INT 15H handles a set of AT-specific services.

AH/AX BIOS Service
                                     
00H   Cassette motor ON   
01H   Cassette motor OFF    Oldest PCs only (descriptions
02H   read cassette         omitted for lack of interest)
03H   write cassette      

4fH   keyboard intercept

53xxH Advanced Power Management (APM) services

(*) 80H   device open
(*) 81H   device close
(*) 82H   program termination
8300H set event wait interval
8301H cancel event wait interval
84H   joystick
(*) 85H   SysReq key
86H   wait
87H   move extended  memory block
88H   get extended memory size
89H   switch to virtual 8086 mode

(*) 90H   device busy loop
(*) 91H   finish interrupt

c0H   get system configuration

(*) services marked with (*) are designed for use with multitasking
operating systems and will only perform the described service if
they have been intercepted by a multitasking operating system.
By default (and when MS-DOS is the active system), these fns
simply clear the Carry Flag and return immediately.

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INT 15H: AT Extended Services / APM