NASM division using variables instead of actual values

I’m learning some basic arithmetic using NASM on Linux. I need to divide two numbers using variables NUMBER1 and NUMBER2. My code works if I type in actual value instead of variables. For example if I type in ‘6’ instead of NUMBER2 and ‘2’ instead of NUMBER1, the program does division and gives me answer of 3. Running code with variables gives FLOATING EXCEPTION (CORE DUMPED). could please explain how correctly use variables in this code? While debugging, I see that the problem is in DIV line. Thank you !

 section .text

global main ;must be declared for using gcc

main: ;tell linker entry point
mov ax,[NUMBER2]
sub ax, '0'
mov bl, [NUMBER1]
div bl
add ax, '0'
mov [res], ax
mov ecx,msg
mov edx, len
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
nwln
mov ecx,res
mov edx, 1
 mov ebx,1 ;file descriptor (stdout)
 mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel

section .data
NUMBER1: dw 2
NUMBER2: dw 6
msg db "The result is:", 0xA,0xD
len equ $- msg

segment .bss
res resb 1

Answer

Because given example is supposed to process ASCII codes for numbers, not numbers themselves. If you enter 6 instead of '6', 6 - '0' evaluates to 65494 (not 6). If you try to divide further by 2, processor is unable to store quotient in lower half of ax register.

If you do not intend to output result to console and only try to learn how division with one byte integer works using assembler, pick your favourite debugger, place breakpoint after division operation and enjoy your result.

section .text

global main ;must be declared for using gcc

main: ;tell linker entry point
mov ax,[NUMBER2]
mov bl, [NUMBER1]
div bl
nop ; quotient at al, remainder at ah
; remove nop instruction if your code is larger than few lines
; place output code here

section .data
NUMBER2: dw 6 ; dw means two bytes
NUMBER1: db 2 ; db means one byte

segment .bss
res resb 1