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The 8051 Instruction Set

Atmel 8051 Microcontrollers Hardware Manual 1-34

4316E–8051–01/07

1.14.13 DECbyte

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.

DEC A

Bytes: 1

Cycles: 1

Encoding: 0 0 0 1 0 1 0 0

Operation: DEC

(A) ← (A) - 1

DEC R

Bytes: 1

Cycles: 1

Encoding: 0 0 0 1 1 r r r

Operation: DEC

) ← (R

) - 1

DEC direct

Bytes: 2

Cycles: 1

Encoding: 0 0 0 1 0 1 0 1 direct address

Operation: DEC

(direct) ← (direct) - 1

DEC @R

Bytes: 1

Cycles: 1

Encoding: 0 0 0 1 0 1 1 i

Operation: DEC

((R

)) ← ((R

)) - 1

1 2 ... 31 32 33 34 35 36 37 38 39 40 41 ... 115 116

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