Memory Layout for
Multiprogrammed System
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
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