Instruction Set Architecture's
The Cortex-A8 SoC we have in our N900 supports several Instruction Set Architecture's (ISA).
These include ARM, Thumb, Thumb2, ThumbEE.
Which is the most powerful and fastest. Every ARM instruction uses exactly 4 bytes of memory.
Note:- *Facebook sharing plugin was compiled by mistake with Thumb2 ISA.
Thumb2 is another instruction set, which has most of the power and speed of ARM ISA, but its instructions could be 2 bytes in length instead of 4 bytes.
According to some research that can be found on internet, using Thumb2 ISA leads to reduction of code size in 30-40% without sacrificing much of the performance (some 2-5% performance decrease).
 Memory Usage
We all know that the major performance bottleneck on the N900 is the size of RAM. Constantly swapping pages in and out of memory leads to stutter and lag.
Part of that RAM is used to hold the executable code, so the smaller the code is, the larger part of the RAM will be free to be used for data.
While it is a simplified explanation, you should be able to get the general idea. Smaller code means better code cache usage, less reading from the storage and therefore faster application start times.
With packages that are so far in CSSU-thumb repository, the code size is reduced by approximately 35MiB.
As more packages are included, more RAM and storage space will be regained in the future.
 The Catch
The SoC revision inside our N900 has silicon erratas or put simply bugs in the hardware. These render Thumb2 compiled binaries unstable if no countermeasures are taken.
A system without fixes for the erratas, that tries to run Thumb2 binaries, crashes randomly, not only in Thumb2 compiled executables/libraries, but in ordinary ARM ones too.
 The Countermeasures
There are two erratas that must be worked around on N900 in order for Thumb2 compiled binaries to not lead to system instability.
These are ARM errata 430973 and ARM errata 657417
 ARM errata 430973
This is worked around in the kernel by clearing a special type of cache (Branch Target Buffer) inside the chip on every context switch made by the CPU.
In theory that slows down the system. I didn't make any benchmarks, but from current testing there is no visible performance loss.
A special call to Secure Monitor (that is a piece of ROM in our SoC programmed by Texas Instruments during chip manufacture) is needed to properly enable the workaround.
Therefore CSSU Thumb requires a kernel which has the workaround. This is provided in Kernel-CSSU or the latest version of Kernel Power
 ARM errata 657417
This is worked around in the in linker, all binaries in the CSSU Thumb repository are compiled using Linaro gcc 4.7.2 and binutils 2.22.
Information on how to setup the CSSU-thumb toolchain (gcc4.7.2-linaro) in scratchbox is linked below
Make sure Nokia and Extras catalogues are enabled in application manager.
The installation process is the same as other CSSU Flavours
CSSU-Thumb is based on CSSU-Testing flavor.
If you have kernel-power v51r1 or above, no custom kernel will be installed, as needed errata workaround is now included in kernel-power.
There is no way to uninstall, i.e. the same rules apply as for the other CSSU flavors. The only way to rollback without messing with apt-get is to Reflash. Or you can downgrade all of the packages manually using apt-get.
This is still in development stage and while so far there are no known issues, I will take no responsibility for any damage and/or data loss caused by using CSSU-thumb repository. It is highly unlikely to happen, but be prepared to reflash if something goes south. Make a full backup of your data and settings before installing.
 15.06.2012 - 21.2011.38-1Tmaemo4-thumb7
- Qt libraries moved back to /opt
- added new packages: qt-mobility, clutter, libhildonfm, osso-gnome-vfs2
 16.06.2012 - 21.2011.38-1Tmaemo4-thumb9
- added new packages: libalarm2, alarmd, libxml2
 20.06.2012 - 21.2011.38-1Tmaemo4-thumb10
- added new packages: xserver-xomap, xserver-xorg-core
- fixed a bug in the kernel : Clear the IT state when invoking a Thumb-2 signal handler http://git.et.redhat.com/?p=kernel-k...62e8de4406ac7e
 19.07.2012 - 21.2011.38-1Tmaemo4-thumb11
- added new packages: microb-engine
 27.07.2012 - 21.2011.38-1Tmaemo4-thumb12
- toolchain upgrade to gcc 4.7.2
- microb-engine - fixed HTTPS TLS error
- Qt libraries rebuild with the new toolchain
 29.07.2012 - 21.2011.38-1Tmaemo4-thumb13
- kernel upgraded
 01.08.2012 - 21.2011.38-1Tmaemo4-thumb14
- fixed a bug in tinymail
 04.09.2012 - 21.2011.38-1Tmaemo4-thumb16
- microb-engine source code was missing some stuff from maemo.org, rebased.
- add libcurl3 (7.26.0-1maemo1+0cssu1-thumb1) - fix for Facebook widget, photo uploader and MfE IPv6 problems
 15.09.2012 - 21.2011.38-1Tmaemo5.1+thumb0
- rebased on top of 21.2011.38-1Tmaemo5.1 CSSU-testing
- changed versioning scheme
- added thumb-compiled camera-ui, alarmd, hildon-application-manager, apt, modest
Because of the versioning scheme change you should follow the instructions here if upgrading from 21.2011.38-1Tmaemo4-thumb16 or older version.
 23.09.2012 - 21.2011.38-1Tmaemo5.1+thumb1
- added missing dependency (system-services (>= 0.3.8-68+0cssu0))
- Thumb2 compiled dosfstools and openssl
 25.09.2012 - 21.2011.38-1Tmaemo5.1+thumb2
- Thumb2 compiled gdigicam and gst-plugins-bad
 03.10.2012 - 21.2011.38-1Tmaemo5.1+thumb3
- Thumb2 compiled sqlite3, gstreamer, gst-plugins-base, gst-plugins-good
 03.11.2012 - 21.2011.38-1Tmaemo5.1+thumb4
- Thumb2 compiled db4.2, pulseaudio, calendar-backend, evolution-data-server
 26.12.2012 - 21.2011.38-1Tmaemo7.1+thumb0
- packages updated to latest CSSU-testing(21.2011.38-1Tmaemo7.1)
- Thumb2 compiled obexd, gnome-vfs-filechooser-backend, ke-recv, libmatchbox2, rtcom-eventlogger-async
 06.01.2013 - 21.2011.38-1Tmaemo7.1+thumb1
- Thumb2 compiled pango, freetype, libpng, zlib, libxml2
- NEON-optimized upstream versions of libpng (~30% faster) and zlib (~8% faster decompression, ~18% faster compression)
- libxml2 recompiled against latest zlib
 07.01.2013 - 21.2011.38-1Tmaemo7.2+thumb0
- packages updated to latest CSSU-testing(21.2011.38-1Tmaemo7.2) - blacklist fraudulent TURKTRUST intermediate certificates
 07.04.2013 - 21.2011.38-1Tmaemo7.2+thumb1
- pixman upgraded to upstream version 0.29
- build microb against system libpng and zlib
- pulseaudio, qt and hildon-desktop upgraded to latest CSSU versions on gitorious
 15.07.2013 - 21.2011.38-1Tmaemo8.2+thumb1
- packages updated to latest CSSU-testing(21.2011.38-1Tmaemo8.2)
- thumb-compile tracker, hildon, hildon-thumbnailer
 3.11.2013 - 21.2011.38-1Tmaemo8.3+thumb1
- osso-systemui-alarm: Fix nsv_sv_play_event call
- maemo-security-certman: Change VeriSign certificate order, fixes supl.nokia.com not working
 12.11.2013 - 21.2011.38-1Tmaemo8.3+thumb2
- upgrade glib to 2.24.1
- upgrade pulseaudio to 0.9.15-1maemo44+0m5+0cssu5
- Thumb2 compile alsa-lib, glib, pulseaudio, telepathy-glib, telepathy-gabble and osso-pdf-viewer
 24.11.2013 - 21.2011.38-1Tmaemo8.3+thumb3
- upgrade hildon-desktop to 1:2.2.153-2+thumb0
- Thumb2 compile cairo, libx11, libxfont
 30.12.2013 - 21.2011.38-1Tmaemo9+thumb1
- clock-ui: Open source rewrite of the nokia clock-ui, thanks to Adekker (ade)
- camera-ui: Added Fallback to DCIM if NOKIA_CAMERA_DIR or NOKIA_MMC_CAMERA_DIR environment variable is not defined
Removed some GLIB Warning messages when changing profile
- dosfstools: Fixed install paths, data will be installed to /usr/share instead of /share
- e2fsprogs: Updated to upstream 1.42.5, fixes fsck for ext* filesystems
- glibc: Enabled kernel implementations of epoll, ppoll and pselect
- fmtx-middleware: Fm TX now keeps running when headphones/usb is plugged in
- gtkfilesystemmemory: Moved development files to osso-memory-backend-dev, frees rootfs space
- hildon-welcome: Fixed not running when /proc/bootreason is missing (ie. an upstream kernel is used)
- initrd-progs: Open source replacement with new feature -G which prints block data to stdout
- ke-recv-extra: Fixed 100% Cpu use on 3.x kernels
- libcpaboutcssu: dpkg-query instead of shell hack, added luf
- maemo-tvout-control: Moved maemo-applet-tvout back into libs section, fixes control panel entry missing
- modest: Moved recursive imap operations into glib main loop, fixes 100% Cpu / freeze on imap sync
- modest-l10n: Added english string for mcen_fi_advsetup_update_all_folders
- osso-af-utils: Replaced call to getbootstate with read of /var/lib/dsme/saved_state,
fixed boot when /proc/bootreason is missing
- osso-calculator: Added support for top_application dbus call
- osso-systemui-tklock: Fixed bug #12701: Ball is moving when touching anywhere along the bar-slide
- profiled: Open source replacement version from Nokia (harmattan/nemo)
- qt4-x11-maemo: Added MeeGo libs, change compiler flags
- status-area-applet-battery: Open source replacement which works with BME hal, bq27200 hal, rx51-battery hal
- upstart: Enabled auto fsck on boot, added support for swap_opts in /etc/default/mount-opts
- Thumb2 compile clock-ui, e2fsprogs, gtkfilesystemmemory, ke-recv-extra, osso-af-utils, profiled
 12.01.2014 - 21.2011.38-1Tmaemo9.1+thumb0
- fmtx-middleware: revert to 0.62, fixes US model issues
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