OS - Ch 1,2

Reference : Operating Systems: Three Easy Pieces, Remzi H. Arpaci-Dusseau and Andrea C. Arpaci-Dusseau

Introduction To OS

• OS provides system calls that allow application programs request to use.
  • OS privdes the calss for program execution, memory access, etc
  • Hence also be referred as providing a “standard library”
• Virtualization allows many programs to share a CPU, and to be executed concurrently.
  • Each program can access to its resourse and instructions.
  • Hence called as a “resource manager”

Virtualization

CPU

• Logical transformation of a/few CPU/s into a number of it to run many programs concurrently.

Memory

• Physical memory is an array of byte which has address.
• Do not forget the insturctions are stored within its memory.
• Run the same program concurrently
  • It will have the same memory allocation (002000000 in the given example)[^1]
    • Each process has its own virtual address space
    • The actual memory address (physical) is different.

[^1]: Make sure address-space randomization is diabled -> enabled for security reason regarding stack-smashing attacks.

Concurrency

• It becomes an issue because
  • CPU executes one insturction at a time. Hence, insturctions are not atomic.

Persistence

• Device like DRAM saves data in volatile^2
  • Disconnection or crash result with data loss.
• OS software manages disk : file system.
  • Unlike CPU or memory, OS does not do virtualization on disk, but rather assume users may want to share file info.
  • For performance, most File System delays writes, and batch them (try) into larger groups.
  • For persistence, most file system incorporate intricate write protocol such as journaling, or copy-on-write.

Design Goals

1. Define Abstractaions to ease the usage of computer system
   • Abstraction allows:
     • Able to write codes in C (higher lang) even if you do not know Assembly.
     • Able to write assembly without considering logic gates.
     • Able to write processes without having knowledge in transisters.
2. Minimize the overhead (performance)
   • Having virtualization and easy-to-use system is meaningful but not necessary.
3. Safety between OS and Application programs.
   • OS needs to ensure the execution of a program does not negatively influence the others
   • Safety is a key concept of Isolation principle.
4. Provides high level reliability
   • OS fails, all the applications bulit on fail.
5. Energy-efficiency, security, mobility and so on…