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I have loaded an idt table with 256 entries, all pointing to similar handlers:
So when the common handler enters, the stack is properly aligned and contains the exception/interrupt number, error code (which may just be a dummy), eflags, cs and eip.
My question regards returning from the interrupt handler. I use iret to return after taking out the exception number and the error code from the stack, but this doesn't work for exception nr 8; if I leave the error code on the stack, then it returns fine!
Questions:
iret determine whether it has to pop an error code or not?
502
In computing, a page fault (sometimes called PF or hard fault) is an exception that the memory management unit (MMU) raises when a process accesses a memory page without proper preparations. Accessing the page requires a mapping to be added to the process's virtual address space.
The divide-error fault occurs during a DIV or an IDIV instruction when the divisor is zero.
Processor exceptions occur when this normal flow of execution is diverted, to allow the processor to handle events generated by internal or external sources. Examples of such events are: externally generated interrupts. an attempt by the processor to execute an undefined instruction.
If the CPU pushed an error code automatically, the handler must pop it before the iret. The iret instruction doesn't know where you're coming from, if it's a fault, a trap or an external interrupt. It always does the same, and it assumes that there's no error code on the stack.
Quoting from the SDM (Software Developer's Manual), Volume 3, Chapter 5, section 5.13 titled Error Code:
The error code is pushed on the stack as a doubleword or word (depending on the default interrupt, trap, or task gate size). To keep the stack aligned for doubleword pushes, the upper half of the error code is reserved. Note that the error code is not popped when the IRET instruction is executed to return from an exception handler, so the handler must remove the error code before executing a return.
You can find the IA-32 Software Developer's Manual here: http://www.intel.com/products/processor/manuals/
Volume 3 part 1, chapter 5, describes exception and interrupt handling. Volume 2 part 1 has the spec for the iret instruction.
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