– At most one I/O request is outstanding at a time, no simultaneous I/O processing. • After I/O starts, control returns to user program without waiting for I/O completion. – System call – request to the operating system to allow user to wait for I/O completion. – Device-status table contains entry for each I/O device indicating its type, address, and state. – Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt.
Operating-System Operations • Interrupt driven by hardware • Software error or request creates exception or trap – Division by zero, request for operating system service • Other process problems include infinite loop, processes modifying each other or the operating system • Dual-mode operation allows OS to protect itself and other system components – User mode and kernel mode – Mode bit provided by hardware • Provides ability to distinguish when system is running user code or kernel code • Some instructions designated as privileged , only executable in kernel mode • System call changes mode to kernel, return from call resets it to user
Transition from User to Kernel Mode • Timer to prevent infinite loop / process hogging resources – Set interrupt after specific period – Operating system decrements counter – When counter zero generate an interrupt – Set up before scheduling process to regain control or terminate program that exceeds allotted time
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- Fall '11
- Central processing unit, X86, Interrupt, Interrupt handler, Non-maskable interrupt