Open Firmware
Open Firmware is the hardware-independent firmware (computer software which loads the operating system) that the XO runs.
It was developed by Mitch Bradley at Sun Microsystems, and used in post-NuBus PowerPC-based Apple Macintosh computers (though it has been dropped with Apple's transition to Intel processors), Sun Microsystems SPARC based workstations and servers, IBM POWER systems, and PegasosPPC systems, among others. On those computers, Open Firmware fulfills the same tasks as BIOS does on PC computers.
For example Fedora and Debian use the YaBoot BootLoader for Open Firmware.
The Open Firmware user interface includes a FORTH-based shell interface. FORTH is a powerful high level language that is remarkably compact. A complete Forth development environment including compiler, decompiler, assembler, disassembler, source level debugger, and assembly language debugger is present in the XO boot ROM (SPI FLASH). With the Open Firmware Forth system, you can directly access all of the hardware devices on the XO, use built-in functions like selftest diagnostics and games, and even write complete applications, without needing any external tools. The bulk of Open Firmware is written in Forth, so the source level debugger can be used to debug Open Firmware itself.
Source Code
The source code for the firmware used in the XO is available in the Open Firmware subversion (SVN) repository.
You can browse the subversion repository:
You can download the source code:
svn co svn://openfirmware.info/openfirmware
To find the software used in a given OLPC release, look on the Firmware page to find the release, then look on the release page for the "svn" revision number. E.g. for OLPC Firmware q2d07 the revision number is 775.
To work backwards from the laptop, get the Ok prompt and type
more rom:sourceurl
and look at the SVN revision number on the end of the line. This is especially useful on development builds, but works on stable builds as well. An M suffix means that the source was modified since the revision number.
Replacing BIOS in the XO-1 - a historical overview -
Inside was a development BIOS and bootloader used for a very short time until we were able to bootstrap our own. At that point, we moved to LinuxBIOS for both low-level hardware initializer and bootloader, which was less than ideal and somewhat unwieldy. In a surprise move, SUNW then opened up their parts of the OFW/OBP code under a BSD license, which allowed Mitch Bradley (who was working for OLPC) to open up his own parts -- that of his company, FirmWorks -- and let us have an acceptably-licensed Open Firmware we can use as a fancy and compact bootloader. LinuxBIOS did low-level hardware initialization, transfered control to OFW, which then also acting as boot loader to load Linux OS.
Since the "c" series of firmware releases starting on April 6, 2007, LinuxBIOS has not been present at all. The low-level init is now done with a few lines of assembly language code and a big table of register values.
Removing LinuxBIOS was what made it possible to get the startup time down to a couple of seconds, and to do the firmware part of resume in a few milliseconds.
Accessing Open Firmware on an XO
There are two ways to get to OFW on an unlocked XO, at boot time and from Sugar.
- See Ok. At boot time, briefly type the Escape key (top left on the XO keyboard, with a white X in a black disc) when you hear the startup jingle. Do not hold it down. In a few seconds, the FORTH ok prompt appears, along with a hint on help.
- In the Terminal activity, execute the command
echo y > /proc/sysrq-trigger
The former method (ESC at startup) is for routine Forth use and experimentation. The latter method (breaking out of Sugar/Linux) is for debugging OS problems and to inspect the state of hardware after the OS has been running, but is not recommended, due to its potential for confusing the OS or losing unsaved data.
See FORTH for a brief introduction to the FORTH programming language, which Open Firmware is written in, plus links to other resources. See Forth Lessons for lessons specifically tuned to Open Firmware on the XO.
Be careful using "flash" and commands ending with "-tag". The "flash" command overwrites the Open Firmware image; it is safe to use with officially-released OLPC Open Firmware images. The "*-tag" commands modify the Manufacturing_Data that identifies the system; use them only if you know what you are doing.
When finished with OFW,
- turn the power off, by holding down the power button,
- type boot to start the operating system,
- type bye to restart the laptop, or
- type power-off to turn off the laptop.