bcd_test.s

 
.include "p24Hxxxx.inc"

.global __reset          ;The label for the first line of code.


         .data            ;unitialized data section

         .bss
;;These start at location 0x0800 because 0-0x07FF reserved for SFRs
u16_k:   .space 2        ;Allocating space (in bytes) to variable.
u8_j:    .space 1        ;Allocating space (in bytes) to variable.

.text                          ;Start of Code section
__reset:                       ; first instruction located at __reset label
       mov #__SP_init, w15     ;Initalize the Stack Pointer
       mov #__SPLIM_init,W0
       mov W0, SPLIM           ;Initialize the stack limit register
       rcall main              ;call main()
       reset                   ;start over
 ;__SP_init set by linker to be after allocated data

main:
      mov #0xFFFF,W0  ; 65535 unsigned, -1 signed
      mov #0x1,W1     ; 1 unsigned, +1 signed

;addition
      mov   #0x39,W0
      mov   #0x28,W1
      rcall bcd_add8  ;BCD (39 + 28 = 67)

;subtraction
      mov   #0x42,W0
      mov   #0x24,W1
      rcall bcd_sub8 ;BCD (42 - 24 = 18)


done:                     ;do not return
       bra done

;;do bcd 8-bit addition, W0+W1=W0
bcd_add8:
      add.b W0,W1,W0  ;W0 = W0 + W1
      daw.b W0        ;decimal adjust
      return

;;do bcd 8-bit subtractin, W0-W1=W0
bcd_sub8:
      mov   #0x99,W2
      sub.b W2,W1,W1  ;do 0x99-W1
      inc.b W1,W1     ;add 1 for 10's complement
      add.b W0,W1,W0  ;W0 = W0 + (-W1)
      daw.b W0
      return




    .end