Operating systems: roles, functions and scheduling
Connect operating-system services to resource management, then compare four CPU scheduling approaches.
The OS sits between applications and hardware resources
- describe how an OS abstracts hardware complexity while managing resources;
- describe memory, file, device, scheduling, security, accounting, GUI, virtualization and networking functions;
- compare first-come first-served, round robin, multilevel queue and priority scheduling;
- select a scheduling approach from system requirements.
Rapid Recall Deck
Attempt each card first. If a term is new, flip and study it, then answer it again without looking. Mark only secure recall as “Know it”.
Functions become easier when tied to resource requests
Map the request to an OS function
For each event, name the primary OS function involved.
- A program requests 500 MB of working memory.
- A user saves a file into a folder.
- A print job is sent to a printer.
- The OS records how much CPU time a user consumed.
- A user signs in and permissions are checked.
- A virtual machine is created.
- Memory management.
- File-system management.
- Device management.
- Accounting.
- Security/authentication/authorization.
- Virtualization.
Abstraction is useful only if the shared system remains controlled
GUI + networking
Describe one role of the GUI and one role of OS networking support.
The GUI provides a graphical interaction layer for users/applications, translating user actions into system operations. Networking support manages interfaces/protocol services and resource access needed for processes to communicate over networks.
System integrity
Explain why background OS operations must preserve system integrity while multiple programs are running.
The OS must prevent one process from corrupting another process or critical system resources, coordinate shared hardware safely and enforce access rules so concurrent activity does not destabilize the system.
Scheduling policy encodes what the system values
FCFS vs round robin
A batch-processing system values simplicity and jobs usually run to completion. An interactive desktop needs regular responsiveness for many active programs. Compare the two scheduling approaches and choose a fit for each.
FCFS is simple and processes jobs in arrival order, but a long job can delay all later jobs. It can suit straightforward batch workloads. Round robin gives each ready process a time slice, improving responsiveness/fairness for interactive workloads, though frequent context switching adds overhead.
Priority vs multilevel queue
Compare a scheduler that ranks individual processes by priority with one that separates process classes into different queues.
Priority scheduling selects work according to assigned priorities, useful when urgent tasks must run first but risking starvation of low-priority work. Multilevel queue classifies processes into separate queues (for example interactive vs batch) that can receive different policies/priorities, giving stronger separation between workload classes but adding scheduling complexity.
A short trace makes the scheduling difference concrete
Mini scheduling trace
Three processes are ready at time 0: P1 needs 5 CPU units, P2 needs 2, P3 needs 1. Ignore context-switch overhead.
A. Under FCFS in order P1, P2, P3, which process finishes first? B. Under round robin with time slice 1, which process finishes first?
FCFS: P1 finishes first because it runs to completion before P2/P3. Round robin (quantum 1): after P1, P2 and P3 each receive one unit; P3 then finishes on its first slice, so P3 is first to complete.
A scheduling answer needs both benefit and risk
Choose a scheduler
A hospital monitoring computer has emergency alarm tasks that must pre-empt ordinary background reporting, while routine processes still need eventual service.
Priority scheduling is a natural fit because emergency tasks can receive higher priority. However, continuously arriving high-priority work can starve low-priority tasks. A scheduler can mitigate this with policies such as aging/priority adjustment or carefully designed priority classes so waiting work eventually receives CPU time.
Paper 1 can move from factual role to comparison
Paper 1-style mini-set
1. Outline the role of accounting in an operating system.
2. Describe two OS functions other than accounting.
3. Compare round-robin and first-come first-served scheduling.
- Accounting records/tracks resource usage such as CPU time, storage or other services for monitoring, quotas, analysis or billing.
- Any two accurate functions with purpose, such as memory allocation/protection, file management, device management, security, scheduling, GUI, virtualization or networking.
- Both allocate CPU execution among processes; FCFS uses arrival order and may let long jobs delay later ones, while round robin cycles through processes using time slices to improve responsiveness/fairness at the cost of switching overhead.
Finish the learning cycle
IA — main task
Continue Criterion D development/testing. Start wrapping Criterion D evidence so each important technique has a reason and test, not just code screenshots.
Paper 1 — short practice
From memory, list all nine OS functions in the Guide and compare FCFS, round robin, multilevel queue and priority scheduling in one table.