forth programming language forth stack based computer programming language environment originally designed language features include structured programming reflection ability modify program structure program execution programming functions composed programmer create new commands acronym language spelled capital letters forth following customary usage earlier years procedural programming language type checking forth features interactive execution commands making suitable shell systems lack formal operating system ability sequences commands later execution forth usually early versions written extremely portable code today generate machine code like language forth open boot space applications spacecraft embedded systems involve interaction hardware dos game written custom forth free software implementation actively maintained commercially supported systems overview forth environment combines interactive shell user defines runs called words words tested source entered program elements including basic operators defined words forth environments vary resulting program stored running program effect manually entering source programming environments use stack control flow structure typically stores local including parameters value system forth local value instead intermediate values kept second stack words operate directly values stack called data stack simply stack second function stack called return stack abbreviated special manipulation functions provided allow temporary storage word pass parameters manipulate data words specified terms effect stack typically parameters placed stack word execution parameters erased replaced return values operators follows rule reverse polish examples illustrating stack usage maintenance forth simple language permit writing high level programs systems forth successfully large complex projects applications developed competent disciplined professionals proven easily maintained evolving hardware platforms decades use forth niche astronomical space applications forth today embedded systems small devices efficient memory use short development time fast execution speed implemented efficiently modern use forth machine language produced uses forth include open boot apple sun based stage boot controller operating system history forth evolved personal programming system continuous development forth exposed programmers early 1970s starting elizabeth national radio work elizabeth formed forth systems dozens platforms decade forth named file holding interpreter labeled fourth 4th generation software operating system restricted file names characters moore saw forth successor link generation programming languages software fourth generation hardware fourth generation programming language term come frequently moved job job career early pressure developing language ease different computer forth system bring new hardware example forth resident software new chip resident development system apple forth inc developed starting later generate forth systems wide finally led language common practice codified facto standards years respectively standards unified commonly referred forth forth popular 1980s suited small time compact portable home computer forth rom resident operating system forth system programming produced single chip resident forth complete family tree version forth graphics expansions forth expansion measuring controlling programmer perspective forth relies heavily explicit use data stack reverse polish commonly operator placed opposed common operator placed makes language easier extend forth flexibility makes static grammar inappropriate extending requires writing new word instead modifying grammar changing underlying implementation result mathematical expression way command line puts numbers implied stack word numbers stack replaces product number placed stack word adds previous product moves output new line purposes omitted output occur line input example finally command prints result user terminal completed successfully point text interpreter prompt moves new line input needing explicit forth structural features stack based example drop code defines new word word term called following commands number stack places stack compares numbers stack input replaces true false value takes true false value chooses execute commands immediately skip drop value stack ends text comment advising word number stack return possibly changed number word equivalent function written programming language operator function written max run word follows interpreter pushes number stack calls pops number pushes result moves output new line finally pops result prints user terminal facilities forth explicit grammar interpreter reads line input user input device word spaces systems recognise additional characters interpreter finds word looks word dictionary word found interpreter code associated word returns rest input stream word isn found word assumed number attempt convert number push stack successful interpreter continues input stream number conversion fail interpreter prints word followed error message indicating word recognised input stream waits new user input definition new word started word ends word semi example word makes dictionary executed console print forth systems include allows specify words facilities lowest level away separate relatively users want use forth use reverse polish parameters instruction instruction designs vary widely specific forth implementation typical reverse polish prepares stack copy instruction memory step forth nature easy define alias registers according role forth system register stack operating system files forth systems run host operating system microsoft windows version use host operating system file system source data files forth standard describes words modern forth systems use normal text files source embedded embedded system resident gets source serial line classic forth systems traditionally use operating system file system instead storing code files source code stored disk blocks written physical disk addresses word block employed translate number sized block disk space address containing data managed automatically forth system block use rare mid-1990s hosted system blocks allocated normal file case commonly cooperative round robin scheduling normally available words support covered forth standard word pause save current task execution context locate task restore execution context task private copies control scratch area tasks simple efficient result forth available simple non standard facilities include mechanism issuing calls host systems provide extensions employ scheduling provided operating system typically larger different set words stand forth pause word task creation suspension destruction modification priority self compilation cross compilation fully featured forth system source code technique commonly called compilation forth programmers term doesn exactly match compilation normally defined usual method handful words place compiled bits memory words use specially named versions store area memory area memory area beginning different address code define words access target computer memory host compiling computer memory store operations code space etc new definitions store effectively code interpreter forth system code compiled version stored memory written disk ways provided load temporarily memory testing new version appears work written previous version numerous variations exist different environments embedded systems code instead written computer technique known cross compilation serial port single bit keeping word names non executing parts dictionary original compiling computer minimum definitions forth words store word commands forth word executed time consuming writing remote port constructing initial program implement store execute modern integrated features eliminate task structure language basic data structure forth dictionary maps words code named data structures dictionary laid memory tree linked lists links proceeding latest recently defined word oldest sentinel value usually found context switch causes list search start different leaf linked list search continues branch merges main trunk leading eventually sentinel root rare cases compilation dictionary isolated stand effect resembles nesting depending context defined word generally consists head body head consisting field link field body consisting code field field head body dictionary entry treated separately example forth program new platform head remain compiling computer body goes new platform environments embedded systems heads occupy memory cross heads target target expected support interactive forth dictionary entry exact format dictionary entry prescribed vary certain components present exact size order vary described structure dictionary entry look way structure flag flags length word array length isn known time address previous link field backward previous word address code execute word array unknown length data words end structure field starts prefix giving length word typically bits flags character representation word follows prefix depending particular implementation forth alignment link field contains previously defined word relative displacement absolute address points oldest sibling code field address word execute code data field beginning machine code execute directly defined words code field points word save current forth instruction return stack load new address continue execution words return instructions structure program consists forth words visible system usable programmer allows programmer change words special purposes time flag field set words time behavior simple words execute code command line embedded code compiling simply places code word classic examples time words control structures forth control structures implemented time words apart rarely control flow words found return forth control flow words executed compilation combinations primitive words branch addresses instance words match set branch branch branch pop value stack branch false counted loop control flow words work similarly set combinations primitive words work counter compilation data stack support control structure balancing nesting branch addresses snippet drop compiled following sequence inside definition lit branch drop lit branch lit numbers branch represent relative jump addresses lit primitive word pushing literal number data stack compilation state interpretation state word creates dictionary entry definition enters compilation state interpreter continues read space words user input device word found interpreter compilation associated word instead interpretation default compilation word interpretation current definition word semi finishes current definition returns interpretation state example word compilation differ default interpretation semi control flow words words meaning inside definitions interactive command line interpreter state changed manually words left right enter interpretation state compilation state respectively words word literal value compilation insert calculated value current definition literal compilation object data stack current definition place object data stack current state interpreter read flag state contains value true compilation state false allows implementation called state smart words behavior changes according current state interpreter immediate words word immediate marks recent definition immediate word effectively replacing compilation interpretation immediate words normally executed compilation compiled programmer state example immediate word word postpone takes compilation named word current definition word marked immediate defined separate words force compilation non immediate immediate words respectively unnamed words execution forth unnamed words defined word following words semi leaves execution token data stack execution token provides handle compiled similar function programming language execution stored word execute takes execution token data stack performs associated word takes execution token data stack associated current definition word takes word returns execution token associated word data stack interpretation state random word execute equivalent words comments words postpone examples words arguments user input device instead data stack example word reads ignores following words including right place comments definition similarly word comments continue end current line correctly separated following comment text structure code forth systems body code definition consists machine language form code original forth follows informal standard called indirect code direct popular modern times fastest modern use insert simple words perform strategies code smaller faster data objects word variable data object points code associated defining word created defining word characteristic defining behavior creating dictionary entry plus possibly data space behavior instance class words constructed defining word examples include variable names cell memory location instance behavior variable returns address stack constant names value specified argument constant instance behavior returns value create names location space allocated location set contain string value instance behavior returns address beginning space forth provides facility programmer define new application specific defining words custom defining behavior instance behavior examples include circular named bits port automatically data objects defined similar words global scope function provided local languages provided data stack forth forth real local forth programming style uses named data objects compared languages typically data objects contain data number words tasks implementation forth enforce consistency data type usage programmer responsibility use appropriate operators store values perform operations data programming words written forth compiled form classical indirect lists addresses words executed turn modern systems generate actual machine code including calls external words code expanded place systems generally speaking forth program saved memory image compiled program single command run executed compiled version loaded development programmer uses interpreter mode execute test little piece developed forth programmers advocate loose design development continuous testing integration design usually separation program high level sets tools write final program designed forth program reads like natural language single solution sets tools attack related problems code examples hello world possible implementation hello hello world hello hello world word carriage return causes following output displayed new line word dot quote reads double quote string code current definition string displayed execution space character separating word string hello world included string needed recognizes forth word standard forth system interpreter output obtained following code fragment forth console hello world dot immediate word string displays word space character separating hello world string word comes text print convention forth interpreter start output new line convention interpreter waits input end previous line prompt implied flush action forth programming languages mixing states compiling interpreting definition word executed single character value character definition written use value character directly text comment ignored word takes value data stack displays corresponding character following uses words left right literal temporarily switch interpreter state value character return compilation state calculated value current definition literal word takes space word places value character data stack word immediate version example definition rewritten like single character definition describing comment immediate postpone defined like postpone literal immediate complete program developed system inc code extremely simple written programmers description array different time array changes controlled initially new add new add array new exchange array code array sum array array setting step getting new adding array key array finally set additions following version uses words value value value value create state array set reset update update swap set set min set loop reset begin update swap update repeat reset update update swap ways test code create test loop test output forth virtual machine simple implement standard reference implementation numerous language addition supporting standard varieties computer systems microsoft windows mac forth systems target variety embedded systems listed prominent systems conform standard portable forth implementation project native code embedded forth inc language forth highly native code forth open standard based forth
Forth (programming language)