Working with the Unix OS

Processes I
8. PROCESSES (I)
Process Subsystem Details
In Unix, a process is the execution of a program and consists of a pattern of bytes containing:
- machine instructions (text)
- data
- stack
Several processes may all be instances of one program.
Processes follow sets of instructions of their own and not of others and may not read or write data or stack of
another process.
Processes
- Created by "fork" system call (all except process 0)
- Invoking process: parent
- New Process: child
- Every process has one parent, but parent may have many children.
- Kernel identifies files by process ID (PID)
- Process 0 created "by hand" at boot. After "forking" a child process, it becomes the 'swapper". Its child is
called "init".
- "init" is the ancestor of all other processes on the system.
- When a user compiles a source program, an executable file is created which contains:
- Set of headers describing the attributes of the file
- Program text
- Initialized data and an indication of the space needed for uninitialized data
- other sections e.g.: symbol table info.
Executables
- Image, etc. loaded into memory during an 'exec' system call.
- When loaded, consists of 3 "regions":
- Text
- Data
- Stack
- Process has 2 stacks: 1 for user mode, 1 for kernel mode
- Processes enter kernel mode by executing "trap" instruction which causes a hardware mode switch.
Kernel Process Table
- One entry per process
- Each process is allocated a "u_area" which contains private data manipulated only by the kernel.
- This points to a "per process region table" which points to a "region table".
- A region is a contiguous area of a process's addressable space ( i.e.: text, data, stack).
- Extra indirection is in place so that data spaces may be "shared" between processes
- Process table entry and u area entry contain the control and status information for a process. The u area is
basically an extension of the process table.
- u_area info is only needed when the process is executing.
- Process table entries are needed by the scheduler.
Context of a Process
- Process's state:
- Text
- Values of global user variables and data structures
- Values of machine registers
- Values in its process table entry and its u_area
- Contents of its user and kernel stacks
- "Context switch" change of active process.
- Interrupts are handled in the context of the current process, not necessarily the originator.
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