Machine virtualization

The UNIX operating system is a 32-bit, virtual memory, demand paging operating system. The most important aspects of this for developers familiar with the DOS architecture are:

• The UNIX operating system is a preemptive multitasking system. This means that the program does not need to relinquish control to the kernel in order to switch tasks.

• When more than one process accesses a file or other resource, the application must be coded to synchronize that access. For example, a file may need to be locked before it is updated, to ensure that another process does not try to update that file at the same time. Both advisory and mandatory file locking are supported on embedded systems.

• The program does not need to be aware of any other programs running at the same time. In particular, it does not need to be aware of concurrent access of devices (such as hard disks) by other programs.

The UNIX kernel supports machine virtualization by:

• Preemptive scheduling of processor time fairly between processes. Each process is given a priority based on a number of parameters. Processes that have used the least CPU time in previous time slices (probably due to waiting on hardware events to occur) have their priorities increased, whereas processor-bound tasks have their priorities decreased.

  Note that the embedded and SCO OpenServer systems include a fixed-priority scheduler that enables you to set some processes to always run at a higher priority than other processes.

• Isolation of memory between processes. Each process can only access memory that has been allocated to it. Under DOS, although a well-behaved program would normally allocate memory before use, there is no enforced protection and bugs can be difficult to find. Under the UNIX system, any attempt to refer to an unallocated memory address results in the program being stopped. An image of the terminated process (called a ``core dump'') is saved for debugging purposes.
  

• Isolation of device address between processes. Any process can access any device through a device driver without being concerned with the state of the device due to any other process accessing it concurrently. For devices such as terminals, this means that two processes accessing a terminal can write to it through the device driver at the same time without worrying about interfering with the UART register setup, although the output from both processes would appear mixed together on the screen. For devices such as hard disks, this means that two processes could write to the disk (through the device driver) without worrying about moving the disk head in the middle of a read or write operation for any other process.

• Isolation of function between kernel and user processes. Only the kernel contains program code for memory management, filesystem management, process management, and device drivers. These are all critical areas for the integrity of the operating system services and so are kept within the kernel and protected from user-level processes. User-level processes issue system calls to interact with these kernel services.