Get Memory Size With INT 12

Get Memory Size With INT 12

I want to get memory size in assembly with int 12 but when i call this
interrupt it is only gives 639.What is 639 means?(I converted integer to
String)
Ex:
bits 16
org 0x0
start:
int 12h;GET MEMORY TO AX (KB)
mov bx,ax ;BX=AX
call int_to_str ;IN:BX(INT)-OUT:BX(STRING)
mov si,bx ;SI=BX
call print_string ;PRINT SI
mov ax,10h ;KEY STROKE
int 16h
ret
This Code Gives Only 639.I have not understood yet.Please Help.Thanks!