CMSC 421 Midterm (with Complete Solutions)

What does time sharing mean? correct answers Time sharing is a logical extension of multiprogramming in which the CPU executes multiple jobs by switching among them so fast that the user can still interact with each program while it's running. Requires an interactive computer system which provides direct communication between the user and the system. Storage device hierarchy? Which is faster? correct answers Top to bottom: registers cache main Memory nonvolatile memory hard-disk drives optical disk magnetic tapes The higher levels are expensive but fast, as we move down, cost per bit generally decreases but access time generally increases Service layers correct answers Services are layered. For example, every user interface call must use the system call. All system calls use the services below. There are no skipping layers. The layer below provides services for the layer above. What is a system call and what does it do? correct answers It's a programming interface to the services provided by the OS. Provides an interface between a process and the OS to allow user-level processes to request services of the OS. Mostly accessed by programs via API. Invokes intended syscall in OS kernel and returns status of syscall and any return values. System calls are the only entry point into the kernel system. What is a system program, sometimes called utilities? correct answers These provide a convenient environment for program development and execution. Some are user interfaces to system calls, others are more complex. Divided into these categories: File management, status information, file modification, programming language support, program loading and execution, communications, and background services.