DOS extender functions
 DosAllocHuge()                Allocate an extended memory block of up to 16 Mb
 DosAllocRealSeg()             Allocate a conventional memory block
 DosAllocSeg()                 Allocate an extended memory block of up to 64 Kb
 DosCreateCSAlias()            Create an executable (code) selector corresponding to a data selector
 DosCreateDSAlias()            Create a data selector corresponding to an executable (code) selector
 DosFreeRealSeg()              Free a selector previously allocated with DosAllocRealSeg()
 DosFreeSeg()                  Free a memory block allocated with DosAllocSeg() or DosAllocHuge()
 DosFreeSelector()             Free a selector allocated with DosMapRealSeg(), DosCreateCSAlias()
 DosGetBIOSSeg()               Get a selector for the 1 Kb BIOS data area
 DosGetExceptionHandler()      Get the protected mode address of an exception handler
 DosGetHostMode()              Get the current DOS extender host mode (DPMI, VCPI, XMS)
 DosGetHugeShift()             Get the left shift count to apply to 1 to get the huge selector increment
 DosGetMachineMode()           Get the current machine mode (real, 286 protected)
 DosGetProtVec()               Get a protected mode pointer to a protected mode interrupt handler
 DosGetRealProtVec()           Get both real and protected mode pointers to the real and protected mode
 DosGetRealVec()               Get a real mode pointer to a real mode interrupt handler
 DosGetSegBase()               Get the base address of a segment selector
 DosGetSegDesc()               Get the descriptor structure of a segment selector
 DosGetSegLimit()              Get the size limit of a segment selector
 DosIsBlinkX()                 Determine whether the program is running under the Blinker DOS extender
 DosLockSeg()                  Lock a segment in memory to prevent it being swapped out by the Blinker
 DosMapRealSeg()               Convert a real mode paragraph value into a protected mode selector
 DosMemAvail()                 Get total available extended memory
 DosProtToReal()               Get a real mode pointer corresponding to a protected mode pointer
 DosRealAvail()                Get total available real mode memory
 DosRealFarCall()              Make a far call to a real mode address
 DosRealIntr()                 Generate a real mode interrupt from protected mode
 DosReallocHuge()              Resize an extended memory block of up to 16 MB
 DosReallocSeg()               Resize an extended memory block of up to 64 Kb
 DosRealToProt()               Get a protected mode pointer corresponding to a real mode pointer
 DosSetExceptionHandler()      Install a protected mode exception handler
 DosSetProtVec()               Install a protected mode interrupt handler
 DosSetRealVec()               Install a real mode interrupt handler
 DosSetSegAttrib()             Set segment attributes for a selector
 DosUnlockSeg()                Unlock a segment previously locked using DosLockSeg()
 DosVerifyAccess()             Get the read/write/code/data flags for a selector
 DosVMAvail()                  Get the amount of available virtual memory