Divide by 3
A lot of times, you need to divide something by 3. One way is to multiply by $55.
; divide 16 bit number by 3 by multiplying by 1/3
; enter with
; A containing the hi byte of the number to be divided by 3
; Y containing the lo byte of the number to be divided by 3
; the hi byte of the partial product is kept in A or saved
; on the stack when neccessary
; the product (N/3 quotient) is returned hi byte in A,
; lo byte in Y
.proc div3_ay
; save the number in lo_temp, hi_temp
sty lo_temp
sty lo_product
sta hi_temp
ldy #$09
clc
bcc ENTER
; each pass through loop adds the number in
; lo_temp, hi_temp to the partial product and
; then divides the partial product by 4
LOOP:
pha
lda lo_product
adc lo_temp
sta lo_product
pla
adc hi_temp
ENTER:
ror
ror lo_product
lsr
ror lo_product
dey
bne LOOP
ldy lo_product
rts
.endproc
; enter with number to be divided in A
; answer returned in A
.proc div3_a
sta temp
lsr
lsr
adc temp
ror
lsr
adc temp
ror
lsr
adc temp
ror
lsr
adc temp
ror
lsr
rts
.endproc