Note: This API calls are shared between DOS and Win16 personality.
DPMI is a shared interface for DOS applications to access Intel 80286+ CPUs services. DOS DMPI host provides core services for protected mode applications. Multitasking OS with DOS support also provides DMPI in most cases. Windows standard and extended mode kernel is a DPMI client app. Standard and extended mode kernel differs minimally and shares common codebase. Standard Windows kernel works under DOSX extender. DOSX is a specialized version of 16-bit DPMI Extender (but it is standard DPMI host). Standard mode is just DPMI client, exnhanced mode is DPMI client running under Virtual Machime Manager (really, multitasker which allow to run many DOS sessions). Both modes shares DPMI interface for kernel communication. The OS/2 virtual DOS Protected Mode Interface (VDPMI) device driver provides Version 0.9 DPMI support for virtual DOS machines. Win16 (up to Windows ME) provides Version 0.9 DPMI support.
DPMI host often merged with DPMI extender. Usually DPMI extender provide DPMI host standard services and DOS translation or True DPMI services.
Int 31H, AH=03H, AL=04H
Free Real Mode Callback Address
AX = 0304H CX:DX = real mode callback address to be freed
if function successful Carry flag = clear if function unsuccessful Carry flag = set AX = error code 8024H invalid callback address
Releases a real mode callback address that was previously allocated with the Allocate Real Mode Callback Address function (Int 31H Function 0303H).
Real mode callbacks are a limited system resource. A client should release any callback that it is no longer using.
Text based on http://www.delorie.com/djgpp/doc/dpmi/