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YOU FORTH LOVE IF HONK THEN

Forth is a ProgrammingLanguage developed by ChuckMoore in the 1960s (see the history of Forth). Forth is a Stack based language with a ReversePolish syntax. A function call is very fast as the arguments are those on the Stack when the program enters and the return value is left on the Stack when it exits. A Forth program consists of many small functions, with just the essentials written in AssemblyLanguage.

Forth is used in embedded systems. It produces very compact code. A whole Forth interpreter and development system will fit into 8 kilobytes, easily, and leave plenty of room for code. Back when the computer with 8 kilobytes of RAM that you were to write programs for was also the the computer you had to write programs on Forth was very popular.

Implementations

DragonForth

Free Forth compiler for PalmPilot

Quartus

A Forth compiler for the PalmPilot

GForth

GNU's Forth. It's unusual for a Forth in that it comes with a manual. Forth programmers aren't big on manuals :-(

PFE

It has a VirtualMachine and kernel (see below) written in C. It might be a good one to study.

RetroForth

Both a ForthOS and a Forth for Linux systems.

bigFORTH

A native code compiler for x86 which includes a GUI library and form editor.

Forth is also the basis for the RPL language used in HewlettPackard calculators and OpenFirmware.

Forth Machines

Sentences long extremely and notation Polish reverse in writing about wrong is what?
— Jarkko Hietaniemi

This is a general description of how Forth works. It's different. If you like Brainf*ck you will like Forth.

Forth functions are called words. which are organised in the user dictionary.

A Forth machine has two stacks. When subroutines (called words in Forth lingo) are called they pop arguments off the parameter stack and push back return values. The return stack holds the return addresses of word calls.

Finally, the input buffer for the Forth parser forms an integral part of a Forth system.

Forth programs are written in ReversePolish notation. Forth words access the parameter stack directly to fetch arguments and leave return values. The stack is also used for temporary storage, as traditionally there are no local variables in Forth. There are words for flipping around values on the top of the stacks to help you with this. As you program in Forth you must keep careful track of what is supposed to be on the stack at every point. This is all part of ChuckMoore's philosophy of brutal simplicity in software engineering, and the most indigestible aspect of Forth.

The user dictionary looks a bit like a stack and is organised in sets called vocabularies. The first vocabulary is a tiny kernel of word definitions written in MachineCode, then there is a standard library vocabulary defined in Forth (compiled to ByteCode), and then optional vocabularies that define a simple text editor, assembler and disk OperatingSystem. (A modern Forth system might have a TCP/IP stack on there somewhere too.)

There is no syntax in Forth, just words for manipulating other words, the input buffer and the user dictionary.

The word : tells Forth to begin compiling the subsequent words found on the input buffer.

Some words have flags that tell Forth to execute them immediately while compiling: these create control structures in the ByteCode of a word. Ordinary words are compiled into call statements. Words that look like integers are compiled into push statements. The word ; pushes the new defintion onto the dictionary and ends compilation. (This is the gist.)

You use : to add new words to the Forth system until you've created a high-level Forth vocabulary that you can easily express your problem in… then you just keep going until you've defined a single word that, when called, executes your entire program. Forth is very much a BottomUp language. :-)

MachineCode statements can be used as Forth ByteCodes so the machine can execute them directly. This is called subroutine threading. It is one of a number of approaches to reduce the overhead of interpretation (which spends a lot of time executing call statements).

Real Forth geeks write their own Forth kernels from the metal up. (I wrote my own, on paper, when I was 14. (I don't think I can do that anymore.) I was working from a Forth book and a Z80 opcode table; I didn't have a computer. I still have pieces of it. God knows if it would have worked.) ChuckMoore creates his own Forth machines from silicon. It's not very hard once you've absorbed the Forth philosophy. There is shockingly little holding Forth up.

Examples

This right here is a lovely example of Forth: LifeInForth.

This more sober example comes from the IRC bot (in ISForth):

\ receive up to 512 bytes from bot socket

: bot-read
  off> #inbuff
  inbuff                \ where to receive into
  begin
    1 over bot-fd recv  \ read one char from bot socket
    1-                  \ result should be 0 now
    if                  \ if its not then we didnt get any input
      drop exit         \ so discard buffer address and exit
    then
    incr> #inbuff       \ we got a character
    dup c@              \ get the character we just read
    $0a <>              \ while its not an eol
    #inbuff 512 <> and
  while
    1+                  \ advance buffer address
  repeat
  drop                  \ discard buffer address
;

Yet another example, a simple text editor (from Retro Native Forth):

0 variable lastBlock
asmbase $1024 + constant blockBase
: block 512 * blockBase + ;
: select dup lastBlock ! ;
: vl dup 40 type cr 40 + ;
: view lastBlock @ block vl vl vl vl vl vl vl vl vl vl vl vl ;
: edit select block s drop drop view ;
: delete select block 512 0 fill view ;
: edit-line 40 * lastBlock @ block + s drop drop view ;
: delete-line 40 * lastBlock @ block + 37 0 fill view ;

As you can see, Forth appeals most to AssemblyLanguage types.


CategoryProgrammingLanguages, CategoryMachineOrientedProgrammingLanguages, CategorySystemsProgrammingLanguages