Z80 assembly: DJNZ

The counted loop. One instruction decrements B and jumps back until B reaches zero.

DJNZ e10FCDecrement B, Jump if Not Zero

B ← B − 1
if B ≠ 0: PC ← PC + e (jump, 13 T-states)
if B = 0: continue (8 T-states)
flags: none change

The second byte, e, is a signed displacement. It is added to the address after DJNZ. Here, FCh (−4) at 0106h gives 0108h − 4 = 0104h.

Loop passes (starting value of B)

Run speed

What happened

Program listing

Address	Machine code	Label	Instruction	Comment

The loop adds 3 to A once per pass. Registers start at 0. The highlighted row is at PC. Each Step executes one instruction fetched from memory.

Registers and flags

Flags

A register is a small storage place in the CPU. PC means Program Counter. A T-state is one CPU clock period. A flag records a result from an instruction. S is sign, Z is zero, H is half-carry, P/V is parity or overflow, N is add/subtract, and C is carry. This page ignores undocumented X and Y bits.

Execution trace

#	Address	Instruction	Result	T-states

DJNZ compared with two instructions

; use B as the loop counter
loop:  DEC B
       JR NZ,loop

Loop control	DJNZ	DEC B + JR NZ

DEC B changes S, Z, H, P/V and N. DJNZ changes no flags. The timing counts here cover loop control only.

Mnemonic glossary

Program mnemonics

Mnemonic	Stands for	What it does here
LD	Load	Loads a value into a register.
ADD	Add	Adds 3 to A and updates all six documented flags.
DJNZ	Decrement B, Jump if Not Zero	Subtracts 1 from B. It jumps back if B is not 0. It does not change flags.
HALT	Halt	Stops this lesson’s CPU until reset. Interrupts are not modeled.
ORG	Origin	Assembler directive, not a CPU instruction. Places the COM-style program at 0100h.