the instruction. Note that there are 12 oscillator cycles to one instruction cycle on a standard 8051. Encoding: Lists the byte encoding for the instruction. Operation: Lists, step-by-step, the operations performed by the instruction. Flags Affected: are highlighted in Bold

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Related topics:
8051 Arithmetic Operation Instructions   |   8051 ADD Instruction   |   8051 ADDC Instruction   |   8051 SUBB Instruction   |   8051 INC Instruction   |   8051 MUL Instruction   |   8051 DIV Instruction   |   8051 DA Instruction

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List of topics: 8051

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