Until recently, the idea of downloading a 700MB file was faintly ridiculous. These days, though, a standard 8Mbit/sec broadband connection can pull down a file that size in around ten minutes. That’s why we’re increasingly seeing large applications – and, in particular, operating systems – delivered as ISO images.
The beauty of an ISO file (as opposed to a standard ZIP file – another compressed file format that’s often used for download distribution) is that it doesn’t simply contain copies of files. An ISO file is in fact an image of the physical layout of an optical disc: more of a clone of that disc than a collection of files. This means it contains not just the files themselves but metadata about them.
More importantly, it means that the special hidden files and disc structure needed for a bootable disc are maintained. When the content of a bootable disc is correctly extracted to an ISO file, and then later burned back to a physical disc, that disc becomes bootable too: crucial for OS installation.
The history of ISOs
The file format itself is a venerable old thing. The file extension is ISO because the format of a CD or DVD is based on the ISO 9660 standard – a standard first mooted in 1986, when optical data storage was in its infancy. It’s one of the true successes of data compatibility: any DVD or Blu-ray drive can still read any old CD-ROM, thanks to the robustness of the format. And modern dual-layer DVD and Blu-ray discs are based on the UDF format, which is fully backward compatible with ISO 9660.
And it isn’t the inefficient file transfer medium it might seem: an ISO file isn’t fixed at the CD or DVD standard sizes such as 700MB or 4.7GB. There’s no requirement for the file to slavishly mirror the blank areas of a disc: as long as the required elements of the filing system are there, the ISO file itself can be very small – just a few megabytes or even less – but can still be burned to a standard CD or DVD.
Burning ISO images
For Windows 7 users, burning an ISO image to a physical disc is simplicity itself. As long as the file has the correct ISO extension, just right-click it and select Burn image from the resulting context menu.
For older Windows versions it isn’t so easy. Windows Vista and XP have built-in CD or DVD-burning abilities, but these are designed for writing files individually, not entire images. When burning an image, you must ensure the software is aware of ISO files, since the burning process is different.
If you just burn an ISO file to a disc using Windows XP’s built-in burning feature, for instance, it will create all the ISO 9660 structure for you, and then simply add the ISO image to the disc as a standard file, which is completely useless. To correctly burn an image, the software needs to explicitly copy the ISO structure embedded in the file – and all its associated filing system structure – onto the disc.
There are two great applications – IsoBuster
– that will allow you to expose and explore the contents of an ISO file without needing to burn it first; to burn ISO images correctly to a disc under any Windows OS; and to create your own ISO files.
The ability to open, alter and resave an ISO file opens up all sorts of possibilities for making life easier. You can, for instance, create a slipstreamed OS installation. Slipstreaming means taking the original installation CD or DVD, extracting its contents, updating it with the latest service packs and patches, compiling it to an ISO image, and then re-burning the modified ISO image to make an updated bootable OS installation disc.
On a simpler level, you can also easily add a folder or two of utilities – two typical old favourites being Paint.NET and FileZilla – to your existing installation media so you have a neat, portable operating system toolkit from which you can boot, install the OS and then the utilities or drivers you need.
That isn’t quite as simple as it initially sounds, though, since you need to make sure you preserve the boot-image areas of the disc; otherwise, you’ll just end up with a standard data disc that you can’t boot from. But the combo of IsoBuster and ImgBurn makes it a simple process, and trying it will give you some insight into the structure of CDs and DVDs too. Check out the walkthrough to see how to make your very own personalised OS installation disc, so you can install everything you need from one place.
Walkthrough: customising with IsoBuster and ImgBurn
Accept IsoBuster’s default installation options, but uncheck the option to install the IsoBuster Toolbar and associated fripperies. Also uncheck the offer to install the pointless Ask toolbar when you install ImgBurn.
Once both are installed, insert the bootable disc you want to modify, then fire up IsoBuster. Select the drive you want to examine from the dropdown list in the top left, or, if you’re working directly from an ISO file on disk, select File | Open Image File.
Once the disc has been examined, the structure will appear in the left-hand pane. We’re using a Windows XP disc here but most bootable discs will look similar, with a single session and a single track, within which are actual files.
Bootable discs such as OS images will have two entries below Track 01. The top one contains the files you’ll see by just browsing the disc in Windows Explorer; the lower Bootable Disc entry gives you access to the all-important bootable sector.
We want to extract both elements, so we can add to them and recreate a bootable disc. First, right-click on the main files entry (in this case, WXPVOL_EN) and select the Extract option. You can also do this using drag and drop in Windows Explorer.
Now right-click on Bootable Disc and select the file with the IMG extension. There will be only one of these and it’s the only file you need from here. Right-click it and select Extract, then save it somewhere convenient.
Now to construct your bootable disc with your added utilities. Close IsoBuster and fire up ImgBurn. Select “Create image file from files/folders”. Click the small “Switch to standard input” button below the free space readout on the left.
In Windows Explorer, browse to the content of the main folder, hit Ctrl-A and drag all the files and subfolders into the ImgBurn window. Note, don’t simply select the top-level WXPVOL_EN folder – you must drag in the individual files and subfolders.
Now we can add any other folders we want to the disc’s structure. Here, we’re adding a selection of installation files for some of our favourite free utilities, including Paint.NET, FileZilla, OpenOffice and the Windows Live Mesh installer.
In the Advanced tab on the right, click the secondary Bootable Disc tab. Check Make Image Bootable and browse for the IMG boot file you extracted in step 6. Leave the other settings as they are. Your image is ready.
Hit the Build button in the bottom left of the window. Enter a file name – it doesn’t matter what – and hit Save. The image will be generated. A few suggestions and warnings may appear. You can safely accept the defaults and ignore the warnings.
Your image is now ready to be burnt to a physical disc. Select Mode | Ez-Mode Picker to get back to ImgBurn’s startup window, and select “Write image file to disc”. Pop a blank disc in the drive, select your ISO image, hit the big Write button and away you go.