Subroutine call and return instructions

In program design, common program segments with certain functions are usually compiled into subroutines. When the subroutine is needed, the CALL instruction is called, and a subroutine return instruction RET is arranged at the end of the subroutine so that after the subroutine is executed, it can return to the main program to continue execution.

1. Call instruction CALL
Before the program transfer occurs, the current value of PC is pushed into the stack, and then the subroutine entry address is sent to PC, so that the CPU switches to executing the subroutine.

2. Subroutine return instruction RET** The subroutine return instruction pops the contents of the two adjacent cells on the top of the stack and sends them to the PC, subtracts 2 from the contents of SP, and the program returns to the instruction pointed to by the PC value. The RET instruction is usually placed at the end of the subroutine so that the program can return from the subroutine to the main program.

3. Interrupt return instruction RETI** The RETI instruction has the same function as the RET instruction, which is to make the subroutine return to the main program. However, the RETI instruction does one more thing than the RET instruction, that is, before the main program returns, the instruction will also clear the corresponding interrupt flag to indicate that the interrupt has been responded to. 

4. No operation instruction NOP** No operation is also a CPU control instruction. It does not have the function of transferring the program. It only consumes one machine cycle, that is, PC+1 at this time. It is often used for program waiting or time delay. ``` ******************************************************************/ * 

【Course 4】 ****Application of MCU Assembly Instruction System*********** * 

【Description】 ****Through this example, you can understand the assembly instruction system of the microcontroller and write programs* 

【Description】 ****Add two immediate numbers FCH and 52H******************************************************************/ ORG 0000H Start: MOV A,#0FCH ;Calculate FCH+52H=? MOV R0,#52H ADD A,R0 JC L1 ;Judge CY, if CY=1, jump to L1 and execute MOV 30H,#00H JMP L2 L1: MOV 30H,#01H L2: MOV 31H,A

  MOV A,#0FCH	
  ANL A,R0 ; Calculate the AND of two numbers
  MOV P0,A
  MOV A,#0FCH	
  ORL A,R0 ; Calculate the OR of two numbers
  MOV P1,A
  MOV A,#0FCH	
  XRL A,R0 ; Calculate the XOR of two numbers
  MOV P2,A
  MOV A,#0FCH
  CPL A ; Negate the FCH number
  MOV P3,A
  RL A ; Shift left 1 bit
  RL A ; Shift left 1 bit
  RL A ; Shift left 1 bit
  RL A ; Shift left 1 bit
  MOV P3,A    
  END