32 bit unsigned substraction from Eddy Gora
;------------------------------------------------------------------------
; This is simple routine for full unsigned 32 bit substract
; a - b and result back in a ( initially a is lost )
; on exit success, CARRY = 0, on fail ( substraction negative ) CARRY = 1
; can be easly reduced / increased for 8 / 16 / 32 / 64 bits
; no optimisations ( tricks ) are made
; enjoy, eddy@hcv.ro from HCV Grup Ltd.,
__Config 3FF1h
include <p16f877a.inc>
a1 equ 0x20
a2 equ 0x21
a3 equ 0x22
a4 equ 0x23
b1 equ 0x24
b2 equ 0x25
b3 equ 0x26
b4 equ 0x27
org 0
START
clrf a1
clrf a2
clrf a3
clrf a4
clrf b1
clrf b2
clrf b3
clrf b4
movlw 0x9c
movwf a1
movlw 0x00
movwf a2
movlw 0x00
movwf a3
movlw 0xE2
movwf a4 ; MSB
movlw 0x9d
movwf b1
movlw 0xff
movwf b2
movlw 0x4f
movwf b3
movlw 0xc0
movwf b4 ; MSB
nop
call SUBB_32
nop
SELF
goto SELF
;------------------------------------------------------------------------
; This is simple routine for full unsigned 32 bit substract
; a - b and result back in a ( initially a is lost )
; on exit success, CARRY = 0, on fail ( substraction negative ) CARRY = 1
; can be easly reduced / increased for 8 / 16 / 32 / 64 bits
; no optimisations ( tricks ) are made
; enjoy, eddy@hcv.ro from HCV Grup Ltd.,
SUBB_32
bcf STATUS,C ; clear our routine return flag
movf b1,W ; substract first LSB bytes
subwf a1,F ; a1 - b1 --> a1
; a > b --> c=1
; a = b --> c=1
; a < b --> c=0 we have BORROW
btfsc STATUS,C ; have BORROW ?
goto NO_BORROW1 ; 1 NO
; 0 YES, so substract 1 from the next LSB
movlw .1
subwf a2,F
btfsc STATUS,C ; have also BORROW here ?
goto NO_BORROW1 ; 1
; 0 YES, so substract more, 1 from next
movlw .1
subwf a3,F
btfsc STATUS,C ; still BORROW ?
goto NO_BORROW1 ; 1
; 0 YES, substract 1 from the last
movlw .1
subwf a4,F
btfsc STATUS,C
goto NO_BORROW1
bsf STATUS,C ; if another BORROW, the substraction result is negative
; so return quick with CARRY = 1
return
NO_BORROW1
movf b2,w ; the same manner ...
subwf a2,f
btfsc STATUS,C
goto NO_BORROW2
movlw .1
subwf a3,F
btfsc STATUS,C
goto NO_BORROW2
movlw .1
subwf a4,F
btfsc STATUS,C
goto NO_BORROW2
bsf STATUS,C
return
NO_BORROW2
movf b3,w
subwf a3,f
btfsc STATUS,C
goto NO_BORROW3
movlw .1
subwf a4,F
btfsc STATUS,C
goto NO_BORROW3
bsf STATUS,C
return
NO_BORROW3
movf b4,w
subwf a4,f
btfsc STATUS,C
goto NO_BORROW4
bsf STATUS,C
return
NO_BORROW4
bcf STATUS,C
return
;-----------------------------------------------------------------------
end
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