What is an operating system and What happens when you run a program.What is responsibility of OS.

1. What is an operating system?

• Middleware between user programs and system hardware.
• Manages hardware: CPU, main memory, IO devices (disk, network card, mouse, keyboard, etc.)

2. What happens when you run a program? (Background)

• A compiler translates high level programs into an executable(".c " to "a.out")
• The exe contains instructions that the CPU can understand and data of the program (all numbered with address)
• Instructions run on CPU: hardware implements an instruction set architecture(ISA)
• CPU also consists of a few registers, e.g,
  -Pointer current instruction .(program counter or PC)
  -Operands of instructions, memory addresses.

3. So, what happens when you run a program?

• To run an exe, CPU.
  ->fetches instruction pointed at by PS from memory.
  ->loads data required by the instruction into registers.
  ->decodes and executes the instruction.
  ->stores result in memory.
• Most recently used instructions and data are in CPU caches for faster access.

4. So, what does the OS do?

• OS manages program memory:– Loads program executable (code, data) from disk to memory.
• OS manages CPU:– Initializes program counter (PC) and other registers to begin execution.
• OS manages external devices:– Read/write files from disk.

5. OS manages CPU:)

• OS provides the process abstraction.
•  – Process: a running program.
•  – OS creates and manages processes.
• Each process has the illusion of having the complete CPU, i.e., OS virtualizes CPU.
• Timeshares CPU between processes.
• Enables coordination between processes.

6. OS manages memory

• OS manages the memory of the process: code, data, stack, heap, etc.
• Each process thinks it has a dedicated memory space for itself, numbers code, and data starting from 0 (virtual addresses.
• OS abstracts out the details of the actual placement in memory translate from virtual addresses to actual physical addresses.

7. OS manages devices

• OS has code to manage disk, network card, and other external devices: device drivers
• Device driver talks the language of the hardware devices :
  – Issues instructions to devices (fetch data from a file)
  – Responds to interrupt events from devices (the user has pressed a key on the keyboard.
• Persistent data organized as a filesystem on disk.

8. Design goals of an operating system

• Convenience, an abstraction of hardware resources for user programs
• The efficiency of usage of CPU, memory, etc.
• Isolation between multiple processes.

9. History of operating systems

• Started out as a library to provide common functionality across programs.
• Later, evolved from procedure call to system call: what’s the difference?
• When a system call is made to run OS code, the CPU executes at a higher privilege level.
• Evolved from running a single program to multiple processes concurrently.