SXMicrocontrollerMathMethod

24x24 multiplication

from Nikolai Golovchenko

```Product         DS      6
Multipland      DS      3
BitCount        DS      1

Multiplier      equ     Product+3       ;3 bytes shared with Product's
;less significant bytes (+3...5)

MULTIPLY_24x24
mov     W, #\$AB
mov     Multipland, W
mov     W, #\$CD
mov     Multipland+1, W
mov     W, #\$EF
mov     Multipland+2, W

mov     W, #\$98
mov     Multiplier, W
mov     W, #\$76
mov     Multiplier+1, W
mov     W, #\$54
mov     Multiplier+2, W

; these values should generate the reply = \$6651AF33BC6C

;24 x 24 Multiplication
;Input:
; Multiplier - 3 bytes (shared with Product)
; Multiplicand - 3 bytes (not modified)
;Temporary:
; Bitcount
;Output:
; Product - 6 bytes

clr     Product ; clear destination
clr     Product+1
clr     Product+2

mov     W, #24
mov     BitCount, W     ; number of bits

rr      Product+3       ;shift out to carry
rr      Product+4       ;next multiplier bit
rr      Product+5

jnc     SKIP_LOOP_24x24 ; if carry is set we must add
; multipland
; to the product

mov     W, Multipland+2 ; get LSB of multiplicand
add     Product+2, W    ; add it to the lsb of the product

mov     W, Multipland+1 ; middle byte
snb     C               ; check carry for overflow
movsz   W, ++Multipland+1 ; if carry set we add one to the source
; that case mulitpland = \$ff->\$00 )

mov     W, Multipland   ; MSB byte
snb     C               ; check carry
movsz   W, ++Multipland
add     Product, W      ; handle overflow

SKIP_LOOP_24x24
; note carry contains most significant bit of

; shift in carry and shift out
; next multiplier bit, starting from less
; significant bit

rr      Product
rr      Product+1
rr      Product+2
rr      Product+3
rr      Product+4
rr      Product+5

decsz   BitCount
ret

```

 file: /Techref/scenix/lib/math/mul/24x24ng_sx.htm, 5KB, , updated: 2004/6/10 14:40, local time: 2019/7/20 06:46, owner: NG--944, TOP NEW HELP FIND:  18.209.104.7:LOG IN

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