[OS] Basics of OS : Computer System Operation

Some basic knowledge of the structure of Computer System is required to understand how Operating Systems work.

→ A modern general-purpose computer system consists of one or more CPUs and a number of device controllers connected through a common bus that provides access to shared memory.

 

• Each device controller is in charge of a specific type of device

• The CPU and the device controllers can execute concurrently, competeing for memory cycles

  → Main memory is limited (2GB, 4GB, 8GB ...), When executed someting, there are loaded into the memory

  → "Concurrently" means all these devices or controllerss can execute all at the same time

• To ensure orderly access to the shared memory, a memory controller is provided whose function is to synchronize access to the memory

Some important terms

1. Bootstrap Program

• The initial program that runs when a computer is powered up or rebooted.

• It is stored in the ROM(Read Only Memory)

• It must know how to load the OS and start executing that system

  → OS is System Software, Bootstrap Program know where OS is, execute OS, load the OS kernerl into main memory

• It must locate and load into memory the OS Kernel

2. Interrupt (= someone disturbs)

• The occurence of an event is usually signalled by an Interrupt from Hardware or Software

  → CPU is always working, sometimes the hardware or software may interrupt the CPU, then interrputed CPU just wait with whatever you are doing and please execute this task(more important) so the CPU has to stop and it has to execute that task which comes up and that is known as an interrupt.

• Hardware may trigger an interrupt at any time by sending a signal to the CPU, usaully by the way of the system bus

3. System Call ( Monitor call)

• Software may trigger an interrupt by executing a special operation called System Call

Interrupt flow

1. When the CPU is interrupted, it stops what it is doing and immediately transfers executing to a fixed location.

• fixed location : usually contains the starting address where the Service Routine(=interrupts want to do) of the interrupt is located

2. The Interrupt Service Routine executes.

3. On completion, the CPU resumes the interrupted computation.

 

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