Working with the Unix OS

Processes I
- Makes sure all processes get a 'fair go'
- If insufficient main memory, then main memory processes swapped to a secondary memory device.
- Two policies for this: Demand paging swapping
- Usually called the 'swapper'
Scheduler Module
- Allocates the CPU to processes
- Processes run till they voluntarily give up the CPU (waiting on a device for example) or until the scheduler
preempts them when time's up.
- Scheduler chooses the highest priority eligible process to run.
Hardware Control
- Responsible for handling interrupts and for communicating with the machine
- Interrupts are handled by special functions in the kernel (as we have discuss recently)
Inter-Process Communication
- Asynchronous signaling of events
- Synchronous transmission of
messages between processes
The Structure of Processes
The process table entry and the u area are
part of the context of a process.
Process states
1. executing in user mode
2. executing in kernel mode
3. is ready to run, resides in main
memory
4. is sleeping, resides in main memory
5. is ready to run, waiting on swapper
6. is sleeping, waiting on swapper
7. is returning from kernel to user mode,
but kernel preempts it
8. is newly created, process exists, but is
not ready to run, nor is it sleeping
executed the exit system call, is a
zombie, but contains an exit code and
timing statistics
Figure 6. Process State
Transition Diagram
Process table fields
- state field
- locate process and its u area
- process size
- user identifiers (UIDs)
- process identifiers (PIDs)
- event descriptor
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- scheduling parameters
- signal field
- various timers