FORTH is a terse, stack-oriented, interpreted/semi-compiled language using reverse Polish notation, like Hewlett-Packard calculators. Forth is at the core of Open Firmware, the boot-loader for the XO-1.
Identifiers, called words, are composed of any sequence of printing characters. Words are separated by any whitespace characters, including space, tab, and new line. The word . prints the item on top of the stack.
For example, the line
2 3 + .
starts by putting 2 on the stack, leaving 3 + . to interpret.
The next step is to put 3 on the stack, giving 3 2, with + . left to interpret.
Then + adds the two top elements of the stack and puts the result back, so that the stack contains just 5, and only . is left.
Finally, . removes the 5 from the stack and prints it
The OK signifies that the line is finished, and FORTH is ready for the next command.
The design goals for FORTH included minimum program size; readability by experienced users, though not always by novices; and extremely high productivity.
We do not have room here to explain the language properly. See references below.
Free Software versions of FORTH include gforth, kforth, pforth, and yforth. Source code is of course available for each of these.
 FORTH for education
FORTH is one of the best programming languages for learning how to build languages. The FORTH kernel was originally about 8K of highly modular code. Students who know an assembly language can read and understand the code for FORTH for that processor in a few weeks. The rest of the FORTH system is written in highly modular FORTH.
Learning FORTH internals is almost the same as learning Virtual Machine architecture. The primary technical difference is that VMs use byte codes (token-threaded code), while compiled FORTH consists mainly of sequences of addresses of subroutines (direct-threaded code). Token-threaded code is shorter, but takes longer to interpret than direct-threaded code.
Most modern Forth implementations now choose to use subroutine-threaded (rather than direct-threaded) code. In subroutine threaded code, the "sequence of addresses" is really a sequence of CALL, BAL, or JSR instructions (or whatever subroutine call instruction is for your platform). This eliminates the need for an "inner interpreter", which on modern microprocessors, gives a substantial performance boost.
 History on the XO
- Mitch Bradley has learned his way around LinuxBIOS, and has brought up a Forth implementation on it. Forth is a very productive environment for test and debugging of hardware, and will come in very handy for easier debug, testing, and exercise of the CAFE chip. A Forth environment allows for immediate turn around of tests on hardware without a compile, link, reboot step that a conventional device driver requires.
- One month ago, without warning, Sun released Open Firmware under a BSD license. After much deliberation, we reached the decision that rather than continuing to use Linux itself as boot loader, we will transition to Open Firmware (OFW), while retaining LinuxBIOS. The date for a transition has not yet been set. Open Firmware has been in use on both Sun and PPC hardware (e.g. Apple) for approaching a decade, and has shipped in quantity millions. Mitch Bradley, currently working for OLPC is its original author.
- Wednesday, we had no fully functioning BIOS for use with CAFE. We had intended to use LinuxBIOS with Linux as bootloader for B1 with a transition to Open Firmware (OFW) as bootloader before B2. We continued with both possibilities in parallel, such that by Friday we had both working. Testing of OFW's new USB stack has succeeded, so we have decided to use OFW immediately. Our efforts will now focus on the LinuxBIOS/OFW combination.
OLPC Firmware q2c01, released on April 6, 2007 introduces the "c" series of firmware, which removes LinuxBIOS and uses only Open Firmware, in order to speed up booting and resumption from sleep.
 See also
 External links
- Forth Interest Group
- Wikipedia: FORTH
- Forth, a programming language for real programmers
- FORTH tutorials
- OpenBoot 4.x Command Reference Manual
- Forth Reference for the XO