Exception and Interrupts Notes of 80386 Programmer manual

1. Interrupts
   • Maskable interrupts, which are signalled via the INTR pin.
   • Nonmaskable interrupts, which are signalled via the NMI (Non-Maskable Interrupt) pin.
2. Exceptions
   • Processor detected. These are further classified as faults, traps, and aborts.
   • Programmed. The instructions INTO, INT 3, INT n, and BOUND can trigger exceptions. These instructions are often called "software interrupts", but the processor handles them as exceptions.

Faults

Faults are exceptions that are reported "before" theinstruction causingthe exception. Faults are either detected beforethe instruction begins to execute, or during execution of theinstruction. If detected during the instruction, the fault isreported with the machine restored to a state that permits theinstruction to be restarted.

Traps

A trap is an exception that is reported at the instruction boundary immediately after the instruction in which the exception was detected.

Aborts

An abort is an exception that permits neither precise location of the instruction causing the exception nor restart of the program that caused the exception. Aborts are used to report severe errors,such as hardware errors and inconsistent or illegal values in system tables.

Table 1. Interrupt and Exception ID Assignments

Identifier   Description

0            Divide error
1            Debug exceptions
2            Nonmaskable interrupt
3            Breakpoint (one-byte INT 3 instruction)
4            Overflow (INTO instruction)
5            Bounds check (BOUND instruction)
6            Invalid opcode
7            Coprocessor not available
8            Double fault
9            (reserved)
10           Invalid TSS
11           Segment not present
12           Stack exception
13           General protection
14           Page fault
15           (reserved)
16           Coprecessor error
17-31        (reserved)
32-255       Available for external interrupts via INTR pin

Basics of Protected Control Transfer

Exceptions and interrupts are both" protected control transfers,"which cause the processor to switch from user to kernel mode(CPL=0) without giving the user-mode code any opportunity to interfere with the functioning of the kernel or other environments.

Interrupt is asynchronous, exception is synchronous. X86 use IDT(Interrupt Descriptor Table, a mini GDT for interrupt) and TSS(Task State Segment, to save the context of current task).

Example

Let's say the processor is executing code in a user environment and encounters a divide instruction that attempts to divide by zero.

1. The processor switches to the stack defined by theSS0 and ESP0 fields of the TSS,which in JOS will hold the values GD_KD and KSTACKTOP, respectively.
2. The processor pushes the exception parameters on the kernel stack, starting at address KSTACKTOP:

                        +--------------------+ KSTACKTOP             
                        | 0x00000 | old SS   |     " - 4
                        |      old ESP       |     " - 8
                        |     old EFLAGS     |     " - 12
                        | 0x00000 | old CS   |     " - 16
                        |      old EIP       |     " - 20 <---- ESP 
                        +--------------------+             
   	

3. Because we're handling a divide error,which is interrupt vector 0 on the x86,the processor reads IDT entry 0 and setsCS:EIP to point to the handler function described by the entry.
4. The handler function takes control and handles the exception,for example by terminating the user environment.

For certain types of x86 exceptions,in addition to the "standard" five words above,the processor pushes onto the stack another word containing an error code.The page fault exception, number 14,is an important example.See the 80386 manual to determine for which exception numbers the processor pushes an error code,and what the error code means in that case.When the processor pushes an error code,the stack would look as follows at the beginning of the exception handler when coming in from user mode:

                     +--------------------+ KSTACKTOP             
                     | 0x00000 | old SS   |     " - 4
                     |      old ESP       |     " - 8
                     |     old EFLAGS     |     " - 12
                     | 0x00000 | old CS   |     " - 16
                     |      old EIP       |     " - 20
                     |     error code     |     " - 24 <---- ESP