1. Exceptional Control Flow p501

<aside> ℹ️ Definition: Mechanisms that change the normal flow of program execution

Hardware-level:
- Exception
System-level:
- Through context switching
Application-level
- Sender process: can send signals to other processes
- Receiver process: nonlocal jumps to exception handler </aside>

2. Exceptions p502

Transfer of control to OS kernel due to an event
Exception number used as index into exception table

Exception Types p504

Type	Reason	Async/Sync	Return Action	Examples
Interrupt	External I/O device signal	Async	Return to $I_\text{next}$	Ctrl-C (I/O interrupt)
Trap	Intentional	Sync	Return to $I_\text{next}$	Syscall, breakpoints
Fault	Potentially recoverable errors	Sync	May Return to $I_\text{next}$	Page Fault
Abort	Unrecoverable errors	Sync	Won’t return	Parity errors

System Calls p507

Unique ID Number	Name	Description
0	`read`	Read File
1	`write`	Write File
2	`open`	Open File
3	`close`	Close File
4	`Stat`	Get info about file
57	`fork`	Create process (clone of all instructions below `fork()`)
59	`execve`	Execute a program
60	`_exit`	Terminate process
62	`kill`	Send signal to process

Understanding fork()

Create a new process running (potentially) concurrently with current process
Fork process inherit ALL instructions immediately below fork()

Faults

Page fault: required page not in memory, must be retrieved from disk
Invalid memory reference: accessing invalid or protected memory (SIGSEGV)

3. Processes p508

<aside> ℹ️ Basics

Definition:
- An instance of a running program
- Identified by unique process ID (PID)
- Process states: running, stopped, terminated

Provides Abstraction:
- Logical Control Flow: creates illusion that each process has dedicated CPU usage (through Context Switching)
- Private Address Space: creates illusion that each process has dedicated use of main memory (via Virtual Memory) </aside>

Context Switches

Save current states in memory
- e.g., register values, memory mappings, open files
Schedule next process for execution