ECS150-MT1

Topics

Lecs

1. Course intro
2. Os-intro
3. Syscalls
4. Os-structure (up to page 8 ‘dynamic/static libraries’)
   “Midterm Menu”

• OS intro - 1 q
• Syscalls - 4 q
• OS Struct - 1q

Review: - Especially all the code example related to syscalls and file descriptors

OS Intro

Bare minimum of computer:

• CPU, Interrupts, Timer, Interconnect, Memory, UART/Serial Controller, HDD/SDD Controller, UI, Persistent Storage

Processor:

• Control Unit, Register, ALU, Memory Access
  Memory:
• Addressable bytes that can hold numerical values
• Speed:
  • Registers > Cache > RAM/Main Mem > Second Level Storage
    I/O:
• Made up of controller and an actual device

OS Roles:

• Referee - resource allocation, isolation, communication
• Illusionist - resource virtualization: mask specificities/scarcity of resources, mask hardware failures
• Glue - Communication services: APIs to hardware, IO operations, filesystem, etc.

Design Principles:

• Reliability
• Security
• Portability
• Performance

System Calls

Categories of libc functions:

1. No priveledged operations
   • memset - only needs access to buf
2. Always need to request priveledged operations (syscalls) from OS
   • open - needs: verification of a file existing, check if user has access, actually reading file
3. Sometimes need priveledged ops
   • printf - prints to stdout, flushing buffer requires OS to write chars (use of syscall write() )

syscalls - Secure API between user applications and kernel

• Categories: Process management, files/dirs, pipes, signals, mem management

Process - a program in execution

• Identified by a PID (process ID)
• Process runs its own mem space
• Process is represented in the OS by a PCB (process control block)
  • Data structure storing info about process
  • PID, state, CPU register copies for context switching, open files, etc.

PCB
-----------|
	PID=1  |
	state  |
	ctxt   |
	files  |
	...    |
------------

Main functions/syscalls

• Process creation execution
  • fork(), exec()
  • exec[lv]p?e? - list vs vector ([lv]), path (p), environment (e)
• Process termination
  • exit() - End running process
  • wait()/waitpid() - Wait for a child process and collect exit code
• Process identification
  • getpid() - get process PID
  • getppid() - get parent process PID

VFS - Virtual File System

Other syscalls:

• File interaction
  • open(), close(), read(), write(), lseek() {move file offset}
• File descriptor management
  • dup2(source, target) - duplicate file descriptor
• File characteristics
  • stat()/fstat() - get file status
• Directory traversal
  • getcwd(): get current working directory
  • chdir(): change directory
  • opendir(): open directory
  • closedir(): close directory
  • readdir(): read directory

File Descriptors:

• Table of open files per process, indexes in PCB
• 0: STDIN
• 1: STDOUT
• 2: STDERR

Signals:

• Many different signals, form of interprocess communication
  • Segmantation fault, signal interrupt, etc

Main related functions/syscalls:
Sending signals

• raise(): Send signal to self
• kill(): Send signal to other process
• alarm()/setitimer(): Set timer for self
  • Receive signal (SIGALRM/SIGVTALRM) when timer is up
    Blocking signals
• sigprocmask(): Examine or change signal mask
• sigpending(): Examine pending blocked signals
  Receiving signals
• sigaction(): Map signal handler to signal
  • Also signal() but not recommended
• pause(): Suspend self until signal is received

OS Structure

OS Layers:

• Application
• Libraries
  --- Divides User from Kernel ---
• Portable OS Layer
• Machine-dependent layer

Libraries:

• Static vs Dynamic

Application Compilation:

• GCC
  • Preprocessor (cpp) transform program before compilation
  • Compiler (cc) compiles a program into assembly code
  • Assembler (as) compiles assembly code into relocatable object file
  • Linker (ld) links object files into an executable