1. Conditional Jumps and Time Delays
Source:

2. The simplest implementation of delay: the instruction “NOP”
Source:

3. Different Interpretations of “NOP”

4. A single-loop time delay
“DJNZ” can be used here.

5. MOV
MOV R0, 0FFH DJNZ R0, $

6. A 2-loop time delay ; **** The 2-loop time delay **** MOV R0, #0FFH
loadR1:
MOV R1, #0FFH DJNZ R1, $ DJNZ R0, loadR1 RET
The overall number of iterations
= N0 × N1

7. A 3-loop time delay Homework:
Write a 3-loop delay procedure based on this flowchart.

8. 2-loop  1-loop = A 3-loop time delay
; *** The 3-loop time delay ***
threeLoopDelay:
MOV R2, #0AH
loop:
CALL twoLoopDelay
DJNZ R2,loop
RET
; ---- The 2-loop time delay ----
twoLoopDelay:
MOV R0, #0FFH
loadR1:
MOV R1, #0FFH DJNZ R1, $ DJNZ R0, loadR1 RET

9. Instructions for conditional branching
JC rel JNC rel
JZ rel
JNZ rel
JB bit,rel
JNB bit,rel
JBC bit,rel
DJNZ byte,rel
CJNE <dest-byte>,<src-byte>, rel

10. JC & JNC
JC rel Function: jump if carry set Description: branches to the specified address if the carry flag is set
JNC rel Function: jump if carry not set Description: branches to the specified address if the carry flag is cleared.
Syntax
JC rel
JNC rel
Steps
(PC) <- (PC) + 2 IF (C) = 1 THEN (PC)< - (PC) + rel
(PC) <- (PC) + 2 IF (C) = 0 THEN (PC) <- (PC) + rel
Machine code
Example
JC LABEL
JNC LABEL

11. JZ & JNZ JZ rel Function: jump if accumulator zero
Description: transfers control to the specified address if the value in the accumulator is 0. Otherwise, the next instruction is executed. 
JNZ rel Function: jump if accumulator not zero Description: transfers control to the specified address if the value in the accumulator is not 0. If the accumulator has a value of 0, the next instruction is executed.
Syntax
JZ rel
JNZ rel
Steps
(PC) <- (PC) + 2 IF (A) = 1 THEN (PC)< - (PC) + rel
(PC) <- (PC) + 2 IF (A) = 0 THEN (PC) <- (PC) + rel
Machine code
Example
JZ LABEL
JNZ LABEL

12. JB, JBC, & JNB JB bit,rel Function: Jump if Bit is set
Description: branches to the specified address if the value of the specified bit is 1. 
JNB bit,rel
Function: Jump if Bit is not set
Description: branches to the specified address if the value of the specified bit is 0.
JBC bit,rel
Function: Jump and clear bit if bit is set
Description: branches to the specified address and clear the specified bit if the value of the specified bit is 1.
Syntax
JB bit,rel
JNB bit,rel
JBC bit,rel
Steps
(PC) <- (PC) + 3 IF (bit-addr) = 1, (PC)< - (PC) + rel
(PC) <- (PC) + 3 IF (bit-addr) = 0, (PC) <- (PC) + rel
(PC) <- (PC) + 3 IF (bit-addr) = 1, (bit-addr) = 0 and (PC) <- (PC) + rel
Machine code
bit, rel
bit, rel
bit, rel
Example
JB P1.2,LABEL
JNB P1.2,LABEL
JBC ACC.3,LABEL

13. DJNZ Syntax DJNZ Rn, rel (Rn = R0, R1, …, R7) DJNZ direct, rel
DJNZ <byte>,<rel-addr>
Function: Decrement and Jump if Not Zero
Description: DJNZ decrements the location indicated by 1, and branches to the address indicated by the second operand if the resulting value is not zero.
Execution steps:
(PC) <- (PC) + instructionSize (byte) <- (byte) - 1 IF (byte) <> 0 THEN (PC) <- (PC) + rel
Syntax
DJNZ Rn, rel
(Rn = R0, R1, …, R7)
DJNZ direct, rel
(“direct” is a location in RAM)
Steps
(PC) <- (PC) + 2 (Rn) <- (Rn) - 1 IF (Rn) <> 0 THEN (PC) <- (PC) + rel
(PC) <- (PC) + 3 (direct) <- (direct) - 1 IF (direct) <> 0 THEN (PC) <- (PC) + rel
Machine code
11011rrr rel
bit add, rel
Example
DJNZ R5,rel
rel
DJNZ 30H, rel
rel

14. CJNE CJNE <dest-byte>,<src-byte>, rel
Function: Compare and Jump if Not Equal.
Description: CJNE compares the magnitudes of the first two operands and branches if their values are not equal.
Execution steps:
PC = PC + 3
IF (dest) <> (src), PC = PC + rel
IF (dest) < (src), C = 1 ELSE C = 0
Syntax
CJNE A,#immediate,rel
CJNE #Ri,#immediate,rel
CJNE Rn,#immediate,rel
, where x = 0 or 1, n = 0, 1, 2, …, or 7.
CJNE A,direct,rel
(“direct” is a location in RAM)
Step
PC = PC + 3
IF A <> immediate, PC = PC + rel
IF A < immediate, C = 1 ELSE C = 0
IF A <> (direct), PC = PC + rel
IF A < (direct), C = 1 ELSE C = 0
IF (Ri) <> immediate, PC = PC + rel
IF (Ri) < immediate, C = 1 ELSE C = 0
IF Rn <> immediate, PC = PC + rel
IF Rn < immediate, C = 1 ELSE C = 0
Machine code
immediate, rel
n immediate, rel
10111nnn immediate, rel
direct, rel
Example
CJNE A,#8EH,LABEL
CJNE @R1,#8EH,LABEL
CJNE R4,#8EH,LABEL
CJNE A,34H,LABEL