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 2FH, AH=16H, AL=80H
Release Current Virtual Machine's Time Slice
AX = 1680H
If function supported by host:
AL = 0
If function not supported by host:
AL = unchanged (80H)
This function is not specific to DPMI hosts. Some operating systems will recognize this call for programs running in real mode. Programmers are encouraged to use this call in all DOS and DPMI-client programs. All DPMI hosts will hook Int 2FH and so a DPMI client can use this API without any other precautions. Non-DPMI programs that can run on DOS 2.xx or earlier should make sure that the Int 2FH vector is non-zero before executing the Int 2FH. When an application calls this function it will regain control at intervals, so it should continue to re-issue this function call so long as it has nothing to do. DPMI client and application vendors are encouraged to use this function. It can significantly improve the performance of a DOS-based multitasking host.
Text based on http://www.delorie.com/djgpp/doc/dpmi/