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. Similar implementation
Flowchart
Program code
Similar implementation
Fill_Acc:
MOV A,#30
Decrement:
DEC A
JZ Empty
JMP Decrement
NOP
Empty:
INC B
JMP Fill_Acc
MOV R0, 0FFH DJNZ R0, $

6. A 2-loop time delay ; **** 2-loop time delay **** MOV R0, #5 loadR1:
MOV R1, #3 DJNZ R1, $ DJNZ R0, loadR1 RET
The overall number of iterations
= N0 × N1

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

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

9. Instructions for conditional branchingJC
Rel
JNC JZ
JNZ JB
Bit,Rel
JNB
JBC
DJNZ
Byte,Rel
CJNE
<dest-byte>,<src-byte>, Rel

10. JC & JNC JC JNC Function Jump if carry set Jump if carry not set
Description
Branches to the specified address if the carry flag is set
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 JNZ Function jump if accumulator zero
jump if accumulator not zero
Description
transfers control to the specified address if the value in the accumulator is 0. Otherwise, the next instruction is executed.
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 JNB JBC Function Jump if Bit is set
Jump if Bit is not set
Jump and clear bit if bit is set
Description
branches to the specified address if the value of the specified bit is 1
branches to the specified address if the value of the specified bit is 0
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 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
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