10. Interrupt Services

• 10.1 Get Real Mode Interrupt Vector
• 10.2 Set Real Mode Interrupt Vector
• 10.3 Get Processor Exception Handler Vector
• 10.4 Set Processor Exception Handler Vector
• 10.5 Get Protected Mode Interrupt Vector
• 10.6 Set Protected Mode Interrupt Vector

These services allow protected mode applications to intercept real and protected mode interrupts and hook processor exceptions.

10.1 Get Real Mode Interrupt Vector

This function returns the value of the current task's real mode interrupt vector for the specified interrupt.

To Call

     AX = 0200h
     BL = Interrupt number

Returns

     Carry flag is clear.
     CX:DX = Segment:Offset of real mode interrupt handler

Programmer's Notes

• The address returned in CX is a segment, not a selector. Therefore you should not attempt to place the value returned in CX into a segment register in protected mode or a general protection fault may occur.
• Note all 100h (256 decimal) interrupt vectors must be supported by the DPMI host.

10.2 Set Real Mode Interrupt Vector

This function sets the value of the current task's real mode interrupt vector for the specified interrupt.

To Call

     AX = 0201h
     BL = Interrupt number
     CX:DX = Segment:Offset of real mode interrupt handler

Returns

     If function was successful:
     Carry flag is clear.

     If function was not successful:
     Carry flag is set.

Programmer's Notes

• The address passed in CX must be a real mode segment, not a selector.
• If the interrupt being hooked is a hardware interrupt then you must lock the segment that the interrupt handler runs in as well as any memory the handler may touch at interrupt time.
• The address contained in CX:DX must be a real mode segment:offset, not a selector:offset. This means that the code for the interrupt handler must either reside in DOS addressable memory or you must use a real mode call-back address. Refer to the section on DOS memory management services in section 9.1 for information on allocating memory below 1 megabyte. Information on real mode call back addresses can be found in section 11.4.

10.3 Get Processor Exception Handler Vector

This function returns the CS:(E)IP of the current protected mode exception handler for the specified exception number.

To Call

     AX = 0202h
     BL = Exception/fault number (00h-1Fh)

Returns

     If function was successful:
     Carry flag is clear.
     CX:(E)DX = Selector:Offset of exception handler

     If function was not successful:
     Carry flag is set.
     The value passed in BL was invalid.

Programmer's Notes

• The value returned in CX is a valid protected mode selector, not a real mode segment.
• 32-bit mode programs will be returned a 32-bit offset in the EDX register.

10.4 Set Processor Exception Handler Vector

This function allows protected mode applications to intercept processor exceptions that are not handled by the DPMI environment. Programs may wish to handle exceptions such as not present segment faults which would otherwise generate a fatal error.

Every exception is first examined by the protected mode operating system. If it can not handle the exception it then reflects it through the protected mode exception handler chain. The final handler in the chain may either reflect the exception as an interrupt (as would happen in real mode) or it may terminate the current program.

To Call

     AX = 0203h
     BL = Exception/fault number (00h-1Fh)
     CX:(E)DX = Selector:Offset of exception handler

Returns

     If function was successful:
     Carry flag is clear.

     If function was not successful:
     Carry flag is set.
     The value passed in BL was invalid.

Programmer's Notes

• The value passed in CX must be a valid protected mode code selector, not a real mode segment.
• 32-bit mode programs must supply a 32-bit offset in the EDX register. If your handler chains to the next exception handler it must do so using a 32-bit interrupt stack frame.
• The handler should return using a far return instruction. The original SS:(E)SP, CS:(E)IP and flags on the stack, including the interrupt flag, will be restored.
• All fault stack frames have an error code. However, the error code is only valid for exceptions 08h, 0Ah, 0Bh, 0Ch, 0Dh, and 0Eh.
• The handler must preserve and restore all registers.
• The exception handler will be called on a locked stack with interrupts disabled. The original SS, (E)SP, CS, and (E)IP will be pushed on the exception handler stack frame.
• The handler must either return from the call by executing a far return or jump to the next handler in the chain (which will execute a far return or chain to the next handler).
• The procedure can modify any of the values on the stack pertaining to the exception before returning. This can be used, for example, to jump to a procedure by modifying the CS:IP on the stack. Note that the procedure must not modify the far return address on the stack -- it must return to the original caller. The caller will then restore the flags, CS:(E)IP and SS:(E)SP from the stack frame.
• If the DPMI client does not handle an exception, or jumps to the default exception handler, the host will reflect the exception as an interrupt for exceptions 0, 1, 2, 3, 4, 5, and 7. Exceptions 6, and 8-1Fh will be treated as fatal errors and the client will be terminated.
• Exception handlers will only be called for exceptions that occur in protected mode.

Call-Back Stack Frames

Stack frame for 16-bit programs:

                      15       0


                          SS

                          SP

                        Flags

                          CS

                          IP

                       Err Code


                        Return
                          CS

                        Return
                          IP

                                 <-- SS:SP

Stack frame for 32-bit programs:

                 31            0


                           SS


                       ESP

                     EFlags

                           CS



                       EIP


                   Error Code

                         Ret CS

                   Return EIP


                                 <-- SS:ESP

Shaded fields should not be modified. Other fields can be modified before returning from the exception handler.

10.5 Get Protected Mode Interrupt Vector

This function returns the CS:(E)IP of the current protected mode interrupt handler for the specified interrupt number.

To Call

     AX = 0204h
     BL = Interrupt number

Returns

     Carry flag is clear.
     CX:(E)DX = Selector:Offset of exception handler

Programmer's Notes

• The value returned in CX is a valid protected mode selector, not a real mode segment.
• 32-bit mode programs will be returned a 32-bit offset in the EDX register.
• All 100h (256 decimal) interrupt vectors must be supported by the DPMI host.

10.6 Set Protected Mode Interrupt Vector

This function sets the address of the specified protected mode interrupt vector.

To Call

     AX = 0205h
     BL = Interrupt number
     CX:(E)DX = Selector:Offset of exception handler

Returns

     If function was successful:
     Carry flag is clear.

     If function was not successful:
     Carry flag is set.

Programmer's Notes

• The value passed in CX must be a valid protected mode code selector, not a real mode segment.
• 32-bit mode programs must supply a 32-bit offset in the EDX register. If your handler chains to the next exception handler it must do so using a 32-bit interrupt stack frame.
• Note all 100h (256 decimal) interrupt vectors must be supported by the DPMI host.