What is an OS?

Some history

Batch systems: one job at a time

Multiprogrammed systems: store multiple jobs in memory, with an operating system that schedules, allocates, multiplexes. but one job after another, with a lot of waiting.

Time sharing: single CPU can be passed between jobs, multitasking, illusion of parallelism.

You are not expected to understand this.

What is an OS?

Kernel vs user mode:

OS is an extended machine — it extends & abstracts over hardware functionality OS is resource manager — protects unsafe use of resources, accounting/limiting

• offers functionality through syscalls
• groups of syscalls offer services
• processes are abstractions to create user’s program
• each program/process has its own address space
• data is in files, these persist over processes

Processes

• process represents instance of a program in execution, with a name
• memory address spaces limit programs to a specific part of memory. layout depends.

Memory address space

very basic layout is stack (frames for function calls), data (variables), text (program code)

Structure of the OS

System calls

• interface offered by OS to apps for service requests
• interface depends on OS and hardware, so encapsulate syscall logic in libc (POSIX standard)