set result to zero
for each character:
    if character not valid handle "invalid character error" somehow
    else
        if result is larger than "max/10" handle "overflow error" somehow
        result = result * 10
        digit = character - '0'
        if result is larger than "max - digit" handle "overflow error" somehow
        result = result + digit

;esi = address of source number
    ;edi = address of destination number
    ;cl = shift count

    mov edx,[esi+12]
    mov eax,[esi+8]
    shld edx,eax,cl
    mov [edi+12],edx
    mov edx,eax
    mov eax,[esi+4]
    shld edx,eax,cl
    mov [edi+8],edx
    mov edx,eax
    mov eax,[esi]
    shld edx,eax,cl
    mov [edi+4],edx
    shl eax,cl
    mov [edi],eax

;esi = address of first source number
    ;edi = address of second source number and destination

    mov eax,[esi]
    add [edi],eax
    mov eax,[esi+4]
    adc [edi+4],eax
    mov eax,[esi+8]
    adc [edi+8],eax
    mov eax,[esi+12]
    adc [edi+12],eax

;eax = first number
    ;edi = address of second number and destination

    add [edi],eax
    adc dword [edi+4],0
    adc dword [edi+8],0
    adc dword [edi+12],0

;eax = first number
    ;edi = address of second number and destination

    sub [edi],eax
    sbb dword [edi+4],0
    sbb dword [edi+8],0
    sbb dword [edi+12],0

;esi = address of first source number
    ;edi = address of second source number

    mov eax,[esi+12]
    cmp [edi+12],eax
    jb .smaller
    ja .larger
    mov eax,[esi+8]
    cmp [edi+8],eax
    jb .smaller
    ja .larger
    mov eax,[esi+4]
    cmp [edi+4],eax
    jb .smaller
    ja .larger
    mov eax,[esi]
    cmp [edi],eax
    jb .smaller
    ja .larger
    mov al,0         ;Values are equal
    ret

.smaller:
    mov al,-1        ;First value is smaller than second
    ret

.larger:
    mov al,1         ;First value is larger than second
    ret