使用SASM进行汇编编程

Question

我需要帮助。我正在尝试在SASM中运行下面的程序（NASM）。

SYS_EXIT  equ 1
SYS_READ  equ 3
SYS_WRITE equ 4
STDIN     equ 0
STDOUT    equ 1

segment .data 

   msg1 db "Enter a digit ", 0xA,0xD 
   len1 equ $- msg1 

   msg2 db "Please enter a second digit", 0xA,0xD 
   len2 equ $- msg2 

   msg3 db "The sum is: "
   len3 equ $- msg3

segment .bss

   num1 resb 2 
   num2 resb 2 
   res resb 1    

section .text
   global _start    ;must be declared for using gcc

_start:             ;tell linker entry point
   mov eax, SYS_WRITE         
   mov ebx, STDOUT         
   mov ecx, msg1         
   mov edx, len1 
   int 0x80                

   mov eax, SYS_READ 
   mov ebx, STDIN  
   mov ecx, num1 
   mov edx, 2
   int 0x80            

   mov eax, SYS_WRITE        
   mov ebx, STDOUT         
   mov ecx, msg2          
   mov edx, len2         
   int 0x80

   mov eax, SYS_READ  
   mov ebx, STDIN  
   mov ecx, num2 
   mov edx, 2
   int 0x80        

   mov eax, SYS_WRITE         
   mov ebx, STDOUT         
   mov ecx, msg3          
   mov edx, len3         
   int 0x80

   ; moving the first number to eax register and second number to ebx
   ; and subtracting ascii '0' to convert it into a decimal number

   mov eax, [num1]
   sub eax, '0'

   mov ebx, [num2]
   sub ebx, '0'

   ; add eax and ebx
   add eax, ebx
   ; add '0' to to convert the sum from decimal to ASCII
   add eax, '0'

   ; storing the sum in memory location res
   mov [res], eax

   ; print the sum 
   mov eax, SYS_WRITE        
   mov ebx, STDOUT
   mov ecx, res         
   mov edx, 1        
   int 0x80

exit:    

   mov eax, SYS_EXIT   
   xor ebx, ebx 
   int 0x80

我遇到了这个错误：

  

[20:53:11]警告！构建中出现错误：
  c：/ program files（x86）/ sasm / mingw / bin /../ lib / gcc / mingw32 / 4.6.2 /../../../ libmingw32.a（main.o）：在函数'main中“：
  C：\ MinGW \ msys \ 1.0 \ src \ mingwrt /../ mingw / main.c：73：未定义引用`WinMain @ 16'

另外，如何限制用户输入最多4位数？