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     The _dos_getvect routine uses system call 0x35 to get the current
     value of the interrupt vector specified by <intnum>.
 
     The _dos_setvect routine uses system call 0x25 to set the current
     value of the interrupt vector <intnum> to the function pointed to
     by <handler>. Subsequently, whenever the <intnum> interrupt is
     generated, the <handler> routine is called.
 
     If <handler> is a C function, it must have been previously
     declared with the _interrupt attribute. Otherwise, you must make
     sure that the function satisfies the requirements for an
     interrupt-handling routine.
 
     If <handler> is an assembler function, it must be a far routine
     that returns with an IRET instead of a RET.
 
     The _dos_setvect routine is generally used with the _dos_getvect
     function. To replace an interrupt vector, first save the current
     vector of the interrupt by using _dos_getvect. Then set the vector
     to your own interrupt routine with _dos_setvect. The saved vector
     can later be restored, if necessary, using _dos_setvect. The user-
     defined routine may also need the original vector in order to call
     it or to chain to it with _chain_intr.
 
     The _interrupt attribute indicates that the function is an
     interrupt handler. The compiler generates appropriate entry and
     exit sequences for the interrupt-handling function, including
     saving and restoring all registers and executing an IRET
     instruction to return.
 
     Registers and Interrupt Functions
 
     When you call an interrupt function, the DS register is
     initialized to the C data segment. This allows you to access
     global variables from within an interrupt function.
 
     In addition, all registers except SS are saved on the stack. You
     can access these registers within the function if you declare a
     function parameter list containing a formal parameter for each
     saved register. The following example illustrates such a
     declaration:
 
          void _interrupt _cdecl _far int_handler ( unsigned _es,
                       unsigned _ds, unsigned _di, unsigned _si,
                       unsigned _bp, unsigned _sp, unsigned _bx,
                       unsigned _dx, unsigned _cx, unsigned _ax,
                       unsigned _ip, unsigned _cs, unsigned _flags )
          {
          ∙ ∙ ∙
          }
 
     The formal parameters must appear in the opposite order from which
     they are pushed onto the stack. You can omit parameters from the
     end of the list in a declaration, but not from the beginning. For
     example, if your handler needs to use only DI and SI, you must
     still provide ES and DS, but not necessarily BX or DX.
 
     You can pass additional arguments if your interrupt handler is
     called directly from C rather than by an INT instruction. To do
     this, you must declare all register parameters and then declare
     your parameter at the end of the list.
 
     The compiler always saves and restores registers in the same fixed
     order. Thus, no matter what names you use in the formal parameter
     list, the first parameter in the list refers to ES, the second
     refers to DS, and so on. If your interrupt routines use in-line
     assembler, you should distinguish the parameter names so that they
     are not the same as the real register names.
 
     If you change any of the register parameters of an interrupt
     function while the function is executing, the corresponding
     register contains the changed value when the function returns. For
     example:
 
          void _interrupt _cdecl _far int_handler( unsigned _es,
                                                   unsigned _ds,
                                                   unsigned _di,
                                                   unsigned _si )
          {
              _di = -1;
          }
 
     This code causes the DI register to contain -1 when the <handler>
     function returns. It is not a good idea to modify the values of
     the parameters representing the IP and CS registers in interrupt
     functions. If you must modify a particular flag (such as the carry
     flag for certain DOS and BIOS interrupt routines), use the OR
     operator (|) so that other bits in the flags register are not
     changed.
 
     When an interrupt function is called by an INT instruction, the
     interrupt-enable flag is cleared. If your interrupt function
     needs to do significant processing, you should use the _enable
     function to set the interrupt flag so that interrupts can be
     handled.
 
     Precautions for Interrupt Functions
 
     Since DOS is not reentrant (a DOS interrupt cannot be called from
     inside a DOS interrupt), it is usually not safe to call from
     inside an interrupt function any standard library function that
     calls DOS INT 21H.
 
     Similar precautions apply to many BIOS functions. Functions that
     rely on INT 21H calls include I/O functions and the _dos family of
     functions. Functions that rely on the machine's BIOS include
     graphics functions and the _bios family of functions. It is
     usually safe to use functions that do not rely on INT 21H or BIOS,
     such as string-handling functions. Before using a standard library
     function in an interrupt function, be sure that you are familiar
     with the action of that library function.
 
     Return Value
 
     This _dos_getvect function returns a far pointer for the <intnum>
     interrupt to the current handler, if there is one. There is no
     return value for _dos_setvect.
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