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Queues are used to hold process control blocks (PCB) that represent processes internally to the OS. Depending on the hardware, this can take 1 to 1000 microseconds (i.e., very slow in computer terms). When the CPU switches to another process, a context switch occurs which involves saving the complete state of the previously executing process before loading the state of the next process to execute into the CPU. Many processes execute concurrently, but only one can be executing on a CPU at a time.
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#DUAL MODE PROCESSING OPERATING SYSTEM CODE#
#DUAL MODE PROCESSING OPERATING SYSTEM SOFTWARE#
A system call (syscall or trap) instruction initiates a software interrupt and is used as follows: Operating system services, such as I/O, are provided through system (/monitor) calls. Modifications to the CPU timer are privileged Remember that only one program (in a single CPU system) can be executing at a time so when the OS turns control over to a user program it has "lost control."