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The double faults post is the 10th post now

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Philipp Oppermann
2017-03-25 16:38:35 +01:00
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blog/content/post/12-double-faults.md → blog/content/post/10-double-faults.md
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@@ -17,7 +17,7 @@ As always, the complete source code is available on [Github]. Please file [issue
## What is a Double Fault?
In simplified terms, a double fault is a special exception that occurs when the CPU fails to invoke an exception handler. For example, it occurs when a page fault is triggered but there is no page fault handler registered in the [Interrupt Descriptor Table][IDT] (IDT). So it's kind of similar to catch-all blocks in programming languages with exceptions, e.g. `catch(...)` in C++ or `catch(Exception e)` in Java or C#.
[IDT]: {{% relref "09-catching-exceptions.md#the-interrupt-descriptor-table" %}}
[IDT]: {{% relref "09-handling-exceptions.md#the-interrupt-descriptor-table" %}}
A double fault behaves like a normal exception. It has the vector number `8` and we can define a normal handler function for it in the IDT. It is really important to provide a double fault handler, because if a double fault is unhandled a fatal _triple fault_ occurs. Triple faults can't be caught and most hardware reacts with a system reset.
@@ -208,7 +208,7 @@ struct InterruptStackTable {
For each exception handler, we can choose an stack from the IST through the `options` field in the corresponding [IDT entry]. For example, we could use the first stack in the IST for our double fault handler. Then the CPU would automatically switch to this stack whenever a double fault occurs. This switch would happen before anything is pushed, so it would prevent the triple fault.
[IDT entry]: {{% relref "09-catching-exceptions.md#the-interrupt-descriptor-table" %}}
[IDT entry]: {{% relref "09-handling-exceptions.md#the-interrupt-descriptor-table" %}}
### Allocating a new Stack
In order to fill an Interrupt Stack Table later, we need a way to allocate new stacks. Therefore we extend our `memory` module with a new `stack_allocator` submodule: