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