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标题: PIC单片机无符号BCD减法子程序 [打印本页]

作者: winnie 时间: 2009-5-6 09:58
标题: PIC单片机无符号BCD减法子程序
;*******************无符号 BCD 减法***************
;
;    This routine performs a 2 Digit Unsigned BCD Subtraction.
; It is assumed that the two BCD numbers to be subtracted are in
; locations Num_1 & Num_2. The result is the difference of Num_1 & Num_2
; ( Num_2 - Num_1) and is stored in location Num_2 and the overflow carry
; is returned in location Num_1.
;
; Performance :
;             Program Memory  :    31
;             Clock Cycles :    21  ( worst case )
;
;*******************************************************************
;
Num_1 equ    8    ; Overflow flow carry overwrites Num_1
result  equ    8
;
Num_2 equ    9    ; Num_2 - Num_1 overwrites Num_2
O_flow  equ    9
;
;其它寄存器自己定义
;
BCDSub
movf Num_1,w
subwf Num_2
clrf Num_1
rlf    Num_1
btfss STATUS,DC
goto adjst1
btfss Num_2,3       ; Adjust LSD of Result
goto Over_1
btfsc Num_2,2
goto adjst1       ; Adjust LSD of Result
btfss Num_2,1
goto Over_1       ; No : Go for MSD
adjst1
movlw 6
subwf Num_2
Over_1
btfss Num_1,0       ; CY = 0 ?
goto adjst2       ; Yes, adjust MSD of result
clrf Num_1
btfss Num_2,7       ; No, test for MSD >9
RETLW 0
btfsc Num_2,6
goto adjst2
btfss Num_2,5
RETLW 0
adjst2
movlw 60             ; add 6 to MSD
subwf Num_2
clrf Num_1
btfss STATUS,CARRY ; test if underflow
RETLW 0
movlw 1
movwf Num_1
Over
RETLW 0
;
;********************************************************************
;    测试程序(注意用法,Num_2-Num_1=Num_2)
;*********************************************************************
main
movlw 23
movwf Num_1    ; Set Num_1 = 23
movlw 99
movwf Num_2    ; Set Num_2 = 99
call BCDSub    ; After subtraction, Num_2 = 76 ( 99-23 )
;                         ;  and Num_1 = 0 ( indicates positive result )
;
self goto self ; 如不是测试程序,本句无用
;
org    1FF
goto main
;
END

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