Jerome Fine replies: NOT a silly question at all!! media use a sector size of 2048 bytes. When a block size of 512 bytes is required, usually the CDROM drive (perhaps in conjunction with the controller or host adapter) manages to make the conversion. This is not a difficult as it sounds since ONLY read requests are used. My personal experience is that it is not trivial. While I understand that Zane Healy has managed this with a PDP-11 system and is able to use a CD in a suitable CDROM drive, I have not been successful myself. BUT, I have never been take concerned or motivated to sole the problem for a PDP-11 system. However, using an emulator under Windows 98 SE on a 750 MHz Pentium III system with a standard motherboard, I am successful in running old RT-11 software. To do so requires the conversion from the 2048 bytes sectors to 512 byte blocks which are totally transparent as far as a hard drive comparison is concerned. In fact, when I recently acquired a DVD reader and the newest version of E11, I am even able to read the first 64 blocks on the CD (16 CD sectors) and boot RT-11 from the CD during a test. The reason that I call it a test is because it is obviously much easier to just use the ISO file for the CD and run that file using the SIMH emulator. What I am trying to say is that it may be necessary to do something special, but the use of a CD when 512 byte blocks are required for use with software which does not understand 2048 byte sectors on a CD should not be impossible if you can find a CDROM drive which will take care of the problem for you. Sincerely yours, Jerome Fine -- If you attempted to send a reply and the original e-mail address has been discontinued due a high volume of junk e-mail, then the semi-permanent e-mail address can be obtained by replacing the four characters preceding the 'at' with the four digits of the current year.