Operating Systems foe ES (Schedulers)

Question 1

High level schedule?

Answer 1

Scheduler allows tasks to be called (periodically or sporadically)

Question 2

Low-level Schedule?

Answer 2

Scheduler can be viewed as a single timer interrupt service routine that is shared between many different tasks

Question 3

Preemptive, non-preemptive?

Answer 3

preemptive - tasks can be interrupted

non-preemptive - tasks always run to completion

Question 4

typecast the pointer: ‘void *aNumber’ to an integer pointer ‘intPtr’

Answer 4

int *intPtr = (int *)aNumber

Question 5

What is difference between:

1. (*intPtr)++; and
2. intPtr++;

Answer 5

referencing and dereferencing.

1. increments the integer saved in *intPtr
2. increments the intPtr address to next entry

Question 6

Round Robin limitations?

Answer 6

• tasks run in fixed order (no skipping/priorities)

- no control over timing

Question 7

What is an Event-driven loop?

Answer 7

• tasks executed upon events
• tasks implemented as ISRs
• infinite while loop which gets interrupted

Question 8

number of tasks in a tasklist tasks[]? Code?

Answer 8

numTasks = sizeof(tasks)/sizeof(tasks[0])

Question 9

Difference between cooperative and preemptive scheduler?

Answer 9

Cooperative: tasks decide themselves when to halt and yield to another task
Preemptive: Scheduler decides when to halt current task and resume other task

Question 10

What does context switch mean?

Answer 10

Changing of running task is called context switch

Question 11

What are the manual steps of context switch?

Answer 11

• context pushed on stack: PC pushed by ISR automatically and r0 to r31, SREG and stack pointer are pushed manually
• restoring context from stack pointer of current TCB (task control block)

Question 12

Define Concurrent Task Model

Answer 12

Describes functionality of system in terms of concurrently executing subtasks disregarding interleaving aspects

Question 13

How to achieve communication between tasks?

Answer 13

• shared memory

- message passing

Question 14

What is mutual exclusion and how is it achieved?

Answer 14

• when tasks enters critical sections all other tasks must be locked out until it leaves critical section
• Mutex and Semaphores