ReactOS Forum

I've got great interest to how the processor/Windows do context-switch/function-calls between different privilege level.
I've read Intel's document and some other articles. I know x86 is capable of hardware task-switch. But wikipedia articles say that software context-switch is dominating the real world because of performance.
I'd like to know how the features of processor help in a multi-tasking system but it seems that the essential part of multi-tasking does not follow the hardware design.
And since I find that the Intel document is a bit obscure on the topic, no real workflow example is given. "CALL or JMP to TSS descriptor invoke a task switch." How the OS avoid using hardware task-switch? Does it disable TSS completely? I haven't read the Intel document throughout yet. Is there any good books/articles describe this issue with example workflow and details?

Thanks.

Hi!

Valeron wrote: I'd like to know how the features of processor help in a multi-tasking system but it seems that the essential part of multi-tasking does not follow the hardware design.

Look at OS sources, how they do it; then take debugger in hands.

There are some places in reactos:

• System calls. KiIntSystemCall and KiFastSystemCall. KiSystemService and KiFastCallEntry. KeReturnFromSystemCall.

• Context switch. KiSwapContext, KiSwapContextInternal

There is nothing magical in it.

"CALL or JMP to TSS descriptor invoke a task switch." How the OS avoid using hardware task-switch?

Simply does not CALL or JMP to TSS descriptor. :)

Does it disable TSS completely?

It is not completely disabled. In reactos it is used for NMI and double fault exception. See some code in Ki386InitializeTss.

Hi, hto, thanks for your reply.

I haven't read the codes you post yet. I've got another question up.
If I'm going to learn multi-tasking principle, e.g. how the OS and hardware incooperate to achieve context-switch/process memory protection/priviledge level protection..etc, in what order I should read the books/articles?

For example, I read x86 document first, the features relate to multi-tasking is just hardware only, an OS completely employee the hardware can definitely function right to set-up and multi-tasking architeture. But in real world, we know it's another world. The OS give up hardware context-switch but employee the memory protection feature of the processor and GDT/LDT/IDT. And we know there's some feature is mandated, e.g. memory segmentation. You know that the x86 document(volume 3: system programming guide) is very looooooong, and with little example. That makes it not easy to be read.
Should I first understand x86's system feature before understand real-world OS?

Valeron wrote: in what order I should read the books/articles?

I don't know what to suggest; join #ReactOS IRC channel, where most of the ros devs live. Maybe someone will point to the right direction.

Also, there is a wiki page with some links / books. Maybe it will be useful.

Should I first understand x86's system feature before understand real-world OS?

In order to understand low-level parts of OS, some knowledge of how the processor works is needed, of course.

You can begin from some OS, and when questions will arise, search answers in books / articles / on the Web / ask people.

Thank you.