d3f4c362b3
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@2457 a1c6a512-1295-4272-9138-f99709370657
117 lines
4.5 KiB
Text
117 lines
4.5 KiB
Text
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.
|
|
|
|
Resume Explained
|
|
|
|
...
|
|
|
|
Charging
|
|
|
|
(Charging concerns Recorder models only, the other models have hardware-
|
|
controlled charging that Rockbox can't affect.)
|
|
|
|
...
|