Microcontroller keyboard scanning program (assembly)

;Keyboard scanning program;This program is more complicated, but if you insist on understanding it, you can still understand it. It is quite classic
;Finally, the key value is output to the digital tube (connected to port P2)
             ORG 0000H
START: MOV R0,#00H ;Initialize the program. The delay at the beginning is to make the hardware ready
              DJNZ R0,$
LOOP: MOV SP,#60H
             CALL KEY
DISPLAY:
             MOV A,R4  
             MOV DPTR,#TABLE ;Define the starting address of the font table
             MOVC A,@A+DPTR ;TABLE is the starting address of the table
             MOV P2,A
             SJMP LOOP

;Subroutine content, the fourth bit of P1 port is the row line, and the upper four bits are the column lines
KEY: PUSH PSW
           PUSH ACC
           MOV P1,#0F0H ;Set all lines to low level, full scan word-P1.0-P1.3, column as input mode
; This section only verifies that a key is pressed, and cannot determine which row it is 
          MOV R7,#0FFH ;Set the count constant as a delay

KEY1: DJNZ R7,KEY1

          MOV A,P1; read the column value of P1 port
          ANL A,#0F0H; determine whether a key is pressed (when a key is pressed, the upper four bits of P1 port are not all 1, and the bottom four bits are still all 0)
  ; the reason for performing phase OR here is to change the bottom four bits of 0000 to 1111, so as to perform negation in the next step
          ORL A,#0FH // This place is not in the original version, it is added again, if it is not added, it is wrong *******
   
          CPL A; after negation, if there is a high level, the key is pressed
          JZ EKEY; transfer if the accumulator is 0 (meaning that after negation, all are 0, if a key is pressed, the upper four bits will be 1 after negation), exit
          LCALL DEL20ms; if a key is pressed, process

;Scan row by row, 1 row at a time  
SKEY: MOV A,#00H
           MOV R0,A ;R0 is used as a row counter, with an initial value of 0
           MOV R1,A ;R1 is used as a column counter, with an initial value of 0
           MOV R2,#0FEH ;R2 is used as a scan temporary word, with an initial value of 1111 1110 (the fourth bit is used as a row scan word)
  
SKEY2: MOV A,R2
             MOV P1,A ;Output row scan word, 1111 1110
             NOP

             NOP
             NOP ;3 NOP operations make the output of port P1 stable
             MOV A,P1 ;Read column value (same as the beginning)
             MOV R1,A ;Temporarily store column value (the first time is **** 1110, that is, one of the upper four bits "may" be 0)
              ANL A,#0F0H ;Get the upper four bits,
             ORL A,#0FH ;Set all fourth bits to 1
              CPL A
BIAOZHI:JNZ SKEY3 ;If the accumulator is not 0, transfer the instruction (which means that it is determined that the key is in this row), and transfer to process
  
              INC R0 ;If the key is not in this row, the row counter is increased by 1
              SETB C ;The carry flag is increased by 1, so that the low bit 0 at the beginning will not appear in the low position when shifting left (after one cycle)
              MOV A,R2
              RLC A ;Shift left 1 bit with carry (form the scan word of the next row, and scan again)
              MOV R2,A
              MOV A,R0
  ;Compare the row counter R0 after adding 1 with the total number of scans (4 comparisons)
             CJNE A,#04H,SKEY2 ;(Has the scan been completed?) There is also an error in this place in the book. The book says: SKEY1
             AJMP EKEY ;If not, exit

;After a key is pressed, the row is scanned, this is the confirmed row
SKEY3: MOV A,R1 ;
             JNB ACC.4,SKEY5 ;Direct addressing bit is 0, how to transfer instructions
             JNB ACC.5,SKEY6
             JNB ACC.6,SKEY7
            JNB ACC.7,SKEY8
             AJMP EKEY //I feel that this command is useless

SKEY5: MOV A,#00H ; save column number 0
             MOV R3,A
             AJMP DKEY

SKEY6: MOV A,#01H ; store 1 column number
             MOV R3,A
             AJMP DKEY

SKEY7: MOV A,#02H ; store column number 2
             MOV R3,A
             AJMP DKEY

SKEY8: MOV A,#03H ; store column number 3
             MOV R3,A
             AJMP DKEY

;Take out the specific row number, add the column number, and finally confirm the key number
DKEY: //MOV R4,#00H
           MOV A,R0    
            MOV B,#04H
            MUL AB ;Let the row number * 4, and put the fourth digit in A (there are 4 rows in total, and the multiplication must be <16, that is, only the fourth digit has a value)
            ADD A,R3 ;Add the row number and column number to finally confirm the specific number of any keyMOV
             R4,A
  
EKEY: POP ACC
           POP PSW  

           RET ;Key scan processing function

DEL20ms:
           MOV R7,#2
DL2: MOV R6,#18
DL1: MOV R5,#255
           DJNZ R5,$
           DJNZ R6,DL1
           DJNZ R7,DL2

           RET
;This is the digital table of common cathode digital tube
TABLE: DB 3FH ;0
             DB 06H ;1
             DB 5BH ;2
             DB 4FH ;3
             DB 66H ;4
             DB 6DH ;5
             DB 7DH ;6
             DB 27H ;7
             DB 7FH ;8
             DB 6FH ;9
             DB 77H
             DB 7CH
             DB 39H
             DB 5EH
             DB 79H
             DB 71H

             END