A protected mode client terminates by executing an Int 21H in protected mode, passing the value 4CH in register AH and a return code in register AL. (This mimics the Int 21H Function 4CH termination used by DOS applications in real mode.> The client has the following responsibilities before termination:

real mode memory that was unlocked for paging with Int 31H Function 0602H must be relocked with Int 31H Function 0603H;
interrupts hooked by the client for real mode with Int 31H Function 0201H must be released by restoring the address of the original owner of the interrupt.
protected mode handlers for real mode exceptions installed with Int 31H Function 0213H should be cleaned up if possible.

When the DPMI host detects an Int 21H Function 4CH termination request, it takes the following actions (the detailed comparison of DPMI version 0.9 and version 1.0 host termination handling is in Appendix C, page 158>:

any extended memory blocks that were previously allocated with Function 0501H or 0504H are unlocked and freed (this is the only cleanup action required by a DPMI V 0.9 host>;
the client's local descriptor table (LDT) is freed in its entirety by DPMI 1.0 hosts. (A DPMI version 0.9 client should clean up its own segment descriptors before its termination since some DPMI version 0.9 hosts may not free the terminating client's segment descriptors if the client is not the primary client);
physical address mappings created with Int 31H Function 0800H are freed;
mappings created with Int 31H Functions 0508H or 0509H are destroyed;
the client's interrupt descriptor table (IDT) is freed in it's entirety and any client exception handlers installed through Functions 0203H, 0212H and 0213H are deregistered;
any real mode regions that were unlocked with Function 0602H are relocked;
any real mode callbacks that were allocated with Function 0303H are deallocated;
the client's shared memory block allocations and serializations are freed;
any debug watchpoints that were set with Function 0B00H are cleared;
the coprocessor state (if any) is restored to the default.

After the DPMI host performs the cleanup activities listed above, it will switch to real mode and re-issue the Int 21H Function 4CH interrupt, passing the return code from the DPMI client down to DOS. DOS will then terminate the client as a real mode process by releasing its DOS memory blocks (whether allocated by real mode Int 21H Function 48H or by DPMI Function 0100H), flushing file buffers, closing file and device handles, and so on.

Clients should only terminate from their main thread of execution, and should not issue the protected mode Int 21H Function 4CH from within a hardware interrupt handler, exception handler, or real mode callback. Client may, however, terminate from within a protected mode routine that has been entered via the DPMI raw mode switch service. Clients which wish to terminate-and-stay-resident to provide services to protected mode clients should use DPMI Function 0C01H rather than Int 21H Function 31H.

Note: Although the DPMI host monitors for Int 21H Function 4Ch in protected mode, it ignores all other Int 21H Functions. DOS Extenders typically install an interrupt 21H handler of their own in order to trap and service DOS function requests by a protected mode application; thus, the DOS Extender's Int 21H handler will always see the Function 4CH termination request first. The DOS Extender should perform any cleanup activities of its own and then pass the termination request to the DPMI host by chaining the original owner of the protected mode Int 21H vector.