In some cases, the CPU was often idle for >80% of the time. The speeds of the mechanical I/O devices is considerably slower than that of the electronic computer and jobs often have to pause when performing I/O. Normal mode also has an instruction to call the OS (hand back control).īatch systems were good for CPU bound scientific computing with minimal I/O, but bad for data processing with high I/O as the CPU is often idle. Some CPUs also implemented protection in Normal mode to prevent the user program overriding the first section of memory in which the OS existed. These systems had a new memory layout and the processor had two modes - Normal, where some instructions (e.g., HALT) are forbidden, which was used by the batch programs, and Privileged, where all instructions are available, this was the level the OS ran in. These systems had a small permenantly resident OS which had initial control and controlled automatic job sequencing. A number of jobs is then done in succession without wasting CPU time. A small computer reads a number of jobs onto magnetic tape and the magtape is run on the mainframe. Optimisations could be made in reducing the setup time by batching similar jobs. Application programs existed (compiler/card reader), but were limited. The library is loaded into the top of memory and stays there. The minimal OS was required to interact with I/O devices and a subroutine library existed to manage I/O interaction. Jobs and programs were written using punch cards and the computers were normally application specific. These systems had minimal operating systems and were typically mainframe computers used to support commercial and scientific applications. There's a trade off required between the extra functionality of the virtual machine and the overheads required to implement this. However, abstraction away from the physical machine to provide ease of use can lead to inefficiencies. The goal of OS design is to provide a useful, simple and efficient system: This is more important than CPU usage - a user doesn't care if the CPU is idle 99% of the time. The OS is designed for ease of use, which is in terms of not having to deal with low level hardware issues. The OS interface describes the calls that an application program can make into the OS. ![]() The program needs an efficient interface to interact with the OS to utilise the resources it needs to accomplish the tasks the user wants to do and to also communicate with the user. When developing an OS, all levels must be considered, not just the OS level.įor users, they interact with the hardware (monitor, keyboard, mouse, etc) to control the system and application programs. Users (people, or other machines and systems) interact with system and application programs (defines the way in which system resources are used to solve the computing problems of the users), which interact with the OS (controls and co-ordinates the use of hardware among the various application programs for the user), which interacts with the hardware (the most basic computing resources - CPU, memory, I/O, etc). Semaphores: More sophisticated methods that utilize the wait() and signal() operations that execute atomically on Semaphore S, which is an integer variable.Ĭondition variables: Utilizes a queue of processes that are waiting to enter the critical section.A computer system can be abstracted into various components. Mutex locks: Software method that provides the acquire() and release() functions that execute atomically. Test_and_set: Uses a shared boolean variable lock and the test_and_set instruction that executes atomically and sets the passed parameter value to true.Ĭompare_and_swap: Same as test_and_set, except that the passed parameter value is set to true if it is equal to expected in the parameter of the compare_and_swap instruction. Most solutions to the critical section problem utilize locks implemented on software. ![]()
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