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All about PIC microcontrollers

Within these pages, you can find many useful pieces of code mostly in assembly for the Microchip PIC micro controller family. A lot of people have spend many hours trying to put the bits and bytes together. If the code is NOT written by a member of the PCB Heaven community, then a link will be added above the code with the original website that this code was found.
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View code
BCD (packed) addition
Author: Scott Dattalo
This code was originally located @ http://www.piclist.com


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;******************************************
;bcd_add
;
; Computes z = x + y
; where x,y,z are all 8-bit packed BCD numbers
; Exits with C=1 if x+y > 0x99 and with z=1 if
; x+y = 0x100.
; Note that z can be aliased to x or y so that
;it's possible to calculate x = x+y or y = x+y
;This routine forms the BCD two's complement of
;y and then uses the bcd_subtract routine to
;find the sum. e.g. z = x - (-y) = x + y
;
; 10 cycles
bcd_add
COMF y, W ;W = ~y
ADDLW 0x9a+1 ;W = ~y + 0x9a +1
;W = (~y + 1) + 0x9a
;W = 0x9a - y
SUBWF x, W ;W = x - (0x9a - y)
;W = x + y - 0x9a
;W = x + y + 0x66
RLF z, F ;Get the carry
SKPDC ;if lsn of x + lsn of y < 10 (dec)
ADDLW -0x06 ; then remove the extra 6 added above
BTFSS z, 0 ;Similarly for the msn
ADDLW -0x60
RRF z, F ;restore the carry
MOVWF z

RETURN

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