comp sci 2

A bunch of instructions stored in a computer that tells it what to do.

Software

Software like Chrome, Word, Zoom.

Application software

Software like Windows, macOS, antivirus.

System software

Helps the parts of a computer work together—manages programs, memory, and devices.

Operating System (OS)

The OS decides which programs can use the CPU and for how long.

Process management

A program that is actively running.

Process

Ready (waiting to run), Running (using CPU), Waiting (paused for input), and Terminated (done).

Main states of a process

Choosing which program gets to use the CPU next when many are ready.

CPU scheduling

The program keeps using the CPU until it's done, no interruptions.

Non-preemptive scheduling

The OS can pause a program to let another one run.

Preemptive scheduling

The total time from when a program enters the system to when it finishes.

Turnaround time

Programs run in the order they arrive. No skipping.

First-Come, First-Served (FCFS) scheduling

If a program freezes or takes forever, everything else gets stuck waiting.

Downside of FCFS

The OS picks the program with the shortest run time next.

Shortest Job Next (SJN) scheduling

You don't always know how long a program will take.

Problem with SJN

Each program gets a fixed time to run. If it's not done, it waits its turn again.

Round Robin scheduling

It's preemptive—the OS can stop a program after its time slice ends.

Round Robin preemptive or non-preemptive

The small amount of time each program gets to use the CPU before the next one gets a turn.

Time slice (or quantum)

Round Robin—it shares time fairly between all programs.

Best CPU scheduling method for fairness

Shortest Job Next (SJN) if job times are known.

Best method for average turnaround time