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This is a project I've worked on and off for the past year. It all started with the MO5SD project by Daniel Coulom (French page). The idea of this is to use an SD card plugged to the tape port of the french Thomson MO5 computer, which happens to use TTL logic levels.

I reused the ideas from MO5SD to build a similar interface for the Amstrad CPC printer port. However, the Amstrad operating system has better capabilities than the MO5 one and is friendly to expansion ROMs. This makes it possible to run a full-blown filesystem rather than a simple bootloader like the MO5 version does.

Getting the SD card read/write code working was the easy part. After spending some time (with help from SyX and Cloudstrife) optimizing the z80 assembler code for SPI bit banging, I started looking for a suitable filesystem. The most common floppy disc ROMs for the CPC are AMSDOS (the one that shipped with the machine) and Parados, which improves support for dual side, 80 tracks floppy drives from PC. None of them easily allow to reroute disk access to anything else than the floppy controller. I heard that RODOS, a less known disk ROM, has such a capability, and that it also has directories, permissions, and some other nice features. However, close inspection of the RODOS ROM showed that there is actually no way to redirect disk access to anything else than the FDC (if you know a way...)

Moreover, the RODOS filesystem design looked like it would be slow. My bit-bang access to the SD card doesn't go very fast, and jumping around between several sectors isn't a good thing. I wanted to keep files without too much fragmentation so I could load them fast. RODOS is also limited to 20MB volumes, which sounded huge in the 1980s but felt ridiculous for my 4GB SD card. Finally, RODOS requires some RAM, and any ROM that does that on the CPC reduces compatibility with some software.

So, I decided to design my own filesystems. The goals are simple:

• Use as few RAM as possible. AMSDOS allocates a 2K buffer, that should be the only space we are allowed to use (and some stack, of course).
• Limit access to the storage medium whenever possible. Try to not read the same sector multiple times.
• Allow the use of big drives. This requires directories, and also some other tricks.
• Be z80-friendly. No 32-bit math is ever done.

I worked on a C++ version first. This allowed me to keep the code readable while I experimented with various things. I did all the testing on PC and will start converting the whole stuff to z80 code only when I'm fairly sure there won't be big changes to it again.

An SD card can hold up to 4GB of data. I decided to split this into 256 volumes that map to the CPC notion of "users". On an AMSDOS floppy, each file belongs to a single user and can't be seen by others. This limits each user to 16 megabytes of data. Since the files are allocated on 512-byte sectors, these can all be addressed with a 16-bit offset. I also limited the number of files and directories to fit them on a 16-bit counter.

The filesystem uses a block map with sector granularity (the 16 first sectors are used for that), and a file/directory list based on extents. When the filesystem is not fragmented, a directory will use just 16 bytes of space (including the 11 character name and up to 256 entries), and a file will use a 16 bytes header to store up to 128K of data. These entries can be extended, so directories have unlimited number of entries, and files have unlimited size.

You can read more about the data structures in the filesystem readme, and also check the source code. They are both available at CPCSDK. The C++ code makes use of some C++ features such as vector, but this could easily be converted into plain C. You will also find a WIP ROM code for the z80 version, mostly done by SyX. I'll start filling it with actual disk access code someday, for now it's just managing the patching of AMSDOS code and forwarding the access to floppy drives.

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