Function  3Dh   Open A File		     Open disk file with handle

entry   AH      3Dh
	AL      access code byte 
  (DOS 2.x)	bits 0-2  file attribute
			000	read only
			001	write only
			010	read/write
	  	bits 3-7 should be set to zero
  (DOS 3.x)	bits 0-2  file attribute
	    		000	read only
	    		001	write only
	    		010	read/write
	    	bit 3	reserved
	    		0	should be set to zero
	    	bits 4-6 sharing mode (network)
	    		000	compatibility mode (the way FCBs open files)
	    		001	read/write access denied (exclusive)
	    		010	write access denied
	    		011	read access denied
	    		100	full access permitted
	    	bit 7	inheritance flag
	    		0	file inherited by child process
	    		1	file private to child process
	DS:DX	address of ASCIIZ pathname string

return  flag CF set on error 
		AX      error code 
		1       error 

	AX      16 bit file handle 
		or error code (1, 2, 4, 5, 0Ch) 

note 1) Opens any normal, system, or hidden file.

     2) Files that end in a colon are not opened.

     3) The rear/write pointer is set at the first byte of the file and the
	record size of the file is 1 byte (the read/write pointer can be
	changed through Int\21f\42). The returned file handle must be used
	for all subsequent input and output to the file.

     4) If the file handle was inherited from a parent process or was
	duplicated by DUP or FORCEDUP, all sharing and access restrictions are
	also inherited.

     5) A file sharing error (error 1) causes an Int\24 to execute with an
	error code of 2.

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