I am trying to write assembly program to multiply two 32-bit signed numbers and store the answer in 64-bit number but my code only gives me the correct answer up to 32-bits. I have goggled it and searched many sites but can't satisfy myself. I can't get the problem although i debugged it many times.This is my first question as a student so I am really sorry if I am unclear.Thanks :)
; 32bit multiplication
[org 0x0100]
multiplicand dd 0,9000008 ; 32bit multiplicand 64bit space
multiplier: dd 45009 ; 32bit multiplier
result: dd 0,0 ; 64bit result
start:
mov cx,32 ; initialize bit count to 32
mov bx, [multiplier] ; load multiplier in dx and bx
mov dx, [multiplier+2]
checkbit:
sar dx,1 ; move right most bit in carry
rcr bx,1
jnc skip ; skip addition if bit is zero
mov ax,[multiplicand]
add word[result],ax
mov ax,[multiplicand+2]
adc word[result+2],ax
mov ax,[multiplicand+4]
adc word[result+4],ax
mov ax,[multiplicand+6]
adc word[result+6],ax
mov ax,[multiplicand+8]
adc word[result+8],ax
mov ax,[multiplicand+10]
adc word[result+10],ax
mov ax,[multiplicand+12]
adc word[result+12],ax
mov ax,[multiplicand+14]
adc word[result+14],ax
skip:
shl word [multiplicand],1
rcl word [multiplicand+2],1
rcl word [multiplicand+4],1
rcl word [multiplicand+6],1
rcl word [multiplicand+8],1
rcl word [multiplicand+10],1
rcl word [multiplicand+12],1
rcl word [multiplicand+14],1
dec cx
jnz checkbit
mov ax,0x4c00 ; terminate program
int 0x21
I think my logic is right but then i can't get what the error is. Any help is appreciated. :)
multiplicand dd 0,9000008 ; 32bit multiplicand 64bit space
You setup 64 bit space but your code modifies 128 bit space!!
Also because of little endeanness you should swap the dwords of the multiplicand.