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     The intdos function invokes the DOS system call specified by
     register values defined in <inregs>. It then returns the effect of
     the system call in <outregs>. The <inregs> and <outregs> arguments
     are unions of type REGS (defined in DOS.H).
 
     To invoke a system call, intdos executes an INT 21H instruction.
     Before executing the instruction, the function copies the contents
     of <inregs> to the corresponding registers. After the INT
     instruction returns, intdos copies the current register values to
     <outregs>. It also copies the status of the system carry flag to
     the cflag field in <outregs>. A nonzero cflag field indicates the
     flag was set by the system call and also indicates an error
     condition.
 
     The intdos function is used to invoke DOS system calls that take
     arguments for input or output in registers other than DX (DH/DL)
     or AL. The intdos function is also used to invoke system calls
     that indicate errors by setting the carry flag. Under any other
     conditions, the function bdos can be used.
 
     Do not use the intdos function to call interrupts that modify the
     DS register. Instead, use the intdosx or int86x function.
 
     Return Value
 
     The intdos function returns the value of the AX register after the
     system call is completed. If the cflag field in <outregs> is
     nonzero, an error has occurred and _doserrno is also set to the
     corresponding error code.
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