INT 28 - DOS 2+ - DOS IDLE INTERRUPT
Invoked each time one of the DOS character input functions loops while
waiting for input.  Since a DOS call is in progress even though DOS is
actually idle during such input waits, hooking this function is necessary
to allow a TSR to perform DOS calls while the foreground program is
waiting for user input.  The INT 28h handler may invoke any INT 21 function
except functions 00h through 0Ch.  Under DOS 2.x, the critical error flag
(the byte immediately after the InDOS flag) must be set in order to call
DOS functions 50h/51h without destroying the DOS stacks.

Notes:  calls to INT 21/AH=3Fh,40h may not use a handle which refers to CON
at the time of the call, the InDOS flag (see INT 21/AH=34h) is normally
set to 01h; if larger, DOS is truly busy and should not be reentered
the default handler is an IRET instruction
supported in OS/2 compatibility box
SeeAlso: INT 2A/AH=84h

See also: 2134

28 - DOS 2+ - DOS IDLE INTERRUPT