Samsung S3F80JB manual Djnz Decrement and Jump if Non-Zero, Djnz r,dst Operation r ← r

S3F80JB INSTRUCTION SET

DJNZ — Decrement and Jump if Non-Zero

DJNZ r,dst

Operation: r ← r – 1

If r ≠ 0, PC ← PC + dst

The working register being used as a counter is decremented. If the contents of the register are not logic zero after decrementing, the relative address is added to the program counter and control passes 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 to be the address of the instruction byte following the DJNZ statement.

NOTE: In case of using DJNZ instruction, the working register being used as a counter should be set at the one of location 0C0H to 0CFH with SRP, SRP0, or SRP1 instruction.

Flags: No flags are affected.

Format:

r opc dst

Bytes		Cycles	Opcode	Addr Mode
			(Hex)	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 is typically used to control 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 is the label for a relative address.

The statement "DJNZ R1, LOOP" decrements register R1 by one, leaving the value 01H. Because the contents of R1 after the decrement are non-zero, the jump is taken to the relative address specified by the LOOP label.

6-39