Under the Hood
This article describes how the advanced features of WinISO work under the hood.
With quick save enabled, WinISO tries to minimize data writes to reduce the time required for saving changes. This feature is ideal for updating small files within a large disc image file, especially on slow storage devices such as HDDs.
However, useless data may be left in the image file. To eliminate them, use Save As to rebuild a new copy.
This feature is enabled by default. It can be configured in the Save tab in the Configurations dialog box. If this feature is disabled, the entire image is rebuilt on each save, and additional space is required for the rebuilding.
The example below shows how it works for common tasks.
When a file is deleted, only the file system data is updated, while the file content is not overwritten. The space occupied by deleted files is considered free space.
The free space is used for storing newly added files.
If the free space is insufficient, new files will be appended to the end of the image file, increasing the image file size.
By saving a new copy or disabling the quick save feature, the entire image is rebuilt. The image file size may be reduced since all data in the free space is discarded.
With file deduplication enabled, WinISO looks for all files with the same content during the save process, and makes them share the same data block. It may reduce the size of the image file, but it takes more time to find duplicate files.
This feature is not enabled by default. It can be configured in the Save tab in the Configurations dialog box.
In the example below, assuming that the content of File A, File B, and File 1 are the same, file deduplication results in a smaller image file.
Bridged and Non-Bridged Modes
An optical disc may contain multiple file systems, typically ISO 9660 and UDF. WinISO offers two authoring modes: bridged mode and non-bridged mode.
In bridged mode, the contents of each file system are the same.
For example, almost all DVD-Video discs use a combination of UDF and ISO 9660 file systems to provide backwards compatibility for operating systems that support only ISO 9660.
In non-bridged mode, the contents of each file system are independent.
For example, the Windows 7 installer disc stores installation files in the UDF file system and a plain text file in the ISO 9660 file system. On operating systems that support only ISO 9660, the plain text file appears instead of the installation files, stating that your operating system does not support UDF.
The example below shows how the two modes differ.
- In bridged mode, adding a file affects both file systems.
- In non-bridged mode, adding a file to one file system does not affect another file system.
UDF Metadata Partition
UDF Revision 2.50 adds the Metadata Partition facilitating metadata clustering, easier crash recovery, and optional duplication of file system information. All metadata like nodes and directory contents are written on a separate partition which can optionally be mirrored. This format is used by some versions of Blu-ray discs.
WinISO places the metadata before the file data and mirrored metadata after the file data. The two metadata partitions are placed as far apart as possible to avoid loss of all data.
However, some disc authoring tools place the two metadata partitions next to each other. In this case, both metadata partitions are still vulnerable to damage, resulting in the loss of all data.
Multi-Boot Image File (.bifx) Format
According to the El Torito specification, a bootable CD-ROM can be configured with multiple boot images. But usually, a boot image file (.bif or .img) represents only a single boot image.
In WinISO 6.4, a new file format (.bifx) was introduced for storing the entire boot information. The .bifx file format is fairly simple.
- It consists of the Boot Catalog sectors followed by the Boot Image sectors.
- The Load RBA (bytes 8–12) of the Boot Entry is the starting block address of the boot image, relative to the beginning of the file.