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=08H, AL=01H
Free Physical Address Mapping
AX = 0801H BX:CX = linear address returned by physical address mapping call
if function successful Carry flag = clear if function unsuccessful Carry flag = set AX = error code 8025H invalid linear address
Releases a mapping of physical to linear addresses that was previously obtained with the Physical Address Mapping function (Int 31H Function 0800H).
The client should call this function when it is finished using a device previously mapped to linear addresses with the Physical Address Mapping function (Int 31H Function 0800H).
Text based on http://www.delorie.com/djgpp/doc/dpmi/