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study/sabre/os/files/ProtectedMode/DPMI1.0Spec/ch4.4.3.html

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+Ordinarily, a handler installed with <a href="api/310205.html">DPMI
+Function 0205H</a> will only service software interrupts that are
+executed in protected mode; real mode software interrupts are passed
+to handlers installed with DOS Int 21H Function 25H issued from real
+mode, <a href="api/310201.html">DPMI Int 31H Function 0201H</a>, or by
+direct manipulation of the interrupt vector table at real mode address
+0000:0000.  However, there are three real mode software interrupts
+that a DPMI host will always reflect to a protected mode handler, if
+one is installed:
+
+<ul>
+Int 1CH - ROM BIOS timer tick interrupt<br>
+Int 23H - DOS Ctrl+C interrupt<br>
+Int 24H - DOS critical error interrupt<br>
+</ul>
+
+Clients should never terminate during the processing of interrupts
+that were reflected from real mode.  Such a termination might prevent
+the DPMI host from cleaning up the client's resources properly. <p>
+
+Protected mode handlers for software interrupts 0-7 are called with
+virtual interrupts disabled and trace flag reset, and these handlers
+should return with interrupts enabled.  All other software interrupts
+do not modify the interrupt flag state. <p>
+
+