CODEWITHSUNDEEP
assemblyoperating-systemprivileges
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Reputation: 3070

How hardware privilege levels are manipulated in x86 processor

I'm not able to understand how the hardware knows that currently a kernel code is running. Do we need to set up some memory address range in some registers indicating the CPU that between these memory address, it is the kernel running.

Upvotes: 1

Views: 788

Answers (2)

user3344003
user3344003

Reputation: 21627

The CPU can go into a higher mode either through an exception or an interrupt. The CPU goes back to a lower mode using a return from interrupt instruction.

The CPU knows what mode it is running from the process status register. The operating system knows it is in kernel mode when it is executing a handler for an exception or interrupt.

The operating system sets up the page tables to define a range of addresses for the kernel. The tables are set up so that they are protected from access when the CPU is not in kernel mode.

Upvotes: 0

Devolus
Devolus

Reputation: 22084

On x86 architectures, this is achieved with descriptor tables. The startup code, installs tables which tells the CPU which memory regions have which privileges. When memory is accessed, the hardware will check if the instruction and/or access methods are valid for the given descriptor of the adress.

Well, that is a very basic description, as this is a rather broad question.

Some posting and link to get you started:

What are Ring 0 and Ring 3 in OS

http://duartes.org/gustavo/blog/post/cpu-rings-privilege-and-protection/

I would also recommend to download (for free) the Intel manuals as this is described in detail there as well.

http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html

Upvotes: 2

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