Interrupt 0F8h  Set Shell Interrupt (OEM)  
                Set OEM handler for Int\21 calls from 0F9h through 0FFh
entry   AH      0F8h 
	DS:DX   pointer to handler for Functions 0F9h thru 0FFh        
note 1) To reset these calls, pass DS and DX with 0FFFFh. DOS is set up to  
	allow ONE handler for all 7 of these calls. Any call to these handlers 
	will result in the carry bit being set and AX will contain 1 if they are 
	not initialized. The handling routine is passed all registers just as  
	the user set them. The OEM handler routine should be exited through an  
	IRET. 
     2) 10 ms interval timer (Tandy?)