CX:DX address of switcher service function handler
Returns: ES:BX 0000:0000 if no instance data chain
Info: This provides a way for application programs (such as TSRs) to
tell the switcher about any "instance data" (memory areas that
are unique for each task). For example, Doskey preserves a
command-line history for each DOS session, even though the
program code need be in memory once.
This fn creates a linked list of Identify Instance Data handlers.
Application programs which need to preserve instance data must
intercept INT 2fH and process fn AX=4b05H as follows:
If you don't need to process Instance Data requests, just JMP to
the previous INT 2fH handler.
If you do need instance data, your INT 2fH handler should:
First, chain to any previously-installed handlers by pushing
the FLAGS register and performing a FAR CALL to the previous
memory and fill it in (including the address of your list of
ES:BX into the pfNextDev field of the structure.
These steps result in a chain of INT 2fH handlers, each of which
can process subsequently-installed handlers.