Segmentation fault in x86 code, using a function

Question

I have trying to write a program in x86 and it's giving me segmentation fault. I have narrowed it down to this segment. Why is this giving a segmentation fault? I did the same thing without the program (that is write the same control flow) and there was no error

 segment .bss

 a resb 4
 b resb 4
 m resb 4
section .text
global _start    ;must be declared for using gcc

toh :

pop eax
mov [a],eax
mov eax, 1
mov [b],eax
mov eax, [b]
push eax
ret

_start:             ;tell linker entry point

mov eax,2
push eax
call toh
pop eax
mov [m],eax

mov eax,1
int 0x80