A conditional branch instruction is a relative transfer when certain conditions are met.
Determine whether the A content is a 0 transfer instruction
JZ rel
JNZ rel
The function of the first instruction is: if (A) = 0, then the transfer, or the order execution (execution of the next instruction of this instruction). Where are you going to move? If you follow the traditional method, it is very troublesome to calculate the offset. Fortunately, we can now assemble it with the help of the machine. So this instruction can be understood as follows: JZ label. That is, transfer to the label. Here is an example:
MOV A, R0
JZ L1
MOV R1, #00H
AJMP L2
L1: MOV R1, #0FFH
L2: SJMP L2
END
If the value in R0 is 0 before executing the above procedure, it is transferred to L1 for execution, so the final execution result is that the value in R1 is 0FFH. If the value in R0 is not equal to 0, the order is executed, that is, the MOV R1, #00H instructions are executed. The final result of the execution is that the value in R1 is equal to zero.
The function of the first instruction is clear, and the second one of course understands well. If the value in A is not equal to 0, it is transferred. Try changing the JZ in the above example to JNZ. What is the result of the program execution?
Comparison transfer instruction
CJNE A, #data,rel
CJNE A, direct, rel
CJNE Rn, #data,rel
CJNE @Ri,#data,rel
The function of the first instruction is to compare the value in A with the immediate data. If the two are equal, execute in order (execute the next instruction of this instruction). If they are not equal, they will be transferred. Similarly, we can rel It is understood as a label, namely: CJNE A, #data, label. Using this command, we can determine whether the two numbers are equal, which is very useful in many situations. But sometimes I want to know which one is bigger and which one is smaller after the comparison between the two numbers. This instruction also has such a function. If the two numbers are not equal, the CPU will also reflect which number is large and which is small. This is done with CY (carry) Bit) to achieve. If the previous number (in A) is large, then CY=0, otherwise CY=1, so after using the CY again after program transfer, it can be judged whether the number in A is larger or smaller than data.
example:
MOV A, R0
CJNE A, #10H, L1
MOV R1, #0FFH
AJMP L3
L1: JC L2
MOV R1, #0AAH
AJMP L3
L2: MOV R1, #0FFH
L3: SJMP L3
There is a single-chip instruction in the above program that we have not learned yet, that is, JC. The prototype of this instruction is JC rel, which is similar to the above JZ, but it is judged whether CY is 0 or 1 is transferred, if CY=1 Then, it is transferred to the label following the JC, and if CY=0, the order is executed (execution of its next instruction).
Analyze the above procedure. If (A) = 10H, the order is executed, that is, R1=0. If (A) is not equal to 10H, go to L1 to continue execution. At L1, judge again. If (A)>10H, then CY=1, the order will be executed, that is, the MOV R1, #0AAH instructions will be executed. If (A) < 10H, it will be transferred to the line at L2, that is, the MOV R1, #0FFH instruction is executed. Therefore, the final result is: (R1)=00H if (R0)=10H, (R1)=0AAH if (R0)<10H, then (R1)=(R1)= before the program is executed. 0FFH.
Understand this instruction, the other ones are similar, the second is to compare the value in A with the value in the direct address, and the third is to compare the value in the direct address with the immediate value, fourth The bar compares the number obtained by addressing the address with the immediate value. It will not be discussed here. Several corresponding routines are given below.
CJNE A, 10H ; compare the value in A with the value in 10H (note the difference from the above question)
CJNE 10H, #35H ; compare the value in 10H with the value in 35H
CJNE @R0, #35H ; take the value in R0 as the address, take the number from this address and compare it with 35H
Loop transfer instruction
DJNZ Rn, rel
DJNZ direct,rel
The first instruction has a detailed analysis in the previous routine, so I won't talk much about it here. The second instruction, just change Rn to a direct address, the other is the same, not much to say, give a routine.
DJNZ 10H, LOOP
Call and return instruction
(1) Main program and subroutine
In the experiment of the previous lights, we have used subroutines, but we did not explicitly introduce them. What is the use of subroutines, why use subroutine technology? For a routine, our data teacher has arranged 10 arithmetic questions. After observation, each question contains a (3*5+2)*3 operation. We can have two choices, the first one, each For a question, we will calculate this formula once. The second option, we can first calculate this result, that is, 51, put it aside, and then use this formula to enter 51 generations. Which of these two methods is better? Needless to say. This is also the case when designing a program. Sometimes a function can be used repeatedly in a different place in the program. We can make this function into a program, and "call" it each time we need to use this function.
(2) call and return process
The main program calls the subroutine. After the subroutine is executed, it must return to the main program to continue execution. It cannot be "going back and not going back". So where do you go back? It is back to the next instruction of the calling subroutine to continue execution (of course, if you return to this instruction, do you have to call the subroutine again? That is endless...). Refer to Figure 1
Call instruction
LCALL addr16; long call instruction
ACALL addr11; short call instruction
The above two instructions are all called subroutines in the main program. There are certain differences between the two, but when you start learning these instructions of the MCU, you can make no difference, and you can use the LCALL label, ACALL label to understand, that is, call Subprogram.
The return instruction says, after the subroutine is executed, it must return to the main program. How to return it? As long as a return instruction is executed, that is, the ret instruction is executed.
Null operation instruction
Nop is a null operation, that is, nothing is done, stop a cycle, generally used as a short time delay.
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