Wednesday, March 23, 2016

8051 Decrement and Jump if Not Zero

Decrement and Jump if Not Zero

DJNZ byte, relative address
DJNZ decrements the location indicated by 1, and branches to the address indicated by the second operand if the resulting value is not zero. An original value of 00H underflows to 0FFH. No flags are affected. The branch destination is computed by adding the signed relative-displacement value in the last instruction byte to the PC, after incrementing the PC to the first byte of the following instruction.

The location decremented may be a register or directly addressed byte.

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.

DJNZ Rn, relative address
Encoding: 1 1 0 1 1 r r r rel. address

DJNZ direct address, relative address
Encoding: 1 1 0 1 0 1 1 1 direct address rel. address

Internal RAM locations 40H, 50H, and 60H contain the values 01H, 70H, and 15H, respectively. The following instruction sequence,


causes a jump to the instruction at label LABEL_2 with the values 00H, 6FH, and 15H in the three RAM locations. The first jump was not taken because the result was zero.

This instruction provides a simple way to execute a program loop a given number of times or for adding a moderate time delay (from 2 to 512 machine cycles) with a single instruction. The following instruction sequence,

MOV R2, # 8

toggles P1.7 eight times, causing four output pulses to appear at bit 7 of output Port 1. Each pulse lasts three machine cycles; two for DJNZ and one to alter the pin.

MOV A, #0 ; Clear accumulator
MOV R4, #12 ; Initialize counter
LOOP :ADD A, #5 ; Add 5 to accumulator
DJNZ R4, LOOP ; Repeat until R4 = 0

Related topics:
8051 Unconditional and Conditional Jump   |   8051 Long Jump   |   8051 Absolute Jump   |   8051 Short Jump   |   8051 Indirect Jump   |   8051 Jump if Accumulator Zero   |   8051 Jump if Accumulator Not Zero   |   8051 Compare and Jump if Not Equal   |   8051 Jump if Carry is Set   |   8051 Jump if Carry is Not Set   |   8051 Jump if Bit Set   |   8051 Jump if Bit Not Set   |   8051 Jump if Bit is Set and Clear Bit

List of topics: 8051

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