Q. Define Stack segment ?

8086 Microprocessor supports Word stack. Stack segment parameters tell the assembler to alert linker that this segment statement states the program stack area.

A program should have a stack area in that computer is continuously carrying on numerous background operations which are completely transparent, even to an assembly language programmer, for illustration a real time clock. Every 55 milliseconds real time clock interrupts.  Every 55 ms CPU is interrupted. CPU records state of its registers and then goes about updating the system clock. When it finishes servicing system clock it has to restore registers and go back to doing whatever it was doing when interruption took place.  All such information gets recorded in stack. If your program has not any stack and if real time clock were to pulse whereas the CPU is running your program there will be no way for CPU to find the way back to your program when it was through updating the clock. 0400H byte is default size of allocation of stack. Please consider if you haven't specified stack segment it is automatically created.