單片機 三字節除法子程序
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AJMP MAIN
ORG 0030H
MAIN:
MOV 77H,#0FH
MOV 78H,#42H
MOV 79H,#40H
MOV 7AH,#00H
MOV 7BH,#03H
MOV 7CH,#0E8H
LCALL DIV_MB
SJMP MAIN
;=================================================
; DIVISION FOR THREE-BYTE DATA
; DIVIDEND FROM 77H&78H&79H
; DIVISOR FROM 7AH&7BH&7CH
; QUOTIENT TO
;ARITHMETICAL COMPLIMENT TO 7DH&7EH&7FH
; 2008_05_08 BY SZCKIC
;=================================================
DIV_MB: MOV 7DH,#00H
MOV 7EH,#00H
MOV 7FH,#00H
MOV R7,#18H
NEXTBIT:
CLR C
MOV A,79H
RLC A
MOV 79H,A
MOV A,78H
RLC A
MOV 78H,A
MOV A,77H
RLC A
MOV 77H,A
;Arithmetical compliment
MOV A,7FH
RLC A
MOV 7FH,A
MOV A,7EH
RLC A
MOV 7EH,A
MOV A,7DH
RLC A
MOV 7DH,A
MOV 5AH,7DH
MOV 5BH,7EH
MOV 5CH,7FH
MOV 5DH,7AH
MOV 5EH,7BH
MOV 5FH,7CH
LCALL TDEC
JNC UPDATE
DJNZ R7,NEXTBITTMP
RET
UPDATE: INC 79H
MOV 7FH,5CH
MOV 7EH,5BH
MOV 7DH,5AH
DJNZ R7,NEXTBITTMP
RET
NEXTBITTMP:LJMP NEXTBIT
;=================================================
; Subtracter for three-byte data
;=================================================
TDEC: CLR C
MOV A,5CH
SUBB A,5FH
MOV 5CH,A
MOV A,5BH
SUBB A,5EH
MOV 5BH,A
MOV A,5AH
SUBB A,5DH
MOV 5AH,A
RET
除法程序寫成了子程序形式，很方便調用，除法中要頻繁用到三字節數據大小的比較，所以也寫成了一個小子程序。
思路很簡單，拋開我們習慣了的乘法口訣，以一個小學二年級學生的思路去做就簡單了，一步一步的比較、移位，只不過這里是二進制而不是習慣了的十進制，當然二進制也有一個優點那就是：
假設A,B是兩個同樣位數的二進制數據（首位不為0），且A>B，則A肯定小于2倍的B，所以對于每次移位只用比較一次。
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