Both Dos and FreeBSD support multitasking. System calls can be carried out in either user mode or kernel mode. Once a program is loaded in RAM and assigned a CPU, it becomes a process. The original program is then removed from the storage device. If a process contains a Stack, then it would not contain a Heap. One program can be the base for several processes. operating system failure can generate a core dump file containing values of the kernel memory. If an OS has a CLI, then it generally would not provide a GUI. When passing parameter via a table in memory, the address of the table generally is passed using a register. Android is based on Linux kernel with some modification, such as power management. For the same function, Windows system call and Unix system call have the same syntax. Microkernel System Structure is the best approach for building operating systems in every aspect