Chapter 1 2 Operating Systems Layered Operating System

Chapter 1. 2 Operating Systems

Layered Operating System model io at ic pl Ap Operating n Hardware System

Absolute Binary Loader Central Memory s 3, t 4 ABL

Command Interpreter Central Memory chess Fmgr. Cmnd. Int. ABL s 3, t 4

Interactive usage

Batch Processing

File Manager • Organizes data on disk • Hides the details of physical files • Offers standardized file organizations – Sequential – Random Access – Indexed Sequential • Maintains File Directories

Random Access Files • Space inefficient • Fast access

Sequential Files • Space efficient • Slow access

Indexed Sequential Files Good compromise: • Space efficient • Access reasonably fast

Command Interpreter chess Cmnd. Int. Fmgr. ABL

Control by Command Interpreter Cmnd Interpreter Program X Start User Pgm Return to CMND. INT

Single-task Systems active task completed I/O completion activation start I/O load waiting for cpu Blocked (= busy waiting)

Interrupts Interrupt Handler

Interrupt Handler Program X Start User Pgm Find cause int. Control Unit Return to int. handlr

Interrupt Handler Based System Command interpreter Interrupt handler User program File manager A. B. L.

Multi-task Systems active preemption task completed start I/O activation load swap out waiting for cpu I/O completion (interrupt) blocked

Multitasking Issues • Process Scheduling Which program becomes active when ? • Job Scheduling (or Queue Management) Which programs reside in Central Memory ? • Memory Management How much memory gets each program ? • Output Spooling How to keep output of programs apart ?

Process Scheduler All programs have a priority and the one waiting with the highest priority becomes active. • Non-preemptive Scheduling: Priorities are compared whenever • the active program blocks itself • a clock tick occurs (typically every 10 m. S) • Preemptive Scheduling Priorities are continuously compared.

Process Priorities active -1 p = p 0 waiting for cpu +1 +1 blocked

Job Scheduler Selects the programs to be loaded in Central Memory • Manual selection (starting an application under Windows) • Automatic selection – based on priorities – Based on the Round Robin algorithm

Round Robin Preempted tasks after t seconds Input queue New tasks Central memory Completed tasks

Memory Management • Allocates memory to each program – Static Allocation • Good Job Scheduling mandatory. – Dynamic Allocation: Virtual Memory • Job Scheduling not so important.

Virtual Memory Page Table 1 cm 004 2 pm 1003 3 pm 1004 4 pm 1007 5 cm 002 6 pm 1009 CM PM

Output Spooling Program A Program B Program C Program D