DPMI clients are loaded in real mode. In order to enter protected mode, the client must first call Int 2FH Function 1687H to test for the presence of a DPMI host and obtain the address of the real-to-protected mode switch entry point. Function 1687H also returns information about the DPMI host's capabilities and the size of a private data area which will be used by the host to hold client-specific data structures. After allocating the required private data area via the normal real mode memory allocation interface (DOS Int 21H Function 48H), the client makes a FAR CALL to the mode switch entry point with the following parameters:

AXflags
BitSignificance
00 = 16-bit application
1 = 32-bit application
1-15Reserved, should be zero
ESreal mode segment of DPMI host private data area (must be at least as large as the size returned in SI by Int 2FH Function 1687H; ES is ignored if the size was zero.

If the Carry flag is set upon return, the mode switch was unsuccessful, the client is still running in real mode, and register AX contains one of the following error codes:

8011Hdescriptor unavailable (cannot allocate descriptors for CS, DS, ES, SS, PSP, and environment pointer)
8021Hinvalid value (32-bit program specified but not supported)

If the mode switch was successful, the mode switch routine returns to the call in protected mode with the Carry flag clear and the following results:

CS16-bit selector with base of real mode CS and a 64KB limit
SSselector with base of real mode SS and a 64 KB limit
DSselector with base of real mode DS and a 64 KB limit
ESselector to program's PSP with a 100H byte limit
FS0 (if running on an 80386 or later)
GS0 (if running on an 80386 or later)

All other registers are preserved, except that for 32-bit clients the high word of ESP will be forced to zero. 32-bit clients will initially run with a 16-bit code segment, but all Int 31H calls will still require 48-bit pointers, and the stack and data descriptors will be 32-bit (the Big bit will be set in the descriptors). Note that if DS+SS at the time of the mode switch call, only one descriptor may be allocated, and the same selector may be returned in DS and SS. The client is allowed to modify or free the CS, DS and SS descriptors allocated by the mode switch routine.

The environment pointer in the client program's PSP is automatically converted to a selector during the mode switch. If the client wishes to free the memory occupied by the environment, it should do so before entering protected mode and zero the word at PSP:2CH (segment address of the environment>. The client may change the environment point in the PSP after entering protected mode but it must restore it to the selector created by the DPMI host before terminating. The client should not modify or free the PSP or environment descriptors supplied by the DPMI host.

Example: The following code illustrates how a DPMI client would obtain the address of the mode switch entry point, allocate the DPMI host private data area, and enter protected mode.

modesw	dd	0			; far pointer to DPMI host's
					; mode switch entry point
	.
	.
	.
	mov	ax,1687h		; get address of DPMI host's
	int	2fh			; mode switch entry point
	or	ax,ax			; exit if no DPMI host
	jnz	error
	mov	word ptr modesw,di	; save far pointer to host's
	mov	word ptr modesw+2,es	; mode switch entry point
	or	si,si			; check private data area size
	jz	@@1			; jump if no private data area

	mov	bx,si			; allocate DPMI private area
	int	21h			; transfer to DOS
	jc	error			; jump, allocation failed
	mov	es,ax			; let ES=segment of data area

@@1:	mov	ax,0			; bit 0=0 indicates 16-bit app
	call	modesw			; switch to protected mode
	jc	error			; jump if mode switch failed
					; else we're in prot. mode now
	.
	.
	.