Int 2FH, AH=4BH, AL=01H

?

BUILD CALLOUT CHAIN

AX = 4B01h
CX:DX -> task switcher entry point (see #02819)
ES:BX = 0000h:0000h

ES:BX -> callback info structure (see #02817) or 0000h:0000h

called by the task switcher

this function is hooked by clients which require notification of task
  switcher activities; the call must first be passed on to the prior
  handler with registers unchanged using a simulated interrupt.	 On
  return, the client must build a callback info structure and store
  the returned ES:BX in the "next" field, then return the address of
  its own callback info structure.
a client program must add itself to the notification chain if it
  provides services to other programs; before terminating, it must
  remove itself from the chain by calling the task switcher's entry
  point with AX=0005h (see #02819)
the task switcher entry point should not be saved, as it is subject to
  change and will be provided on any notification call
the Windows 3.1 Standard Mode supports this API

AX=AX=4B02h

Text based on Ralf Brown Interrupt List Release 61