How do you transition from user mode to kernel mode?

Answer 1

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ANSWER

It depends on your operating system. A kernel is basicaly a line of code, different kernels control different operations for different systems within the computers program.

The closest thing to obtainging a true "kernel mode" is to run an open source Operating system like linux.

With something like windows you can obtain through administrative tools access to operating system programms, you can't truly rewrite (easily) the programs within your system.

A process transitions from user mode to kernel mode usually with some form of software interrupt. On x86 CPU's, the "INT 0x80" instruction is commonly used for running system calls. The kernel traps this software interrupt, processes the request, and returns to user mode by returning from the interrupt. This describes how real operating systems like Linux and FreeBSD work. Windows may work similarly to this, but it's not a real operating system anyway.

A modern symmetrically multi-processing operating system typically, today, consists of millions of lines of code. Most of these, eg Windows, Linux, FreeBSD, NetBSD, OpenBSD among many others, are written in the 'C/C++' programming language.

The above mentions INT 0x80 as being used on x86 CPUs, the author above is absolutely correct, though the answer could be further refined to say that INT 0x80 is specific to the Linux operating system kernel as being the maskable software interrupt used to invoke a system call (whether that be on ring -1, ring 0 or another ring if other rings are available on a given architecture - eg, VMware probably supports ring -1 on architectures supporting ring -1 so that guest operating systems may have free access to ring 0). That may well be the longest sentence I've ever written. :)

As INT 0x80 is a Linux maskable interrupt, it would also be used on other architectures as well such as Sun Microsystems SPARC or Digital Equipment Corporation VAX machines.

System calls (or SysCall/SysEnter) are used for many other purposes, particularly to access various hardware devices such as locally attached disk drives, USB devices, SCSI devices, Network (eg Ethernet [including Arcnet or Token Ring, assuming such things still exist]) cards or other bus attached devices etc.

I must agree with the above author's last sentence as well, though a Rockwell International engineer I once knew classified Windows as a 'virus' as opposed to a 'real' operating system like Linux (probably because Windows tends to proliferate as well as wreak havoc upon it's users).