From d3f4c362b368a9c65ff8b6d3eaec3929c4025dbc Mon Sep 17 00:00:00 2001 From: Daniel Stenberg Date: Tue, 1 Oct 2002 08:20:33 +0000 Subject: "Rockbox From A Technical Angle", take 1 git-svn-id: svn://svn.rockbox.org/rockbox/trunk@2457 a1c6a512-1295-4272-9138-f99709370657 --- docs/TECH | 117 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 117 insertions(+) create mode 100644 docs/TECH diff --git a/docs/TECH b/docs/TECH new file mode 100644 index 0000000000..523a3622a9 --- /dev/null +++ b/docs/TECH @@ -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.) + + ... -- cgit v1.2.3