INT 20H Terminate a program             superseded 
INT 21H DOS Services

INT 22H Terminate address                
INT 23H Control-Break address             Address Vectors Only
INT 24H Critical Error Handler address   

INT 25H/26H Absolute Disk Read/Write

INT 27H Terminate but Stay Resident     superseded 
INT 28H DOS Idle (safe to pop up)       wasUndoc 
INT 29H DOS Internal Fast Screen Write  undocumented 
INT 2eH Perform DOS Command             undocumented 

INT 2fH Multiplex (DoubleSpace, spooler, TSR control, other APIs)

INT 31H DPMI DOS Protected Mode Interface Services
INT 33H Mouse Support
INT 67H EMS Expanded Memory Manager (HIMEM.SYS)
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DOS Interrupts