All programs or processes when running are executed in User mode. Processes do not interact directly with the kernel but they just give instructions in form of system calls and the kernel mode will take care of the rest.
Switching to kernel mode requires one of these to happen, an interrupt, a trap and I/O or event occurrence.
The kernel mode responds to the request for example handling of device drivers, memory allocation and others which we refer to as privileged instructions, and the process is switched back to user mode where we find less privileged instructions.
"When the computer boots up, Windows calls the KERNEL, the
Switching to kernel mode requires one of these to happen, an interrupt, a trap and I/O or event occurrence.
The kernel mode responds to the request for example handling of device drivers, memory allocation and others which we refer to as privileged instructions, and the process is switched back to user mode where we find less privileged instructions.
"When the computer boots up, Windows calls the KERNEL, the
main kernel-mode program that allows all the other programs
to run, even the user-mode programs".http://www.allinterview.com/showanswers/67083.html
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