118 lines
4.5 KiB
Text
118 lines
4.5 KiB
Text
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Rockbox From A Technical Angle
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==============================
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Background
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Bj<42>rn Stenberg started this venture back in the late year 2001. The first
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Rockbox code was committed to CVS end of March 2002. Rockbox 1.0 was
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released in June.
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Booting and (De)Scrambling
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The built-in firmware in the Archos Jukebox reads a file from disk into
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memory, descrambles it, verifies the checksum and then runs it as code. When
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we build Rockbox images, we scramble the result file to use the same kind of
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scrambling that the original Archos firmware uses so that it can be loaded
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by the built-in firmware.
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CPU
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The CPU in use is a SH7034 from Hitachi, running at 11.0592MHz or 12MHz.
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Most single instructions are excuted in 1 cycle. There is a 4KB internal ram
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and a 2MB external ram.
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Memory Usage
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All Archos Jukebox models have only 2MB ram. The ram is used for everything,
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including code, graphics and config. To be able to play as long as possible
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without having to load more data, the size of the mpeg playing buffer must
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remain as big as possible. Also, since we need to be able to do almost
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everything in Rockbox simultaneously, we use no dynamic memory allocation
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system at all. All sub-parts that needs memory must allocate their needs
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staticly. This puts a great responsibility on all coders.
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Playing MPEG
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The MPEG decoding is performed by an external circuit, MAS3507D (for the
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Player/Studio models) or MAS3587F (for the Recorder models).
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...
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Spinning The Disk Up/Down
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To save battery, the spinning of the harddrive must be kept at a minimum.
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Rockbox features a timeout, so that if no action has been performed within N
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seconds, the disk will spin-down automaticly. However, if the disk was used
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for mpeg-loading for music playback, the spin-down will be almost immediate
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as then there's no point in timing out. The N second timer is thus only used
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when the disk-activity is trigged by a user.
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FAT and Mounting
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Rockbox scans the partitions of the disk and tries to mount them as fat32
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filesystems at boot.
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Directory Buffer
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When using the "dir browser" in Rockbox to display a single directory, it
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loads all entries in the directory into memory first, then sorts them and
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presents them on screen. The buffer used for all file entries is limited to
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maximum 16K or 400 entries. If the file names are longish, the 16K will run
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out before 400 entries have been used.
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This rather limited buffer size is of course again related to the necessity
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to keep the footprint small to keep the mpeg buffer as large as possible.
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Playlist Concepts
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One of the most obvious limitations in the firmware Rockbox tries to
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outperform, was the way playlists were dealt with.
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When loading a playlist (which is a plain text file with file names
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separated by newlines), Rockbox will scan through the file and store indexes
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to all file names in an array. The array itself has a 10000-entry limit (for
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memory size reasons).
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To play a specific song from the playlist, Rockbox checks the index and then
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seeks to that position in the original file on disk and gets the file name
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from there. This way, very little memory is wasted and yet very large
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playlists are supported.
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Playing a Directory
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Playing a full directory is using the same technique as with playlists. The
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difference is that the playlist is not a file on disk, but is the directory
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buffer.
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Shuffle
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Since the playlist is a an array of indexes to where to read the file name,
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shuffle modifies the order of these indexes in the array. The randomness is
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identical for the same random seed. This is the secret to good resume. Even
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when you've shut down your unit and re-starts it, using the same random seed
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as the previous time will give exactly the same random order.
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Saving Config Data
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The Player/Studio models have no battery-backuped memory while the Recorder
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models have 44 bytes battery-backuped.
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To save data to be persistent and around even after reboots, Rockbox uses
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harddisk sector 63, which is outside the FAT32 filesystem. (Recorder models
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also get some data stored in the battery-backuped area).
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The config is only saved when the disk is spinning. This is important to
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realize, as if you change a config setting and then immediately shuts your
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unit down, the new config is not saved.
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Resume Explained
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...
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Charging
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(Charging concerns Recorder models only, the other models have hardware-
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controlled charging that Rockbox can't affect.)
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...
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