Home » Forums » Algorithms & Source Code » Adding 256 bit integers

Adding 256 bit integers
I am trying to add two 256-bit integers together, but can only get 64-bit ints to work. Any help? Here is what I have:

<CODE>

TITLE Addition          (main.asm)

INCLUDE Irvine32.inc

.data
op1 QWORD 0A2B2A40674981234h
op2 QWORD 08010870000234502h
sum DWORD 3 dup(0FFFFFFFFh) ; = 0000000122C32B0674BB5736

.code
main PROC

mov esi,OFFSET op1 ; first operand
mov edi,OFFSET op2 ; second operand
mov ebx,OFFSET sum ; sum operand
mov ecx,2          ; number of doublewords
call Extended_Add

; Display the sum.
mov eax,sum + 8 ; display high-order dword
call WriteHex
mov eax,sum + 4 ; display middle dword
call WriteHex
mov eax,sum ; display low-order dword
call WriteHex
call Crlf

exit
main ENDP

Extended_Add PROC
pushad
clc                ; clear the Carry flag

L1: mov eax,      ; get the first integer
adc eax,      ; add the second integer
pushfd                          ; save the Carry flag
mov ,eax      ; store partial sum
add esi,4        ; advance all 3 pointers
add edi,4
add ebx,4
popfd              ; restore the Carry flag
loop L1          ; repeat the loop

mov dword ptr ,0 ; clear high dword of sum
adc dword ptr ,0 ; add any leftover carry
popad
ret
Extended_Add ENDP
END main

</CODE>
Posted on 2010-03-02 10:54:08 by program_man
Re: Adding 256 bit integers
Do you understand how your data is laid out?
Since you are doing addition left to right, lowest dword of your 256 integer should be leftmost, highest rightmost.
op1 dd LOWDWORD, ? , ? ,  ? ,  ? ,  ? ,  ? , HIGHDWORD
op2 dd LOWDWORD, ? , ? ,  ? ,  ? ,  ? ,  ? , HIGHDWORD
sum dd LOWDWORD, ? , ? ,  ? ,  ? ,  ? ,  ? , ? , HIGHDWORD

Besides that i can't see anything wrong with your code.
Posted on 2010-03-02 11:53:44 by drizz
Re: Adding 256 bit integers
I understand the concept, but I cannot get two 256-bit integers to add and output.
Posted on 2010-03-02 12:01:01 by program_man
Re: Adding 256 bit integers
Your code should work fine. I don't see the problem.

  mov   esi,OFFSET op1      ; first operand
   mov   edi,OFFSET op2      ; second operand
   mov   ebx,OFFSET sum      ; sum operand
   mov   ecx,8              ; number of doublewords
   call   Extended_Add

; Display the sum.

   mov   eax,sum + 8*4      ; display high-order dword
   call   WriteHex
   mov   eax,sum + 7*4      ;
   call   WriteHex
etc...
   mov   eax,sum         ; display low-order dword
   call   WriteHex
   call   Crlf
Posted on 2010-03-02 13:24:25 by drizz
Re: Adding 256 bit integers
program_man,

Your code works (you've linked it with /subsystem:console option, haven't you?), though it can be improved.

lea ebx, leaves eflags untouched, you may replace adds with it.

There is a way to address both sources and destination with only one register being increased: load ebx with address of destination, esi with source1 - destination, edi with source2 - destination and address sources with / .

mov / adc dword ptr , 0 to save one-bit carry? sbb eax, eax / neg eax / mov , eax looks better.
Posted on 2010-03-03 11:20:32 by baldr