INT 21 U - Headroom - API(Application Program[ming] Interface) The defined set of calls which a program may make to interact with or request services of the operating system or environment under which it is running.  Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes.
	AX = 5758h
	BL = function
	    00h ???
	    01h get Headroom location
		Return: CF clear if installed
			    AX = PSPsee Program Segment Prefix segment of Headroom TSR(Terminate and Stay Resident) A program which remains in memory after terminating in order to provide services to other programs or the user.  The name comes from the name of the DOS function call used to remain in memory after termination.
			    BX = paragraphs of memory used by Headroom
			CF set if not (normal DOS return)
		Note:	this function is also used as an installation check
	    02h get INT 21 handler
		Return: CF clear
			ES:BX -> Headroom's INT 21 handler
		Note: also sets unknown flag
	    03h launch application???
		DS:SI -> 233-byte application record
		Return: ???
	    04h ???
		???
		Return: CF clear
	    05h get swap directory
		Return: CF clear
			DX:AX -> ASCIZA NUL-terminated ASCII string.	The ASCIZ string "ABC" consists of the four bytes 41h, 42h, 43h, and 00h.  Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL. swap directory name
	    06h ???
		DX = ???
		Return: CF clear
	    07h ???
	    08h ???
	    09h get current application
		Return: BX = application number
	    0Ah ???
		DX = application number
		DS:SI = ???
		Return: ???
	    0Bh ???
	    0Ch ???
		DX = application number
		???
		Return: ???
	    0Dh ???
		DX = application number
		???
		Return: ???
	    0Eh get ???
		Return: CF clear
			AX = ???
	    0Fh set ??? flag
	    10h clear ??? flag
	    11h find application by name
		DS:SI -> ASCIZA NUL-terminated ASCII string.	The ASCIZ string "ABC" consists of the four bytes 41h, 42h, 43h, and 00h.  Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL. application name
		Return: CF clear
			AX = application number or FFFFh if not loaded
	    12h ???
		DX = application number
		Return: CF clear
			???
	    13h ???
		Return: CF clear
	    14h ???
		same as function 13h
	    15h set ???
		DX = ???
	    16h get ???
		Return: AX = ??? set by function 15h
	    17h get ???
		Return: BX = ???
			CX = ??? (may be pointer in BX:CX)
	    18h BUG: branches incorrectly due to fencepost error
Program: Headroom is a TSR(Terminate and Stay Resident) A program which remains in memory after terminating in order to provide services to other programs or the user.  The name comes from the name of the DOS function call used to remain in memory after termination./task switcher by Helix Software
SeeAlso: AX=4C57h,AX=5757h,INT 2F/AX=5758h