INT 15,90 - Device Busy


	AH = 90h
	AL = type code:
	   = 00  disk
	   = 01  diskette
	   = 02  keyboard
	   = 03  pointing device
	   = 80  network (ES:BX = NCB)
	   = FC  fixed disk reset (PS/2)
	   = FD  diskette motor start
	   = FE  printer
	ES:BX = pointer to network control block if waiting for network

	on return:
	CF = 0 if wait not satisfied
	   = 1 if wait time satisfied
	AH = when CF set, 80h for PC & PCjr, 86h for XT (BIOS 11/8/82)

	- tells the OS a a program is about to wait for a device
	- used for multitasking OS development