The operating system is responsible for listing and managing the computer’s resources, making them available to programs running on the computer, and making sure they only use what they are allowed to.
Speculative execution is a feature that let’s the CPU speed up execution if it correctly predicts a decision point. The CPU carries out the operations along the predicted decision branch and loads the results if it predicts correctly.
Meltdown and Spectre require the programs executing them to have access to kernel memory space. Kernel address isolation attempts to prevent the program from even having access to the kernel address space in the first place. TLB flushing changes the virtual-to-physical memory mapping, disrupting Spectre’s reliance on a consistent virtual-to-physical memory mapping.
Shared memory helps to reduce the amount of memory needed by all the applications running on an operating system. It also allows applications to send data to each other, and to communicate.
Web apps require the browser to request memory on their behalf, and thus their memory usage shows up under the browser process in the OS Task Manager. Web apps use this data to store a more convenient (but larger) representation of the webpage document, and to store the data needed by the app.
Mobile apps, unlike web apps, can bundle resources and libraries to be installed to a mobile device. They can also request access to storage, and typically have a higher memory limit than web apps.
A laptop app can do practically anything, if it is running through the Administrator/root account. Sandboxing is carried out through permission control.
An app crashes when it encounters a situation it can’t handle, or when it attempts to perform an operation that is disallowed by the operating system.
Applications are assigned a thread by the OS for running a sequence of instructions. The instructions are executed sequentially, and the app cannot proceed if it gets stuck on any instruction.
A race condition happens when threads depend on instructions happening with coincidental timing for success. When instructions are not executed with appropriate timing, one or more threads can get stuck waiting on a response that never comes.
The OS takes care of booting up, login and user management, window management, memory allocation, storage interfaces, background services, peripheral management, and much more. Access to these services, where allowed, is provided in the form of software libraries that developers can use.
A hard disk is organised into sectors, which are the smallest unit of storage. The OS’s filesystem determines how and where to store each file on the hard disk. The filesystem manages the file metadata in a file table, separate from the actual contents of the file.
When a computer is booted up, it runs the BIOS from a chip on the motherboard. The chip checks that core parts are present, checks for a storage disk containing a bootloader, loads it into memory, and hands over control. The bootloader loads the operating system kernel. The operating system kernel then does whatever it needs to do to get the system ready for use.
f you can’t get to a BIOS screen, it is likely a hardware problem and has to be solved by a technician. If you can’t get the OS loading screen, it’s a bootloader problem and needs to be solved with more geekery. If something goes wrong with OS loading, and fails to fix itself on subsequent reboots, it’s probably time for a system refresh or reinstall.
Embedded operating systems are unlike user operating systems. They are designed to run the software needed for an appliance’s operation, and are not meant to be used by users directly. Since they are considered somewhere between software and hardware, they are usually referred to as firmware.
When you shut a computer down, it sends an exit signal to all running programs to get them to do their exit routine. This process can sometimes take a long time. To preserve the data configuration in memory while minimising power draw, a computer can go into standby mode: all hardware except the memory gets powered down, until the computer is woken up from standby.
Hibernation mode causes the computer to store the data configuration into a hibernation file on disk. When powered up, the OS reads the data configuration from the file back into memory. This lets the system avoid having to do a full shutdown and bootup; it performs a shorter version of these two sequences instead.
Operating systems use a page file on the storage disk as a complement to physical memory. This allows OSes to behave more performantly than they would if they did not have a page file. Data that is rarely accessed is moved to the pagefile (“paged out”), and can be paged in when it is needed later, albeit with a performance hit.
Programs do not usually deal with the gnarly details of hardware, but instead access it through an interface. They access storage devices through a filesystem, and access hardware through drivers.