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I am writing a small OS that will execute some code in user mode (privilege level 3). From that user level code, I want to call an interrupt back to the OS that prints a message. Right now I don't really care how my interrupt handler takes arguments or anything like that, I really just want an interrupt handler to inform me (the user) that the code has executed. My question is: how do I run code in user mode? I have a function that sets up a Local Descriptor Table with a code segment and data segment (both with user mode privileges). What I dont understand is how I am supposed to load these segments into cs , ss , and ds . I successfully load the my LDT, but I do not know how to actually use it. I have heard that I should use iret , but I don't understand exactly how. Another question that I have is how my interrupt handler should work. Let's say I install an interrupt handler for vector number 0x40, which I want to print "hello, user mode!". I know how to setup an interrupt handler, but I don't exactly understand how the context will be switched when entering a kernel interrupt handler from user mode. I know that the cs register must change, since my routine will be running from the code segment specified in my IDT entry. I also understand that the stack selector probably changes as well, but I cannot be certain of this. Could someone please explain to me what context changes are made when an interrupt gate is called?

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