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Nucleus (kernel)

• core of the OS
• lowest level of software on the central processor (CPU)
• four crucial functions: process management, short-term scheduling (CPU scheduling), interrupt handling, and interprocess communication (IPC)
• there are routines that manage the registers (central storage), time, and devices (interrupt handling)
• responds to requests and errors from processes (which are received as ``traps'' (sometimes called exceptions or software interrupts)) and requests from devices (which are received as interrupts (true interrupts or hardware interrupts))
• some processors provide a special syscall instruction so that system calls can be executed without causing traps
• provides the environment in which processes exist
• every process depends ultimately on the nucleus
• the term ``kernel'' is used in UNIX documentation to refer to the combination of the nucleus, I/O, memory management, and file system layers

			PROCESSES 

			    |
			    |	request (sent as a trap or syscall instruction after a system call)
                            |   or error (sent as a trap)
			    |
			   \|/
			    V
________________________________________________________________________
|                                                                       |     
|    			N U C L E U S                                   |
|_______________________________________________________________________|
			    .
			   /|\
			    |
			    |	request (sent as an interrupt)
			    |

			 DEVICES 

Process

• a program in execution (simple definition)
• a unit of program execution with at least one thread of control (less simple definition!)
• a process is an active entity, while a program is a static entity

Process Control Block (PCB)

• data structure used to represent one process
• the set of pcb's is called the process table
• this data structure is maintained by the nucleus
• contains all the information required to describe the current state of a program in execution
• when a process is suspended, the OS saves the current context for execution in the process control block for that process
• when a process is to be resumed, the OS loads the current context for execution from the process control block for that process
• contains:
  • process identifier (pid), which is the unique identifier for the process--it may be implicitly known from position,
  • program counter (PC), which gives the address in main memory of the next instruction to be executed
  • processor status (PS),
  • (other) register values,
  • process state, which might be running, ready, waiting, etc.
  • ppid = the process identifier of the parent process, which is the process that created this one
  • scheduling information, such as priority and pointers to scheduling queues
  • memory management info, such as limits (base and limit) and pointers to segment or page tables,
  • accounting information,
  • open file pointers,
  • device descriptions

Example Process Control Block

Process State Graph

Switch for Interrupt Handling

• switch from running a process to running an interrupt handler, typically followed by a switch back to running the same process
• In this type of context-switch, the goal is to temporarily interrupt the running process to allow an interrupt to be serviced.

Steps:

1. when an interrupt occurs, the CPU first finishes the current instruction
2. switch the CPU to its privileged mode (system mode) -- this switch occurs automatically when the CPU receives an interrupt (after finishing the current instruction)
3. copy the register values for the old process to a safe place, such as a stack (stored in memory)
4. run the interrupt handler
5. restore the values for the registers form the safe place, such as the stack
6. switch the CPU back to user mode

• The timer interrupt may cause the scheduler to run, which may change the states of processes from RUNNING to READY or vice versa.

Privileged mode (system mode) - all commands (instructions) can be executed (e.g. halt) and all memory can be accessed, including the system area

Context Switch between Processes

• switch the use of the CPU from the running process (the old process) to a ready process (the new process).
• a ready process is a runnable process that currently does not have the CPU, i.e., one that could run if given the CPU.
• In a context-switch, the goal is to remove the one process from the CPU and replace it with a different process.
• Typically, the part of the OS called the CPU scheduler decides when a context switch between processes should occur.

Steps:

1. switch the CPU to its privileged mode (system mode)
2. copy the register values for the old process to the process table (stored in the OS's part of memory)
3. load register values from the process table for the new process
4. switch the CPU back to user mode

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