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79 lines
2.5 KiB
Markdown
79 lines
2.5 KiB
Markdown
# Using the filesystem
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The Collapse OS filesystem (CFS) is a very simple FS that aims at implementation
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simplicity first. It is not efficient or featureful, but allows you to get
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play around with the concept of files so that you can conveniently run programs
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targeting named blocks of data with in storage.
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The filesystem sits on a block device and there can only be one active
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filesystem at once.
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Files are represented by adjacent blocks of `0x100` bytes with `0x20` bytes of
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metadata on the first block. That metadata tells the location of the next block
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which allows for block iteration.
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To create a file, you must allocate blocks to it and these blocks can't be
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grown (you have to delete the file and re-allocate it). When allocating new
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files, Collapse OS tries to reuse blocks from deleted files if it can.
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Once "mounted" (turned on with `fson`), you can list files, allocate new files
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with `fnew`, mark files as deleted with `fdel` and, more importantly, open files
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with `fopen`.
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Opened files are accessed a independent block devices. It's the glue code that
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decides how many file handles we'll support and to which block device ID each
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file handle will be assigned.
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For example, you could have a system with three block devices, one for ACIA and
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one for a SD card and one for a file handle. You would mount the filesystem on
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block device `1` (the SD card), then open a file on handle `0` with `fopen 0
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filename`. You would then do `bsel 2` to select your third block device which
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is mapped to the file you've just opened.
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## Trying it in the emulator
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The shell emulator in `tools/emul/shell` is geared for filesystem usage. If you
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look at `shell_.asm`, you'll see that there are 4 block devices: one for
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console, one for fake storage (`fsdev`) and two file handles (we call them
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`stdout` and `stdin`, but both are read/write in this context).
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The fake device `fsdev` is hooked to the host system through the `cfspack`
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utility. Then the emulated shell is started, it checks for the existence of a
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`cfsin` directory and, if it exists, it packs its content into a CFS blob and
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shoves it into its `fsdev` storage.
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To, to try it out, do this:
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$ mkdir cfsin
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$ echo "Hello!" > cfsin/foo
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$ echo "Goodbye!" > cfsin/bar
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$ ./shell
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The shell, upon startup, automatically calls `fson` targeting block device `1`,
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so it's ready to use:
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> fls
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foo
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bar
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> fopen 0 foo
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> bsel 2
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> getb
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> puth a
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65
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> getb
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> puth a
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6C
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> getb
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> puth a
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6C
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> getb
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> puth a
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6F
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> getb
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> puth a
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21
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> fdel bar
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> fls
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foo
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>
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