User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast.

System goals – operating system should be easy to design, implement, and maintain, as well as flexible.

- Process control

- File management

- Device management

- Information maintenance

- Communications

- Protection

- to ensure efficiency system operations

provide the interface between arunning program and the operatingsystem

- User interface

- Program execution

- I/O operations

- File-system manipulation

- Communications

- Error detection

Design and Implementation of OS not “solvable”, but some approaches have proven successful.

 Start by defining goals and specifications

The design of the system will be affected by the choice of hardware and the type of system: batch, time shared, single user, multiuser, distributed, real time or general purpose.

- Extensibility

- Flexibility

- Reliability

- Portability

- In layered approach, the Os is dividedinto a number of layers, each built ontop of lower layers.

- Advantage

- Disadvantage

- Layered Operating System

- small operating system core

- contain only essential core operatingsystem function

Many services traditionally included inthe operating system are now external subsystems

- Message passing

- Shared memory