SCAN_MATRIX_KEYBOARD:
     acall   DELAY ; calls the 30 MS delay subroutine
     mov     a, p1 
     push    acc   ; value of P1 is preserved
     anl     a, #0fh ; higher nibble is made low ie ROW to check 
                     ; whether a coumn is low or not
     cjne    a, #0fh, AHEAD ;if the Lower Nibble ie Coulmn reading is not high then 
                            ; it means any columnis low
                            ; jump to subroutine AHEAD
     sjmp    SCAN_MATRIX_KEYBOARD ; otherwise LOOP

AHEAD:
     pop     acc ; get back the original value of p1 ie the keypressed
     anl     a, #0f0h ; Make lower Nibbles ie column low to check whether
                      ; a ROW has been presse
     cjne    a, #0f0h, PROCESS_SCAN ; if the higher nibbles ie ROW are not high
                                    ; it means any key is pressed and
                                    ; correspodingly the ROW is low
                                    ; then jump to PROCESS_SCAN
     sjmp    SCAN_MATRIX_KEYBOARD ; otherwise LOOP

PROCESS_SCAN:
      mov    a, p1 ;get the value of keypressed in A 
      cpl    a ; compliment it and find whether actually a key is pressed or not
               ; if all the rows and coulmns are high it means no key is being pressed
      jz     SCAN_MATRIX_KEYBOARD ;if no key is pressed then jump SCAN_MATRIX_KEYBORAD
                                   ; by seeking value zero in A 
      cpl    a ; get the original value of p1
      push   acc ; preserve this value of p1
      anl    a, #0fh ;for again checking whether any coumn is low or not
      cjne   a, #0fh, ROW_SCAN_1 ; if any column is low then jump to find the corresponding row

      sjmp   SCAN_MATRIX_KEYBOARD

ROW_SCAN_1:
      pop   acc ; get the original value of keypressed again to check which row is pressed
      anl   a, #0f0h ; make higher nibbles high so that the row which is low can be trassed
                     ; as upto now it is scanned that a column is low
      cjne  a, #0e0h, ROW_SCAN_2 ; if ROW 1 has been pressed then jump to ROW_1 
                                 ; otherwise jump to ROW_2 checking
      sjmp  ROW_1

ROW_SCAN_2:
     cjne   a, #0d0h, ROW_SCAN_3
     sjmp   ROW_2

ROW_SCAN_3:
     cjne   a, #0b0h, ROW_SCAN_4
     sjmp   ROW_3

ROW_SCAN_4:
     cjne   a, #70h, SCAN_MATRIX_KEYBOARD
     sjmp   ROW_4

ROW_1:
     mov    dptr, #CODE1
     sjmp   FINAL

ROW_2:
     mov    dptr, #CODE2
     sjmp   FINAL

ROW_3:
     mov    dptr, #CODE3
     sjmp   FINAL

ROW_4:
     mov    dptr, #CODE4

FINAL:
     mov     a, p1 ; get the original value of p1 so that the COlumn can be sestimated from here
     clr     c ; clear carry flag

SUB_FINAL:
     rrc     a ; rotate right untill carry is clear
     jnc     MATCH ; if the carry is clear then jump to MATCH
     inc     dptr ; if carry is not clear then increment the DPTR
     sjmp    SUB_FINAL ; LOOP

MATCH:
     clr     a ; clear acc
     movc    a,@a+dptr ; move the value of dptr to ACC
     ;mov    r6, a
     push    acc ; preserve this value
     ret


CODE1: db '1','2','3','4'
CODE2: db '5','6','7','8'
CODE3: db '9','10','11','12'
CODE4: db '13','14','15','16'