PIC Microcontoller Math Method

BCD (packed) subtraction

From: Scott Dattalo

 ;****************************************** 
;bcd_subtract 
; 
; Computes z = x - y 
; where x,y,z are all 8-bit packed BCD numbers 
; Exits with C=1 (and DC=1 too) if x>=y 
; and with z=1 if x==y. 
; Note that z can be aliased to x or y so that 
;it's possible to calculate x = x-y or y = x-y 
; 9 cycles (+ return) 
bcd_subtract 
 MOVF y, W ;W = y 
 SUBWF x, W ;W = x-y 
 RLF z, F ;lsb of z has the carry 
 SKPDC ;if lsn of x < lsn of y 
 ADDLW -0x06 ; then convert lsn of the 
 ; result to BCD. 
 BTFSS z, 0 ;Similarly for the msn's 
 ADDLW -0x60 
 RRF z, F ;Get the carry 
 MOVWF z ;and save the result 
 RETURN 
 

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