This section tells you how to change from 16:16 to 0:32-bit mode, or vice
versa.
There are two parts to thunking, the address transformation, and properly
aligning the stack, if necessary. The stack alignment is usually done by
a subroutine which detects the need to do this, and builds an 'extra' frame
in the new mode, properly aligned by making a copy of the incoming parms,
transforming the addresses as part of this process.
This works only because the specific implementation within OS/2 which was designed to use address tiling for LDT selectors.