加权平均4个数时的浮点异常（SIGFPE）

Question

我正在Linux上使用 NASM 创建汇编语言程序。我正在尝试对4个数字进行加权平均，其中输入的第4个数字是其他数字加权的两倍。如果我使用数字30,40,50,60，我将加权平均值计算为（30 + 40 + 50 + 60 + 60）/ 5。我的代码是：

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

segment .data 

   msg1 db "Enter exam grade 1: ", 0xA,0xD  
   len1 equ $- msg1 
   msg2 db "Enter exam grade 2: ", 0xA,0xD 
   len2 equ $- msg2 
   msg3 db "Enter exam grade 3: ", 0xA,0xD 
   len3 equ $- msg3 
   msg4 db "Enter final exam grade (worth double: ", 0xA,0xD 
   len4 equ $- msg4 
   msg5 db "The sum is: "
   len5 equ $- msg5

segment .bss

   num1 resb 3 
   num2 resb 3 
   num3 resb 3 
   num4 resb 3 
   res resb 4    

section .text
   global main    

main:            
   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, 3
   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, 4
   int 0x80       

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

   mov eax, SYS_READ 
   mov ebx, STDIN  
   mov ecx, num3 
   mov edx, 3
   int 0x80            

   mov eax, SYS_WRITE        
   mov ebx, STDOUT         
   mov ecx, msg4          
   mov edx, len4         
   int 0x80

   mov eax, SYS_READ  
   mov ebx, STDIN  
   mov ecx, num4 
   mov edx, 3
   int 0x80           

   mov eax, SYS_WRITE         
   mov ebx, STDOUT         
   mov ecx, msg5          
   mov edx, len5         
   int 0x80

   mov eax, [num1]      ; move num1 to register eax
   sub eax, '0'         ; sub '0' to convert eax bl from ASCII to decimal
   mov ebx, [num2]      ; move num2 to register ebx    
   sub ebx, '0'         ; sub '0' to convert ebx from ASCII to decimal
   mov ecx, [num3]      ; move num3 to register ecx
   sub ecx, '0'         ; sub '0' to convert ecx from ASCII to decimal
   mov edx, [num4]      ; move num4 to register edx
   sub edx, '0'         ; sub '0' to convert edx from ASCII to decimal
   add eax, ebx         ; add ebx to eax
   add eax, ecx         ; add ecx to eax
   add eax, edx         ; add edx to eax
   add eax, edx         ; add edx to eax again

   mov bl, '5'          ; sub '0' to convert bl from ASCII to decimal
   sub bl, '0'          ; make bl a decimal

   div bl               ; divide by bl
   add eax, '0'         ; add '0' to convert the sum from decimal to ASCII
   mov [res], eax       ; storing the sum in memory location res

   ; print the sum 
   mov eax, SYS_WRITE        
   mov ebx, STDOUT
   mov ecx, res         
   mov edx, 3        
   int 0x80

exit:    

   mov eax, SYS_EXIT   
   xor ebx, ebx 
   int 0x80

如果输入以下数字，我可以产生此异常：

Enter exam grade 1:
30
Enter exam grade 2:
40
Enter exam grade 3:
50
Enter final exam grade (worth double:
60
The sum is: Floating point exception

是什么原因导致我的程序显示浮点异常（SIGFPE）以及如何解决此问题？