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study/sabre/os/files/ProtectedMode/DPMI1.0Spec/ch4.4.2.html

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+The programmable interrupt controllers are mapped by the DPMI host to
+the system's default interrupt assignments.  On an IBM AT-compatible
+system, for example, the master interrupt controller (IRQ0 through
+IRQ7) is programmed to use a base interrupt level of 8 and the slave
+controller (IRQ8 through IRQ15) uses a base interrupt level of 70H.<p>
+
+All of the code and data that may be touched by hardware interrupt
+handlers must reside in locked memory to avoid page faults at
+interrupt time.  The handler will always be called on a locked stack.
+As in real mode, hardware interrupt handlers are called with virtual
+interrupts disabled and the trace flag reset.  In systems where the
+CPU's interrupt flag is virtualized, <tt>IRET</tt> may not restore the
+interrupt flag.  Therefore, clients should execute a <tt>STI</tt>
+before executing <tt>IRET</tt> or else interrupts will remain
+disabled. <p>
+
+Protected mode hardware interrupt handlers that call a real mode
+routine must either ensure that the real mode code will not modify
+segment registers or user the DPMI state save/restore services (see
+page 94).  However, any interrupt handler that executes completely in
+protected mode, or uses the translation services (<a
+href="api/310300.html">Int 31H Functions 0300H</a>, <a
+href="api/310301.html">0301H</a>, or <a
+href="api/310302.html">0302H</a>), does not need to save the real mode
+register state. <p>
+
+Personal computers with two programmable interrupt controllers usually
+have a BIOS that redirects one of the interrupts from the slave
+controller into the range of the master controller for compatibility
+with older, 8086/88-based systems.  For example, devices jumpered for
+IRQ2 on PC/AT-compatible computers actually interrupt on IRQ 9 (Int
+71H), but the BIOS on these systems converts Int 71H to Int 0AH yet
+sends the EOI command (appropriately) to the slave controller.  A
+protected mode client that needs access to the redirected interrupt
+might use a variation on one of the following techniques:
+
+<ul>
+
+<li> Install only a real mode handler for the target interrupt, taking
+advantage of the built-in redirection.  This method is robust on
+systems where other software has reprogrammed the interrupt
+controllers, or where the slave interrupt controller may be
+absent. <p>
+
+<li> Install both real mode and protected mode handlers for the target
+interrupt.  In such cases, the program must send the EOI command to
+both the slave and master interrupt controllers since the BIOS is
+never called.  This method is more efficient in that there are not any
+unnecessary switches to real mode. <p>
+
+</ul>
+
+