PIC microcontroller assembly language learning (I)

1. The statement format of assembly language:

label opcode (instruction mnemonic) operand; comment
(label) (opcode) (operand) (comment)

2. Common pseudo-instructions

a.EQU——Symbol name assignment pseudo-instruction
format: symbol name EQU nn

b.ORG——Pseudo instruction format for defining the program starting address
: ORG nnnn

c.END——Program end pseudo instruction
format: END

d.LIST——List option pseudo-instruction
format: LIST [optional, optional, ...]

e.INCLUDE: Call in external program file pseudo-instruction
format: INCLUDE "file name"

2. Branching Program Structure

——A few explanations on the use of instructions in the program:
(1) For any logical operation (AND, OR, XOR) and arithmetic operation (addition, subtraction) that requires two numbers to participate, one of the operands must be placed in W in advance. Special attention should be paid to the subtraction instruction used here. The subtrahend should be placed in W in advance. In other words, the number placed in W in advance is used as the subtrahend in the operation, and the number in the register is used as the minuend.
(2) A conditional jump instruction often needs to be followed by an unconditional jump instruction to achieve long-distance transfer and program branching.
(3) The instruction system of the PIC microcontroller does not have a dedicated stop instruction, which can be achieved by using an unconditional jump instruction GOTO to jump to itself.

3. PIC microcontroller instructions
consist of three basic types of instructions:
a. Byte operation instructions
b. Bit operation instructions
c. Immediate and control operation instructions

For byte operation instructions, f——> file register identifier, d——> target register identifier

Description: The target identifier specifies the storage location of the operation result:
d=0 The operation result is stored in the W register
d=1 The operation result is stored in the specified file register. The default value of d is 1

 4. Instruction Set

5. Examples

  1 ;--------------------------------------------------------

  2 

  3; Sequential program structure

  4; Take out the lower 4 bits of the 20H unit and store them in 21H, and take out the upper 4 bits and store them in 22H

  5; Key points: ANDLW and SWAPF

  6 

  7;---------------------------------------------------------

  8 MOVF 20H,0 ; Send the content of 20H unit to W

  9 ANDLW 0FH ;W high 4 bits cleared, low 4 bits remain unchanged

 10 MOVWF 21H ; send the lower 4 bits after splitting to 21H

 11 SWAPF 20H,0 ; Swap the high and low nibble of the 20H unit content and send it to W

 12 

 13 ANDLW 0FH ; Clear the high four bits of W to 0 and keep the low four bits unchanged

 14 MOVWF 22H ; send the split high four bits to unit 22H

 15 

 16 

 17;--------------------------------------------------------

 18 

 19 ;Branch program structure

 20; Two numbers are stored in the 20H and 21H units in the RAM. Find the larger number and store it in the 22H unit.

 21 ; Key points: Subtract two numbers and determine the value of flag C

 twenty two 

 twenty three ;---------------------------------------------------------

 24 STATUS EQU 03H ; define the STATUS register address as 03H

 25 C EQU 0 ; define the carry/borrow flag C in STATUS to get the address 0

 26 MOVF 20H 0 ; Send the content of unit 20H to W

 27 SUBWF 21H 0 ; Subtract the content in W from the content in 21H, and store the result in W

 28 BTFSS STATUS, C; if C=1, no borrow, then the number in unit 21H is large, jump to F21BIG

 29 GOTO F20BIG ; If C=0, there is a borrow, and the number in unit 20H is larger, then jump to F20BIG

 30     

 31 F21BIG MOVF 21H,0 ; store the content of 21H into W register

 32 MOVWF 22H; then transfer it to unit 22H

 33 GOTO STOP ; skip the following two instructions to the end of the program

 34         

 35 F20BIG MOVF 20H,0 ; store the content of 20H into W register

 36 MOVWF 22H ; then transfer it to unit 22H

 37 

 38 STOP GOTO STOP; Task completed, shutdown, stay where you are

 39 

 40 

 41 ;--------------------------------------------------------

 42 

 43 ;Loop program structure

 44 ; In the data memory, all 50 cells starting from address 30H are written to 00H

 45 ;Key points: The indirect addressing register FSR is used as an address pointer

 46 

 47;---------------------------------------------------------

 48 COUNT EQU 20H; Designate unit 20H as the loop count counter (i.e. loop variable)

 49 FSR EQU 04H ; define FSR register address as 04H

 50 INDF EQU 00H ; Set the INDF register address to 00H

 51 MOVLW D50 ; send the initial value of the counter 50 to W

 52 MOVWF COUNT ; Transfer 50 to the counter (as the operation value of the loop variable)

 53 MOVLW 30H ; send 30H (starting address) to W

 54 MOVWF FSR ; then transfer 30H to register FSR (used as address pointer)

 55         

 56 NEXT CLRF INDF ; Clear the unit specified by the FSR content as the address to 0

 57 INCF FSR,1 ;Address pointer content plus 1, pointing to the next unit

 58 DECFSZ COUNT,1 ;Decrement the count value by 1. If the result is 0, skip to the next instruction to STOP

 59 GOTO NEXT ; Jump back and execute the next loop

 60 STOP GOTO STOP ; After the loop ends, execute this statement to stop the machine.

 61 

 62;--------------------------------------------------------

 63 

 64 ;Subroutine structure

 65; The largest number of 3 is placed in the 40H unit

 66 

 67;---------------------------------------------------------

 68 STATUS EQU 03H

 69 C EQU 00H

 70 X EQU 20H

 71 Y EQU 21H

 72 Z EQU 22H

 73 ;--------------------------------------------------------

 74 

 75 ; Main program

 76 

 77 ;---------------------------------------------------------

 78 

 79 MAIN MOVF 30H,0

 80 MOVWF X

 81 MOVF 21H,0

 82 MOVWF Y

 83 CALL SUB

 84 MOVF Z,0

 85 MOVWF X

 86 MOVF 32H,0

 87 MOVWF Y

 88 CALL SUB

 89 MOVF Z,0

 90 MOVWF 40H

 91 STOP GOTO STOP

 92 ;--------------------------------------------------------

 93 

 94 ; Subroutine: (input parameters: X and Y, output parameter: Z)

 95 

 96 ;---------------------------------------------------------

 97 SUB MOVF X,0 ; send the contents of X to W

 98 SUBWF Y,0 ; Subtract the content of W from the content of Y, and store the result in W

 99 BTFSS STATUS, C; If C=1, no borrow occurs, execute the next line, otherwise jump

100 GOTO X_BIG        

101         

102 Y_BIG MOVF Y,0 ; Send the data in Y to W

103 MOVWF Z ; then save it to Z

104 GOTO THEEND ; skip the following two to the end

105         

106 X_BIG MOVF X,0 ; send the data in X to W

107 MOVWF Z ; then save it to Z

108 THEEND RETURN ; Subroutine returns