- Author
Daniel Keep <daniel.keep@gmail.com>
- Version
1.0
- Licence
SSFS is designed to be an as-simple-as-possible filesystem for use with the DCPU-16. Its simplicity is based on a number of restrictions:
- no directoties,
- 8.4 filenames,
- no more than 512 files,
- no files of > 64kw,
- no file permissions or attributes,
- no timestamps,
- no redundancy,
- no error-checking, and
- no volume label.
On the bright side, SSFS should be roughly as simple a filesystem as you can reasonably have, making it easy to implement.
This document uses the following sytax conventions:
lp"..."Indicates a little-endian packed ASCII string.
SSFS has just three structures.
The header (which doubles as the boot sector) contains the basic metadata for the filesystem. It is laid out as follows:
| Words | Use |
|---|---|
| 0 - 1 | Jump to boot loader, zero if disk is not bootable. |
| 2 - 3 | lp"SSFS". |
| 4 | Version number = 1.0. Major in 8 msb, minor in 8 lsb. |
| 5 | First free sector; zero if no free sectors. |
| 6 | First directory sector; must be >= 1. |
| 7 - 31 | Reserved, zero-filled. |
| 32 - 512 | Boot loader (if present). |
The header is always in the first sector. It can be followed by zero or more sectors containing the remainder of the bootloader which could not fit into the first sector; these sectors are considered part of the header.
Note that the first free sector is the head of a linked list which lists all unused sectors. Thus, formatting the disk involves writing out this linked list.
The filesystem uses eight directory sectors laid out sequentially; each sector has 64 directory entries, giving 512 entries total. Each entry has the following form:
| Words | Use |
|---|---|
| 0 - 3 | Little-endian, zero-filled, packed file name. |
| 4 - 5 | Little-endian, zero-filled, packed file extension. |
| 6 | Sector at which file data starts; zero if file is empty. |
| 7 | File length in words. |
The location of the directory sectors is technically arbitrary, although it will typically come immediately after the header (and any additional boot loader sectors).
A directory is empty if the first word of the entry is zero or 0xffff. A zero entry indicates that the entry is empty and that there are no further entries in the directory. A 0xffff entry indicates that the entry is empty but that there might be further entries in the directory.
The 0xffff form should be avoided if practical, but can be used when removing the final entry in a sector without needing to read the following sector(s) to see if there are more entries.
Each non-empty file is made up of one or more sectors linked together into a list. All unused sectors are also linked together in the same manner. Each of these sectors has the following form:
| Words | Use |
|---|---|
| 0 | Next sector in list; zero if this is the last sector in the chain. |
| 1 - 511 | Sector data. |