Chapter 6 - Implementing Processes, Threads and Resources Kris Hansen Shelby Davis Jeffery Brass 3/7/05 & 3/9/05 Kris Hansen Shelby Davis Jeffery Brass 3/7/05 & 3/9/05

What’s Covered Different Process and Thread Types Hardware Processes The Abstract Machine Interface Process Abstraction Thread Abstraction State Diagrams Resource Managers

What’s Covered Different Process and Thread Types Hardware Processes The Abstract Machine Interface

Processes Two Definitions of Process Classic Process: a program in execution in a von Neumann computer Modern Process: an OS abstraction that defines the execution environment for a thread or set of threads

Thread Definition: unit of computation with the minimal internal state and resources Threads keep track of code execution in a given process Modern operating systems are built with modern processes and threads in mind

The Abstract Machine The modern OS uses multiprogramming, the illusion that applications each reside inside of there own computer The OS manages the virtual machines, via services Services are invoked by calling API Functions

APIs API stands for Application Programming Interface API functions are implemented by different parts of the OS: Device Manager Process Manager Memory Manager File Manager

Adding Threads In a classic system, only one thread can be running per processor, the base thread In a modern OS, additional threads can share the host process’ resources When an abstract machine is given it’s own multiprogrammed OS, then it is a user space thread implementation the OS implements classic processes, and the user space thread library runs on top to simulate multiprocessing

Adding Threads Other modern OSs are built for threads, i.e. Windows, and support kernel threads When kernel threads are supported, the notions of threads and processes are completely separate by managing both as separate entities

Resources A resource is anything that a process can request If not available, the process is blocked A request for a resource is made via an API call Allocation results in the resource being configured for the abstract machine Each resource has a unique system-wide resource identifier

Process Address Space The process address space is the collection of addresses that a thread can reference Addresses link to memory locations or other abstract machine elements Memory-mapped resources - those resources bound to a collection of addresses in the address space The address space provides a uniform access method for getting memory-mapped resources

Processes Address Space Each resource manager must bind (associate, or link) addresses to resources The OS uses the address space system to control what processes have access to resources Modern OSs usually have 2 32 address (about 4GB worth,) with some supporting (or planning to support) 2 64 addresses

OS Families The abstract interface is determined by the host’s hardware and set of functions released by the OS The definition of an OS is important, both for how it works on the technical side and what software is available on the business side

Process Manager Responsibilities Process Creation and Termination Thread Creation and Termination Process/Thread Synchronization Resource Allocation Resource Protection Cooperation w/ Device Manager on IO Address Space Implementation

Final Process Composition Address Space Program Data Resources Process Identifier

Final Thread Composition Host Process Environment Thread Specific Data Thread Identifier

The Hardware Process When a system boots up, there are no distinctions like processes and threads On booting, all the computer knows is its basic hardware start up instructions, known as a bootstrap This single thread of execution is known as the hardware process

Bootstrap & Loader The bootstrap must execute first Executing the bootstrap leads to the loader, which starts loading the OS After loading, the OS can initialize, polling and initializing hardware and data structures Only after this can threads and processes be handled, by way of launching thread and process managers

Initial Structures When the OS loads, before normal execution can take place, an initial process and thread are created It acts as a placeholder for all of the other threads and processes, it actually does nothing This is also the idle thread (or idle process,) as this process is executed when no other processes are running

The Abstract Machine Interface In an OS, there are two types of instructions: user mode instructions and OS (supervisor mode) instructions This protects from a program being able to access resources completely unchecked When a certain resource is needed, or a system call is necessary, then a call must be made to the OS, which directs the call into the supervisor mode via a trap instruction

Comparing System Call Interfaces Linux Kernel version has 166 system functions Kernel version 2.4 has over 200 Win32 has over 2,000 The number of system calls increases the functionality of the OS, along with the overhead needed to run it

Source Nutt, Gary. Operating Systems. Boston: Pearson Education, 2004.