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        Open disk file with handle (INT 21h, 3Dh)

        Entry:
          AH = 3Dh
          AL = access code
                 0 = Read Only
                 1 = Write Only
                 2 = Read/Write
          AL bits 7-3 = file-sharing modes
                bit 7    = inheritance flag, set for no inheritance
                bits 4-6 = sharing mode
                            000 compatibility mode
                            001 exclusive (deny all)
                            010 write access denied (deny write)
                            011 read access denied (deny read)
                            100 full access permitted (deny none)
                bit 3    = reserved, should be zero
           DS:DX = address of ASCIIZ filename
        Return:
          CF = 1 if error
                  AX = Error Code
          CF = 0 successful
                  AX = file handle