DOS Extenders such as PharLap's (used in Lotus 1-2-3) and many popular
compilers support DPMI-aware libraries and toolkits.  You are advised to
use such library calls rather than access DPMI services directly.

Write or call... Intel Literature JP26  (Phone: 800-548-4725)
3065 Bowers Ave.
P.O. Box 58065
Santa Clara CA 95051-8065

...for a printed version of the DPMI spec (with required background and
some semi-tutorial information).

  DPMI INT 31H Services are only available while in protected mode.
Use INT 2fH 1687H to obtain the address you must call to switch to
protected mode.
  Set up the registers and call that address (see example, below).
  When in protected mode, use INT 31H DPMI Services.
  To exit back to real mode, use DOS Fn 4cH (terminate).

The following code fragment illustrates switching into protected mode:

;--------------------- get the DPMI entry-address for mode switch
dd   pfpDpmiSwitch    ;we'll put the mode-switch entry address here
.
.
.
mov  AX, 1687h        ;see INT 2fH 1687H (get switch fn addr)
int  2fH
cmp  AX,0
jne  no_DPMI          ;bail out -- no DPMI host present

mov  pfpDpmiFns,di    ;save the addr of API entry point
mov  pfpDpmiFns+2,es
;--------------------- allocate the memory needed by the host
mov  BX,SI            ;required memory, in paragraphs
mov  AH,48H           ;DOS fn 48H allocates memory
int  21H
jc   err_exit         ;bail out, no mem available

mov  ES,AX            ;AX is the segment of the allocation
mov  AX,0             ;specify that caller is a 16-bit client
call pfpDpmiSwitch    ;call the switch address (see INT 2fH 1687H)
jc   cantSwitch       ;bail out on error

;--------------------- try a fn call
mov  AX,0400H         ;see INT 31H 0400H (get DPMI version)
int  31H
jc   errHandler
.
.
.
mov AX,4c00H          ;DOS fn 4cH terminates prog & switches to real mode
int 21H

See Also: DPMI Services
INT 2fH 16xxH (Enh386 Windows Services & Notifications)
- -

Using DPMI Services