Question

What is a process control block? What kind of values does it store? When are the times it is used? What is an equivalent model for threads? 2)Group 1 contains some CPU scheduling algorithms and Group 2 contains some applications. Match entries in Group 1 to entries in Group 2. Give a brief explanation of your matching criteria. Group l (P) Shortest Job First Scheduling Group II (1) Thread Scheduling (2) Static Process (Q) Process Contention Scope (PCS) and System Scheduling with Contention Scope (SCS) Throughput a based Scheduling Average Waiting Time Requirements

Answer 1

1. While creating a process the operating system performs several operations. To identify these process, it must identify each process, hence it assigns a process identification number (PID) to each process. As the operating system supports multi-programming, it needs to keep track of the all the processes. For this task, the process control block (PCB) is used to track the process’s execution status. Each block of memory contains information about the process state, program counter, stack pointer, status of opened files, scheduling algorithms, etc. All these information is required and must be saved when the process is switched from one state to another. When the process made transitions from one state to another, the operating system must update information in the process’s PCB.

A process control block (PCB) contains information about the process, i.e. registers, quantum, priority, etc. The process table is an array of PCB’s, that means logically contains a PCB for all of the current processes in the system.

• Pointer – It is a stack pointer which is required to be saved when the process is switched from one state to another to retain the current position of the process.
• Process state – It stores the respective state of the process.
• Process number – Every process is assigned with a unique id is known as processed which stores the process identifier.
• Program counter – It stores the counter which contains the address of the next instruction that is to be executed for the process.
• Register – These are the CPU registers which includes: accumulator, base, registers and general purpose registers.
• Memory limits – This field contains the information about memory management system used by operating system. This may include the page tables, segment tables etc.
• Open files list – This information includes the list of files opened for a process.

For threads, we use Thread Control Block.

Thread Control Block (TCB) is a data structure in the operating system kernel which contains thread-specific information needed to manage it. The TCB is "the manifestation of a thread in an operating system."

An example of information contained within a TCB is:

• Thread Identifier: Unique id (tid) is assigned to every new thread
• Stack pointer: Points to thread's stack in the process
• Program counter: Points to the current program instruction of the thread
• State of the thread (running, ready, waiting, start, done)
• Thread's register values
• Pointer to the Process control block (PCB) of the process that the thread lives on

2. P) Shortest Job First -> 2) Static process Scheduling with Throughput and Average Waiting Time RequirementS

Explanation: SJF scheduling is used to schedule static process with the aim to minimise throughput and average waiting time.

Q) Process Contention Scope and Process Contention Scope based Scheduling -> 1) Thread Scheduling

Explanation: Process Contention Scope is one of the two basic ways of scheduling threads. Both of them being: process local scheduling (known as Process Contention Scope, or Unbound Threads—the Many-to-Many model) and system global scheduling (known as System Contention Scope, or Bound Threads—the One-to-One model).