understanding heap memory structure

I'm currently learning heap exploitation and overflows. But currently i'm stuck with some issue. I'm using glibc v2.19 , when i try to examine the heap memory with gdb.

Here is the code that i'm using.

    char *p;
    p = malloc(20);
    strcpy(p,"AAAAAAAA");
    free(p);

This is the gdb output for the allocated chunck.

(gdb) x/4xw 0x0804b008-8
0x804b000:  0x00000000  0x00000019  0x41414141  0x41414141

This is the gdb output for the unallocated chunck.

(gdb) x/4xw 0x0804b008-8
0x804b000:  0x00000000  0x00000019  0x00000000  0x41414141

I can see that the size field do not change after deallocation, so how this could be and how the allocator know that is this a free chunck or not? because as i know the first 3 bits of the size is used as status flags.

Why the next pointer "0x0000000" is not pointering any where ?

And why the pointer to prevoius chunck "0x41414141" is not used?

And how in this situation the allocator looks for free chuncks?

i also have tried to free two continious chuncks and after i freed the second one the prevoius size field which should be used as the size of the prevoius free chunk is not also used it is just zerod.