Further EDSK extensions

I’ve been involved with various disk preservation groups over the last few years. A large part of that has been for Spectrum +3 and Amstrad CPC disks, with SAMdisk extended to support copy-protected disks. The +3/CPC disks are usually stored in the Extended DSK (EDSK) image file format, designed to hold (almost) any format compatible with the uPD765 floppy controller.

Many problem disks have been reverse-engineered to discover why they didn’t work. A few required emulator enhancements to improve hardware accuracy, but most were missing details from the original disks, due to some creative floppy controller use by the copy protection checks. Not all of these could be supported by the original EDSK specification.

Back in October 2005, I suggested a few EDSK enhancements, designed to address some known limitations of the format. The extensions didn’t involve anything too radical, to maintain as much backwards compatibility as possible.

It’s now three years later, and a number of new gap-related CPC protections have been identified, which are beyond the scope of even the extended Extended DSK format! I’ve made further changes to address the new requirements, as well as a correction to a previous one.

My development version of SAMdisk includes support for all the new features, and will be released if the extensions are approved. Other programs will need similar enhancements to take advantage of them, particularly emulators wanting to run some of the difficult disks.

See the updated extensions document for further details. There are also sample disk images showing each extension.