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Operating Systems/Overview.md

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+---
+type: theoretical
+---
+
+## Operating System
+- Manages hardware and software
+- Resource allocator
+	- Manages and routes resources and requests
+- Control program
+	- Prevents errors and improper use
+
+## User v. System
+
+### User
+
+```mermaid
+graph TD;
+
+	A["user"] <--> B["application programs"]
+	B <--> C["operating system"]
+	C <--> D["computer hardware"]
+
+```
+
+
+
+### System
+An operating system is a control program
+
+
+## Major OS Components
+
+
+| Name                     | Description                                        |
+| ------------------------ | -------------------------------------------------- |
+| Kernel                   | Core part of the OS; manages CPU, memory, I/O      |
+| Process manager          | Handles multitasking, scheduling, and execution    |
+| Memory Manager           | Allocates/deallocates RAM, manages virtual memory. |
+| File System              | Organizes, stores, and secures files.              |
+| Device Manager           | Interfaces with hardware peripherals               |
+| User Interface (CLI/GUI) | Allows users to interact with the OS               |
+
+
+## Management of resources
+
+### CPU
+- A program can do nothing unless its instructions are executed by a CPU
+- An executing program becomes a **process**
+- The process needs resources
+- All processes can potentially be executed concurrently by multiplexing on one CPU core or in parallel across multiple
+
+
+### Memory
+- For a program to be executed, it must be loaded into memory.
+- To improve the CPU utilization, several programs are kept in memory.
+
+OS should:
+- Know which parts of memory are currently being used and which process is using htem
+- Allocate and deallocate memory accordingly
+- Decide which processes and data moves in and out of memory
+
+### Cache
+- Cache operates at different levels (L1, L2, L3) and is managed primarily by hardware (CPU cache controllers), not the operating system.
+- **HOWEVER**, the OS can influence this by
+	- Managing process scheduling which affects cache locality[^1]
+	- Optimizing memory access patterns
+	- Supporting cache-aware algorithms[^2]
+
+
+### File System
+
+- The operating system abstracts from the physical properties of its storage devices to define a logical storage unit, the file
+
+### Mass Storage
+The operating system is responsible for:
+-  Mounting and unmounting
+- Free-space management
+- Storage allocation
+- Disk scheduling
+- Partitioning
+- Protection
+
+### I/O
+
+The I/O subsystem consists of several components:
+- A memory-management component that includes buffering, caching, and spooling
+- A general device-driver interface
+-  Drivers for specific hardware devices
+
+
+
+
+## User and Operating System Interface
+![](Pasted%20image%2020250204103541.png)
+
+
+## Command Interpreters
+Special programs that run when a user first logs on. They read and interpret user commands and execute programs.
+
+On Linux, the command interpreter is called a **shell**. The most common shell is the **Bourne Again Shell (bash)**
+
+## System calls
+- System calls are the interface between the user and the operating system
+- They are the only way a user program can request a service from the operating system
+- They are typically written in a high-level language (C) and are accessed via a library
+
+E.g.
+
+```c
+#include <stdio.h>
+#include <unistd.h>
+
+int main() {
+	write(1, "Hello, World!\n", 14); // 1 is the file descriptor for stdout, 14 is the number of bytes to write
+	return 0;
+}
+```
+![example syscall](assets/image.png)
+
+
+## APIs in the OS
+
+- APIs are a set of functions that allow the programmer to interact with the OS
+- They are typically written in a high-level language (C) and are accessed via a library
+- They are used to interact with the OS and its services
+
+
+i.e. Under the hood, widely used libraries like `stdlib.h` and `stdio.h` are just APIs that interact with the OS
+
+
+
+## Performance Optimization
+A computer system executes a program by:
+- Loading the program into memory
+- FDE cycle (Fetch, Decode, Execute)
+- Repeat until the program is done
+- The OS must manage resources to ensure that the program runs efficiently
+
+
+### Instruction Types
+- CPU-bound: spends most of its time executing instructions
+- I/O-bound: spends most of its time waiting for I/O operations to complete
+
+Keyword - **waiting**, hence I/O operations are very slow.
+
+### How do we sped it up?
+
+- Replace slow I/O devices
+- Perform I/O operations independently of the CPU
+- Multiprogramming/multitasking
+
+
+#### Spooling
+- Simultaneous Peripheral Operations On-Line
+- A technique that uses a buffer to hold data for devices that are not currently in use
+
+```mermaid
+graph TD;
+
+	A["User"] --> B["Spooling"]
+	B --> C["Printer"]
+	B --> D["Disk"]
+```
+
+#### Multiprogramming
+- The OS keeps several jobs in memory simultaneously
+- Needed for efficiency
+- Single users cannot keep the CPU and I/O devices busy at all times
+- Scheduling is key
+- 
+```mermaid
+graph TD;
+
+	A["User"] --> B["Multiprogramming"]
+	B --> C["Job 1"]
+	B --> D["Job 2"]
+```
+
+#### Multitasking/Timesharing
+- The OS switches between jobs so quickly that it appears as if they are running simultaneously
+- If several jobs are ready to run at the same time, the OS must decide which one to run
+
+```mermaid
+graph TD;
+
+	A["User"] --> B["Multitasking"]
+	B --> C["Job 1"]
+	B --> D["Job 2"]
+```
+
+
+
+[^1]: is the tendency of a processor to access the same set of memory locations repetitively over a short period of time
+
+[^2]: https://stackoverflow.com/questions/473137/a-simple-example-of-a-cache-aware-algorithm