System calls are usually invoked by using
System calls are Virtual Instructions. Instructions can be invoked by Interrupts.
Explanations are already given for options (A) and (D).
(C) is wrong.It is jump to fixed locations.
(B) is wrong.polling is the approach where the CPU repeatedly checks the status of a device for exchanging data.
Note: Implementing system calls requires a transfer of control from user space to kernel space, which involves some sort of architecture-specific feature. A typical way to implement this is to use a software interrupt or trap. Interrupts transfer control to the operating system kernel, so software simply needs to set up some register with the system call number needed, and execute the software interrupt.
Ans) A Software interrupt
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