My study notes for the whole CS2850 module that took place at Royal Holloway, University of London. This is for the theory part of it - with topics including Processes and Threads, IPC with and without busy waiting, CPU scheduling, Interactive System Scheduling, and all taught algorithms related to...
Operating Systems
-Created to act as a layer between end user applications and the hardware
Kernel mode allows all instructions whereas user mode only allows a specific set
CPU
● Has a specific instruction set
● Fetches instructions from memory and executes them
● Uses registers for storing temporary values and control information - super fast
access
Modern CPU’s use a pipeline in order to increase performance meaning while executing an
instruction, the next can be decoded and the following one fetched.
Superscalar CPUs have multiple units of the same type. Allows for multiple operations at the
same time.
CPU power has increased due to:
● Ability to keep shrinking transistor size
● Multithreading - allows different tasks to take place in a CPU and allows these tasks
to be switched between. Eg if one is waiting for a value from memory
● Multicore - CPUs have several cores
,Memory
Main memory - RAM (volatile - loses data when power source is lost)
Magnetic disk - HDD
Two levels of cache
-Level 1 resides inside the CPU
-Level 2 is larger than L1 but slower. It can be assigned to a specific core or shared
Problems with cache include:
When you want to add a new item, which item should you remove?
Where to put this newly evicted item into the larger memory?
I/O Devices
● Consists of a controller and the device
● Controller is a hardware component that accepts commands from the OS
● Each controller is different so to minimise OS complexity, they use a device driver
● The vendors of the hardware creates the device driver
Interrupts
● A signal sent to the CPU to alert attention.
● Interrupt controller (physically attached to the CPU) is a chip that handles several
devices. It will assert a pin on the CPU chip if it is ready to accept the interrupt. (step
3) It puts the number of the device on the bus for the CPU to read it and know what
device has finished. (step 4)
Interrupt processing involves taking the interrupt, running the interrupt handler and returning
to the user program. When the CPU accepts the interrupt, the currently running program is
suspended and the CPU is switched to kernel mode.
,How can I/O take place?
Busy Waiting
1. Program issues a system call
2. The OS translates the call to the device driver
3. Driver initiates I/O and continuously polls the device to check if the operation has
been completed
4. Once it’s complete, it returns any result and the OS passes it to the original caller
Main disadvantage of this is the CPU is tied up during the polling process
Alternative
● Device driver to initiate a request to the device and ask to be notified when the
operation is finished.
● Whilst the device is completing the operation, the OS can block the original caller and
do other work.
● When the controller detects the end of the original operation, it generates an
interrupt.
Direct Memory Access (DMA)
● A chip that can get instructions from the CPU (number of bytes to transfer, direct and
memory addresses involved)
● When the DMA chip completes the operation, it will generate an interrupt
● Further minimises CPU intervention
OS Concepts
Process - A program in execution
-Associated with an address space and a set of resources
-Processes can create child processes and these can be shown in a tree diagram
Mounting in UNIX - Adding an external file to the root file so it is part of the file hierarchy
Week 2
Processes
The OS keeps a common set of data for a process:
● Process ID
● Process name
● User running the program
● Executable file associated with the process
● Current values of the registers, variables and program counter
In a multiprogramming system, a CPU quickly switches from process to process. Although
the CPU is only running one process at a time, this gives the illusion of parallelism. Called
pseudo-parallelism. There is only one program counter in the CPU.
, Process Creation
● System initialisation - on system startup
● Execution of a process creation system call by a running process
● A user request to create a new process
● Initiation of a batch job - happens in mainframes where users submit jobs and the OS
will select a job from the input and create a process-
POSIX (Portable Operating System Interface for UNIX)
● A new process is created by a system call called fork
● It creates a clone of the calling process, called the “child process”
○ Shares same memory, resources
○ Address space is separate (if a var gets changed in the child, parent does not
change)
● fork() returns twice
○ Parent process returns the child process id
○ Child process returns 0
Windows
● Process is created in a single step called CreateProcess call
● The process is created and initiates the execution of a program
Process Termination
● Normal exit (voluntary) - process finishes work
● Error exit (voluntary) - exit due to an error handled by the program itself
● Fatal Error (involuntary) - error caused by the process
● Killed by another process (involuntary)
Process Types
● Interact directly with users independently of how they started
○ Eg file explorer, web browser
● Daemons - working in the background and react to occurrences
○ Eg process checking for OS updates
Process States
1. Running - process is using the CPU right now
2. Ready - runnable, temporarily stopped to let another process run
3. Blocked - unable to run until some external event to happen
1 happens when a process is waiting for
something else to continue eg I/O input
2 occurs when the scheduler decides a process
has ran long enough
3 occurs when a process can use the CPU again
4 occurs when the event the process was waiting
for happens
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