"Rockbox From A Technical Angle", take 1
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@2457 a1c6a512-1295-4272-9138-f99709370657
This commit is contained in:
parent
18f6ea638e
commit
d3f4c362b3
117
docs/TECH
Normal file
117
docs/TECH
Normal file
|
@ -0,0 +1,117 @@
|
|||
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.)
|
||||
|
||||
...
|
Loading…
Reference in New Issue
Block a user