在masm x86中使用扩展加法方法，实现256位数的加法可以通过如下代码实现：

; ---------------------------------------------
; Adding 256-bit numbers using Extended Addition 
; ---------------------------------------------

.486
.model flat, stdcall
option casemap :none

;-----------
; Constants
;-----------
SZ_QWORD          EQU  8
SZ_DWORD          EQU  4

;------------------
; Function Prototypes
;------------------
PUBLIC Add256
PUBLIC ReversedAdd256


;------------
; Externals
;------------
extern _memcpy : proc

;--------------------------
; Structure - Quadword Pair
;--------------------------
STRUCT PAIR
   value  QWORD      ?
   reserve QWORD      ?
ENDS

;----------------
; Macro Functions
;----------------
repeat 2                       ;  (1)
   MACRO LOADPAIR dest,src
      mov     eax, src
      mov     edx, [eax + SZ_QWORD]
      mov     eax, [eax]
      mov     dest.value, eax
      mov     dest.reserve, edx
   ENDM
endm

repeat 2                       ;  (2)
   MACRO SAVEPAIR dest,src
      mov     eax, dest
      mov     edx, dest.reserve
      mov     [eax + SZ_QWORD], edx
      mov     [eax], dest.value
   ENDM
endm

;-----------
; Add256
;-----------
PUBLIC Add256
Add256 PROC pDst:PTR QWORD, pSrc:PTR QWORD

    LOCAL Dest:PAIR
    LOCAL Src:PAIR
    LOCAL Carry:PAIR
    LOCAL Sum:PAIR

    MOV     EDX, pDst
    MOV     ECX, pSrc

    CALL    LOADPAIR         ; Load dst and src pairs 1
    CALL    LOADPAIR         ; Load dst and src pairs 2

    ; Zero the High dword
    xor     ecx,ecx
    xor     edx,edx
    xor     ebx,ebx

    ; Do addition using Extended method. 
    add     ebx, ax          ; eax -= ZeroFill(ebx)
    adc