Transcript Lecture 2
Operating System Structure • A key concept of operating systems is multiprogramming. – Goal of multiprogramming is to efficiently utilize all of the computing resources. – When a job issues an I/O request (e.g., open a file, read data from a file), it cannot continue until the request is fulfilled. – The CPU then becomes idle (the job is blocked on the request). Operating System Structure • Basic idea of multiprogramming: – Keep multiple jobs in memory. – When one job blocks on I/O (or other events), the operating system: • Starts the I/O operation. • Switches to another job that is ready to execute. • Now the CPU and I/O device are executing in parallel. – When the I/O device has completed request, it generates an interrupt to inform the CPU. – Virtually all general purpose computers support multiprogramming. • First developed for batch systems in the 60s. – Go to the computer center and give them your program (stored on punch cards). – The computer operator “batched” several jobs together and loaded them into the computer. – Come back at 5:00 to get the results of your program. • Batch systems are non-interactive. Time-Sharing Systems (Multitasking) Logical extension of multiprogramming termed multitasking. Quite often sitting at terminal using a “command line” interface to interact with computer. Types in commands from keyboard. A system program called a shell reads command from the command line and makes OS system calls to carry out commands. OS switches between user’s programs very quickly, generally in round-robin fashion. User 2 User 3 Shared Computer User 4 User1 User 2 User 3 Shared Computer User 4 User1 User 2 User 3 Shared Computer User 4 User1 User 2 User 3 Shared Computer User 4 User1 User 2 User 3 Shared Computer User 4 User1 Time-Sharing Systems (Multitasking) Switching between users is very fast. Goal is to give the illusion that each user has own machine. Think about using a text editor, or getting a directory listing. Response time is a priority. Protection of System Resources • • • • • I/O Devices Memory CPU Files Operating System Protection of System Resources • Based on dual-mode execution: – kernel mode and user mode. – Privileged instructions can be issued only in kernel mode. – Mode bit in Process Status Word (PSW), checked on every instruction. User process executing Continue Execution System Call Mode Bit=1 Trap, mode bit = 0 Execute System Call Return. Mode bit = 1 Protection of I/O Devices • All I/O instructions are privileged instructions. • Only accessed through system calls. Memory Protection • Must provide memory protection for the interrupt vector, interrupt service routines, and other applications address space. • Two registers that determine the range of legal addresses a program may access: – Base register – holds the smallest legal physical memory address. – Limit register – contains the size of the range • Memory outside the defined range is protected. Use of A Base and Limit Register Hardware Address Protection limit CPU (and OS) Protection • Keep user from monopolizing CPU. • Ensure OS regains control of CPU. CPU Protection • Timer – interrupts computer after specified period to ensure operating system maintains control. – Timer is decremented every clock tick. – When timer reaches the value 0, an interrupt occurs. • Timer commonly used to implement time sharing. Privileged Instructions • Load base and limit registers? Privileged Instructions • Load base and limit registers? • Set the system timer? Privileged Instructions • Load base and limit registers? • Set the system timer? • Read the system clock? Privileged Instructions • • • • Set the system timer? Read the system clock? Load base and limit registers? Open a file? Privileged Instructions • • • • • Load base and limit registers? Set the system timer? Read the system clock? Open a file? Compile a program and create executable? Privileged Instructions • • • • • • Load base and limit registers? Set the system timer? Read the system clock? Open a file? Compile a program and create executable? Enable/disable interrupts?