Dispatch Mechanism OS keeps system-wide list of processes. Each process in one of three modes; Running: On the CPU (only one in uniprocessor system), Ready: Waiting for the CPU, Blocked: Waiting for I/O or synchronization with another thread. Dispatch loop:
while (1) {
run a process for a while
stop process and save its state context-switch
load state of another process context-switch
}
How does Dispatcher gain Control?
must change from user mode to system mode; the CPU can only do one thing at a time. While a user process is running, dispatcher cannot run, thus the operating system may lose control
two ways operating system gains control;
Traps: Events internal to user process (System calls, Errors, Page faults)
Hardware interrupts: Events external to user process (Character typed at terminal, Completion of disk transfer, Control given to OS interrupt service routine (ISR))
Dispatcher must track state of process when not running; On every
trap or interrupt, save process state in Process Control Block
(PCB)
