Salient points about Unix/Linux

• Time sharing system
• Each user file is simply a sequence of bytes.
• Disk files and I/O devices are treated as similarly as possible.
• CPU scheduling is simple priority algorithm.
• For unix development intent was to have the kernel and libraries provide a small set of facilities that was sufficiently powerful to allow a person to build a more complex system if one were needed.
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• The kernel is not a separate set of processes that run in parallel to user processes, but is part of each user process.
  
• The kernel permanently resides in main memory as does the currently executing process (or part of it at least).
  
• Interrupt handler runs in the context of currently running process.
• All system calls return -1 on failure.
• When a process executes a system call, the execution mode of the process changes from user mode to kernel mode.
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• Every process except process 0 is created when another process executes the fork system call. Process 0 is a special process that is created bby hand when the system boots. After forking a child process (Process 1), process 0 becomes a swapper process. Process 1, known as init, is the ancestor of every other process.
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• Kernel cannot preempt a process and switch context to another process while executing in kernel mode
• Kernel masks out interrupts when executing a critical region of code if an interrupt handler could corrupt kernel data structures.
  
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Linux Terms/Concepts

Bottom Halves