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S3F84B8_UM_REV 1.00
6 INSTRUCTION SET
6-38
6.3.26 DJNZ — DECREMENT AND JUMP IF NON-ZERO
DJNZ
r,dst
Operation
: r
r – 1
If
r
0, PC
PC + dst
The working register, which is used as a counter, is decremented. If the contents of register are
not logic zero after decrementing, the relative address is added to the program counter and
control is passed to the statement whose address is now in the PC. The range of the relative
address is +127 to –128, and the original value of the PC is taken as the address of instruction
byte following the DJNZ statement.
NOTE:
While using the DJNZ instruction, the working register (which is used as a counter) should be set at the one
of the locations 0C0H to 0CFH with SRP, SRP0, or SRP1 instruction.
Flags
:
No flags are affected.
Format
:
Bytes Cycles Opcode
(Hex)
Addr Mode
dst
r | opc
dst
2
8 (jump taken)
rA
RA
8 (no jump)
r = 0 to F
Example
:
Given R1 = 02H and LOOP is the label of a relative address:
SRP
#0C0H
DJNZ
R1,LOOP
DJNZ controls a “loop” of instructions. In many cases, a label is used as the destination operand
instead of a numeric relative address value. In the example, working register R1 contains the
value 02H, and LOOP specifies the label for a relative address.
The statement “DJNZ R1, LOOP” decrements register R1 by one, leaving the value 01H. Since
the contents of R1 after the decrement are non-zero, the jump is taken to the relative address
specified by the LOOP label.