Operating System Support and Memory and Architecture and Management Notes

Convenience: Makes the computer easier to use.
Efficiency: Allows for better use of computer resources.
Resource Manager: The OS manages the movement, storage, and processing of data; it is a program executed by the processor that frequently surrenders control to user programs.
Key Services: - Program creation (utility programs). - Program execution (loading instructions and data into main memory). - Access to I/O devices (simplifying details into reads and writes). - Controlled access to files (handling formats and protection). - System access (managing shared/public resources). - Error detection and response. - Accounting (collecting usage statistics).

Instruction Set Architecture (ISA): Defines machine language instructions and serves as the boundary between hardware and software.
Application Binary Interface (ABI): Defines standards for binary portability and the system call interface to the OS and hardware.
Application Programming Interface (API): Provides high-level language (HLL) library calls to access hardware resources and services, ensuring software portability.

Early Systems ( $1940s$ to mid- $1950s$ ): No OS; programmers used sign-up sheets to reserve time and manual console controls, leading to wasted setup time and idle processor time.
Batch Systems: Programs from multiple users are grouped and submitted by an operator. A Resident Monitor (early OS) handles setup and scheduling.
Job Control Language (JCL): A special language (e.g., instructions starting with "\ ") used to provide instructions to the monitor.
Multiprogramming: Designed to maximize processor use by keeping multiple jobs in memory so the processor can switch tasks when one waits for I/O.
Time-Sharing Systems: Designed to minimize response time for multiple interactive users by interleaving execution in short bursts or "quantums."

Memory Protection: Prevents user programs from altering the monitor's memory.
Timer: Prevents a single job from monopolizing the system.
Privileged Instructions: Can only be executed by the monitor (e.g., I/O instructions).
Interrupts: Provide the OS flexibility to regain control from user programs.

Long-term Scheduling: The decision to add a process to the pool for execution.
Medium-term Scheduling: Part of the swapping function; decides which processes are partially or fully in main memory.
Short-term Scheduling (Dispatcher): Determines which available process the processor will execute next.
I/O Scheduling: Determines which pending I/O request will be handled by an available device.
Five-State Process Model: Includes states for New, Ready, Running, Exit, and Blocked.
Process Control Block (PCB): Contains identifiers, state, priority, program counter, and context data for a process.

Fixed Partitioning: Dividing memory into fixed-sized regions, which can be equal or unequal (e.g., partitioning a $64\,Mbyte$ memory).
Dynamic Partitioning: Allocates exactly the amount of memory required by a process.
Swapping: Moving processes between main memory and disk storage to manage the degree of multiprogramming.
Address Types: - Logical Address: Relative to the beginning of the program. - Physical Address: Actual location in main memory. - Base Address: The current starting location of the process.

Paging: Memory is divided into fixed-size frames; processes are divided into pages. A Page Table maps pages to frames.
Virtual Memory (Demand Paging): Pages are brought into memory only when needed.
Principle of Locality: Program execution often stays confined to a small section of code.
Page Fault: Triggered when a program references a page not currently in main memory.
Thrashing: A state where the processor spends more time swapping pages than executing instructions.
Translation Lookaside Buffer (TLB): A cache used to speed up the translation of virtual addresses to physical addresses.

Segmentation: Programmer-visible address spaces that simplify handling growing data structures and allow independent recompilation.
Protection Levels: ARM and other architectures use privilege levels ranging from Level $0$ (most protected/clearance) to Level $3$ (least protected/classification).
ARM Specifics: - Supports $16$ domains to maintain protection between processes. - Access Control bits (AP) designate regions as No access, Read only, or Read-write. - Roles include Clients (observe permissions) and Managers (bypass permissions).