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=00H, AL=01H
Frees an LDT descriptor.
AX = 0001H BX = selector for the descriptor to free
if function successful Carry flag = clear if function unsuccessful Carry flag = set AX = error code 8022H invalid selector
Each descriptor allocated with Int 31H Function 0000H must be freed individually with this function, even if it was previously allocated as part of a contiguous array of descriptors.
Under DPMI 1.0 hosts, any segment registers which contain the selector being freed are zeroed by this function.
Refer to the rules for descriptor usage in Appendix D.
Text based on http://www.delorie.com/djgpp/doc/dpmi/