1 The Manual was Scanned, OCR and pdf by Stephen Parry
Contents
1. ui MOD LOOP ORIGIN gt 41 addr1 42 addr2 Convert the ascil text beginning at addr1 1 with regard to BASE The new value is accumulated into double number d1 being left as 42 Addr2 is the address of the first unconvertible digit Used by NUMBER nl n2 prod LO Leave the signed product of two signed numbers nl n2 n3 n4 LO Leave the ratio n4 n1 n2 n3 where all are signed numbers Retention of an intermediate 31 bit product permits greater accuracy than would be available with the sequence n1 n2 n3 nl n2 n3 n4 n5 LO Leave the quotient n5 and remainder n4 of the operation n1 n2 n3 31 bit intermediate product is used as for nl n2 sum LO Leave the sum of nl n2 n addr LO Add n to the value at the address Pronounced plus store nl n2 n3 Apply the sign of n2 to nl which is left as n3 nl run Used in a colon definition in the form 1 LOOP At run time LOOP selectively controls branching back to the corresponding DO based on n1 the loop index and the loop limit The signed increment nl is added to the index and the total compared to the limit The branch back to DO occurs until the new index is equal to or greater than the limit nl gt 0 or until the new index is equal to or less than the limit n1 lt 0 Upon exiting the loop the parameters are discarded and execution continues ahead n2. Delete definition named from the dictionary with all entries physically following it In SPECTRUM Forth an error message will occur if the CURRENT and CONTEXT vocabularies are not currently the same P LI The name of the primary vocabulary Execution makes FORTH the CONTEXT vocabulary Until additional user vocabularies are defined new user definitions become a part of FORTH Forth is immediate so it will execute during the creation of a colon definition to select this vocabulary at compile time n Controls overprinting n 0 Character obliterated by other characters n 1 New characters XOR with previous characters addr LO Leave the address of the next available dictionary location LO Set the numeric conversion base to sixteen hexa decimal addr LO A user variable that holds the addresses of the latest character of text during numeric output conversion C LO Used between lt and gt to insert an ASCII character into a pictured numeric output string e g 2E HOLD will place a decimal point Move cursor to top left of screen n C LO Used with a DO LOOP to copy the loop index to the stack Other use is implementation dependent See R 37 ID IMMEDIATE INK INTERPRET INV KEY LATEST addr print a definition s name from its name field address f run time P C2 LO Occurs in a colon definition in form IF 6 ENDIF IF tp ELSE fp ENDIF
3. addr Leave the memory address relative to the origin parameter area n is the minimum address unit either byte or word n LO Store n into the next available dictionary memory cell advancing the dictionary pointer comma nl n2 diff LO Leave the difference of nl n2 Continue interpretation with the next screen P LO Pronounced next screen 28 DUP FIND nl nl if zero nl nl nl non zero LO Reproduce n1 only if it is non zero This is usually used to copy a value just before IF to eliminate the need for an E LSE part to drop it pfa b tf found ft not found Accepts the next text word delimited by blanks in the input stream to HERE and searches the CONTEXT and then CURRENT vocabularies for a matching entry If found the dictionary entry s parameter field address its length byte and a boolean true is left Otherwise only a boolean false is left TRAILING addr addr n2 29 MOD 0123 0 Adjusts the character count n1 of a text string beginning address to suppress the output of trailing blanks i e the characters at addr n1 to addr n2 are blanks n LO Print a number from a signed 16 bit two s complement value converted according to the numeric BASE A trailing blank follows Pronounced dot Used in the form p LO Compiles an in line string CCCC delimited by the trailing with an execution procedure to transmit the te
4. pfa nfa Convert the parameter field address of a definition to its name field addr d Convert a character string left at addr with a preceding count to a signed double number using the current numeric base If a decimal point is encountered in the text the position will be given in DPL but no other effect occurs If numeric conversion is net possible an error message will be given nl n2 or LO Leave the bit wise logical or of two 16 bit values addr U A user variable that contains a value incremented by EMIT The user may alter and examine OUT to control display formatting nl n2 nl n2nl LO Copy the second stack value placing it as the new top addr LO Leave the address of the text output buffer which is a fixed offset above HERE n Control paper background colour Makes all temporary colours permanent 40 PLOT PFA QUERY QUIT R R W RO REPEAT RSMUDGE ROT 5 gt D SO nl n2 Prints an ink spot at 1 n2 and moves the PLOT position nfa pfa Convert the name field address of a compiled definition to its parameter field address Input 80 characters of text or until a return from the operators terminal Text is positioned at the address contained in TIB with IN set to zero Clear the return stack stop compilation and return control to the operators terminal No message is given n Copy the top of the return stack
5. The decimal point has no other purpose than to force this action See NUMBER n Controls inversion of characters normal n 1 inverse video C LO Leave the ASCII of the text terminal key struck addr Leave the name field address of the topmost word in the CURRENT vocabulary C LO 38 LIMIT LIST LIT LITERAL LOAD LOOP Force termination of a DO LOOP at the next opportunity by setting the loop limit equal to the current value of the index The index itself remains unchanged and execution proceeds normally until LOOP or LOOP is encountered pfa lfa Convert the parameter field address of a dictionary definition to its field address n A constant leaving the address just above the highest memory available for a tape buffer Usually this is the highest system memory n LO Display the ASCII text of screen n on the selected out put device SCR contains the screen number during and after this process n C2 LO Within a colon definition LIT is automatically com piled before each 16 bit literal number encountered in input text Later execution of LIT causes the contents of the next dictionary address to be pushed to the n compiling P C2 LO It compiling then compile the stack value n as a 16 bit literal This definition is immediate so that it will execute during a colon definition The intended use is XXX calculate LITERAL Compilatio
6. removes and tests the number on the stack this executes this if number non zero true ELSE that executes that if the number is zero false THEN continue continues from both lines IF marks the place where the top of the stack is popped and examined If the value is non zero everything up to ELSE is executed and at ELSE execution skips to THEN On the other hand if the stack value is zero everything up to ELSE is skipped after ELSE is executed The ELSE that bit is optional and may be omitted if not needed The truth value on the stack is often the result of a comparison that uses one of the Forth comparison operators i e lt gt etc See Chapter 1 The two truth values may be combined by the Forth words AND OR XOR For example AND leaves true if top two truth values on stack are true OR leaves true 1 or both are true XOR leaves true if 1 is true and other is false FOR example 1 1 AND gives 1 1 O AND gives See truth tables in Table 6 For example Suppose we wish to define a Forth word to mark examination papers say 50 Pass less than 50 Fail We may define a word as follows EXAM 50 lt IF FAIL ELSE PASS THEN CR ENTER OK Then to use the word use Mark EXAM ENTER gt If less than 50 then computer prints FAIL If mark greater than or equal to the computer prints PASS 18 4 2 INDEFINITE LOOPS Forth also includes a series of Looping structures which repeat a se
7. shift V 84 T 0 85 U 1 86 V 2 87 VV 3 88 X 4 89 V 5 90 7 16 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 0092 X989 lt 40 C 550 50950C 65 RE Soe TB th e Symbol Symbol Symbol Symbol Symbol shift shift shift shift shift 58C5 17 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 165 164 User Definable Graphics 255 Key Words in Basic not used 4 0 CONDITIONAL BRANCHES AND LOOPS Forth provides conditional branching statements which alter the order in which commands are executed depending upon a given condition Forth also provides looping atructures to repeat a sequence of commands a given number of times N B Condition branches and loops cannot be executed directly and must be included with a definition 4 1 CONDITIONAL BRANCHES 3 compiling words IF ELSE ENDIF or THEN are used to compile conditional branches in a definition In Forth the IF command examines the top of the stack t amp determine which branch will be taken A conditional branch has the following structure DEFINITION condition IF true this ELSE false that THEN continue where DEFINITION begins the definition condition places a condition non zero zero on the stack IF
8. APPENDIX B USEFUL ROUTINES The INPUT routine listed here inputs a number from the keyboard when executed terminated by an ENTER leaving it on the stack INPUT PAD 1 64 EXPECT PAD NUMBER DROP DROP INPUT will input a double precision number INPUT PAD 1 64 EXPECT PAD NUMBER DROP 25 SPECTRUM FORTH GLOSSARY This glossary contains all of the word definitions in Release 1 of SPECTRUM Forth The definitions are presented in the order of their ASCII sort The first line of each entry shows a symbolic description of the action of the procedures on the parameter stack The symbols indicate the order in which input parameters have been placed on the stack Three dashes indicate the execution point any parameters left on the stack are listed In this notation the top of the stack is to the right The symbols include addr memory address b 8 bit byte 1 0 hi 8 bits zero c 8 bit ASCII character hi 8 bits zero d 32 bit signed double integer most significant portion with sign on top of stack f boolean flag 0 false non zero true ff boolean false flag 0 n 16 bit signed integer number u 16 bit unsigned integer tf boolean true flag non zero The capital letters on the right show definition characteristics C May only be used within a colon definition A digit indicates number of memory addresses used if other than one E Intended for execution only LO Level Zero definition of FOR
9. is a routine listed in the Appendix to input a number called INPUT Try and write a routine to do this yourself 12 3 3 OUTPUT AND PRINTING Forth offers you several different ways of outputting information The most frequently used is output of a line of text The Forth word used for this is followed by the message and terminated by For example THIS IS A LINE OF TEXT lt ENTER gt prints THIS IS A LINE OF TEXT on the screen Any further output will appear on the same line The Forth word CR performs an ENTER or Carriage Return and all proceeding output starts at the beginning of a new line The Spectrum Forth word AT is the same as Basic and may be used to position the cursor for output e g to print 3rd line 2nd column 32 HELLO Spectrum Forth automatically scrolls the screen if the current position is at the bottom of the screen A further way to print a character can be achieved by the Forth Word EMIT which prints the character whose ASCII value is at the top of the Parameter stack This may have been put there by the KEY command This also allows you to print a character which is not directly available from the keyboard For example 143 EMIT lt gt OK prints the character with ASCII value 143 which in this case 1 a graphic space Output may also be sent to the printer There is a variable in Spectrum Forth called PRINT If this variable is set to then the output is sent to the
10. most recently referenced by LIST nd d LO Stores an ASCII sign just before a converted numeric output string in the text output buffer when n is negative n is discarded but double number d is maintained Must be used between lt and gt Used during word definition to toggle the smudge bit in a definitions name field This prevents an uncompleted definition from being found during dictionary searches until compiling is compiled with out error A computer dependent procedure to initialize the stack pointer from SO addr A computer dependent procedure to return the address of the stack position to the top of the stack as it was before SP was executed e g 12 SP would type 2 2 1 Transmit an ASCII blank to the output device LO n LO Transmit n ASCII blanks to the output device addr LO U A user variable containing the compilation state A non 2ero value indicates compilation The value itself may be implementation dependent nl n2 n2nl LO Exchange the top two values on the stack A no operation word which can mark the boundary between applications By forgetting and re compiling an application can be discarded in its entirety An alias for ENDIF P CO LO addr U A user variable containing the address of the terminal input buffer addr b Complement the contents of addr by the bit pattern b addr1 n addr2 Move across the name field of a ZX FORTH variable leng
11. screen If it is set to 1 then it is sent to the printer and the screen For example to send the output to the printer type 1 PRINT I lt gt and PRINT I lt ENTER gt to turn the printer off The equivalent to Copy in Basic is also supplied by the Spectrum Forth word COPY Type COPY lt gt to make a copy of the screen onto the printer 13 3 4 NUMBER PRINTING The simplest way to print a number is to use the Forth word dot which you have already met This prints the top number on the stack in the minimum field width Le no proceeding zero s and one space after the number Number formatting may be achieved by the following Forth words R prints the number in a right justified field of a given width For example 103 4 R ENTER 1030K prints 103 in a field of 4 characters Forth also gives you the ability of pictured output which enables you to format the output as required lt starts the pictured output definition and it expects a double precision number on the stack As you usually use single precision numbers a single precision number may be converted to a double precision one by the Forth word S gt D which converts the top number on the stack to a double precision one Within the pictured output you may use the following Forth words puts the next digit into the output buffer starting from the lowest value i e 112 first puts 2 into buffer then the next puts 1 into the buffe
12. screen number and consists of 16 lines of 64 characters However so that Forth can use these efficiently it is necessary to terminate the screen by special Forth words gt at the end of the final line of the screen commands SPECTRUM Forth to LOAD the next consecutive screen when the screen is loaded from tape using the LOAD command This is used when programs occupy more than one Forth screen SPECTRUM Forth compiles the screen in memory and then prints READY CASSETTE to LOAD on the next screen 5S at the end of the final line of the screen terminates the LOAD command This is used for the last screen of program Failure to place a terminator at the end of the screen could cause the system to crash N B Only one screen may be stored in memory at any time and will be overwritten when another screen is loaded 6 0 OTHER USEFUL COMMANDS As Forth has such a rich vocabulary it is impossible to list them all here The only way to get used to them is practice by reading the GLOSSARY and trying the commands FORGET This word is used to forget a definition It is used in the form FORGET word lt gt This will forget the word word and all the following words defined after it For example if we define the following words WD1 HELLO ENTER WD2 HI lt ENTER gt WD3 BYE lt ENTER gt then FORGET WD2 lt ENTER gt OK will remove WD2 and WD3 from the Dictionary but not WD
13. sorte computina 48K RAM The Manual was Scanned OCR and pdf by Stephen Parry Thomas For ZX Spectrum users everywhere and to preserve the manual 22 Feb 2004 CONTENTS Page 1 Installation Instructions Page I 1 0 Basic Operations Page 1 1 1 Getting Started Page 1 1 11 Screen Editor Page 2 1 2 Words Page 2 1 3 Numbers Page 3 1 4 Parameter Stack Page 3 1 5 Arithmetic Page 4 1 6 Stack Manipulations Page 5 1 7 Definitions Page 5 1 8 Modes Page 7 Table 1 Page 8 Table 2 and Table 3 Page 9 2 0 Data Declaration Page 9 2 1 Constants Page 9 2 2 Variables Page 10 2 3 Altering Number Bases Page 10 2 4 Arrays Page 10 2 5 Other Memory Operations Page II Table 4 Page 12 3 0 Input and Output Page 12 3 1 Character Set amp User Definable Characters Page 12 3 2 Input of Information Page 13 3 3 Output and Printing Page 14 3 4 Number Printing Page 14 3 5 Other Printing Operations Page 15 3 6 Colour Hi Resolution and Sound Page 16 Table 5 Page 18 4 0 Conditional Branches and Loop Page 18 4 1 Conditional Branches Page 19 4 2 Indefinite Loops Page 20 4 3 Return Stack Page 20 4 4 Controlled Loops Page 21 4 5 Nesting Structures Page 21 Table 6 Page 22 5 0 Tape Storage Page 22 5 1 Saving Programs Page 22 2 2 Loading Programs Page 23 5 3 Screen Format Page 23 6 0 Other Useful Commands Page 25 Appendix a Error Codes Page 25 Appendix b Useful Routines Page 26 Glossar
14. stack Conversion is performed according to the current BASE A blank follows Pronounced D dot dn Print a signed double number d right aligned in a field n characters wide d ud Leaves the absolute value ud of a double number Set the numeric conversion BASE for decimal input output LO bl b2b3 b8nl Define a user definable graphic DEFINITIONS Used in the form LI DIGIT DLITERAL DMINUS DEFINITIONS Set the Current vocabulary to the CONTEXT vocabulary In the example executing vocabulary name cccc made it the CONTEXT vocabulary and executing DEFINITIONS made both specify vocabulary cccc cnl n2tf ok c nl ff bad Converts the ASCII character c using base n1 to its binary equivalent n2 accompanied by a true flag If the conversion is invalid leaves only a false flag d d executing d compiling P If compiling compile a stack double number into a literal Later execution of the definition containing the literal will push it to the stack If executing the number will remain on the stack 41 d2 Convert d1 to its double number two s compliment 34 DO DOES gt DP DPL DRAW DROP DUP ELSE nl n2 execute P C2 LO Occurs in a colon definition in form DO LOOP DO LOOP At run time DO begins a sequence with repetitive execution controlled by a loop limit n1 and an index with initial value n2 DO removes these from the stack Upon r
15. 1 This word is used to exit SPECTRUM Forth and return to Basic IMMEDIATE Normally when a word is encountered within a definition it is compiled as part of the definition If you require a word to be executed when it is encountered even within a definition then the word may be declared to be Immediate by following its definition This word is dummy definition which is con ventionally used to start a program so whenever you wish to forget a program you know where to forget from VLIST This word will list all the words in the Dictionary The most recently defined word is listed first The listing can be stopped at any time by use of the break key SHIFT SPACE 23 For example WD3 IT IS NOW COMPILING IMMEDIATE lt ENTER gt OK This then prints the message whenever WD3 is encountered and it is not compiled and and LITERAL Sometimes it is convenient to calculate a constant with a definition without having to calculate it each time the definition is executed The Forth word temporarily places the computer into interpret mode and anything typed will be executed immediately The word places the computer back into compile mode in a definition The Forth word LITERAL places the top value on the stack into the current definition as a constant Literal is immediate and so executed when encountered For example The following two definitions are equivalent PT1
16. 3 13 2 LITERAL ENTER PT2 3 8 lt ENTER gt This is used when the result of a calculation is not known and saves you from working it out VOCABULARY Forth lets you create your own vocabularies so that all the words for one program may be kept together Vocabularies should be declared as immediate For example to define a vocabulary called mine VOCABULARY MINE IMMEDIATE lt ENTER gt To place definitions within your vocabulary you type vocabulary name definitions For example MINE DEFINITIONS lt gt definitions preceding this will be placed in the vocabulary MINE until you again change vocabulary From within a vocabulary you may assess all the words within the vocabulary and all words in the vocabulary the current vocabulary was defined in If you wish to use a word in a different vocabulary then you precede the word with the vocabulary name The basic vocabulary is FORTH MEM This word prints out your remaining free memory The number of bytes printed in the current base 24 APPENDIX SPECTRUM FORTH ERROR CODES ERROR CODE ERROR MESSAGE COMMAND NOT IN DICTIONARY STACK EMPTY DICTIONARY FULL OUT OF MEMORY INCORRECT ADDRESS MODE WARNING NAME NOT UNIQUE STACK FULL OUT OF MEMORY WORD MUST BE USED IN DEFINITION EXECUTION ONLY UNMATCHED CONDITIONAL DEFINITION NOT FINISHED IN PROTECTED DICTIONARY USE ONLY WHEN LOADING OFF CURRENT EDITING SCREEN DECLARE VOCABULARY
17. At run time IF selects execution based on a boolean flag If f is true non zero execution continues ahead through the true part If f is false zero execution skips till just after ELSE to execute the false part After either part execution resumes after ENDIF ELSE and its false part are optional if Missing false execution skips to just after ENDIF Mark the most recently made definition so that when encountered at compile time it will be executed rather than being compiled i e the precedence bit in its header is set This method allows definitions to handle unusual compiling situations rather than build them into the fundamental compiler The user may force compilation of an immediate definition by preceding it with COMPILE addr LO A user variable containing the byte offset within the current input text buffer terminal or disc from which the next text will be accepted WORD uses and moves the value of IN nl Set ink foreground colour The outer text interpreter which sequentially executes or compiles text from the input stream terminal or disc depending on STATE If the word name cannot be after a search of CONTEXT and then CURRENT it is converted to a number according to the current base That also failing an error message echoing the name with will be given Text input will be taken according to the convention for WORD If a decimal point is found as part of a number a double number value will be left
18. ENTER gt OK TEN TIMES begins definition 10 gives terminating value gives starting value DO transfers loop Parameters to the Return stack Process LOOP repeats the loop 1 times Within the loop the loop index may be accessed by the Forth word L Suppose we wish to print the numbers 9 on the screen one per line we can define a Forth word NO to do this NO 10 0 DOCR I LOOP ENTER gt OK If you wish to increment by a step other than 1 you may use the DO LOOP structure For example BY2 10 0 DO 2 LOOP lt ENTER gt OK 1 step where BY2 begins definition 100 Parameters of loop DO begin loop as before 2 puts step onto stack LOOP removes step from stack and adds to current loop index If result is less than terminating value loop is repeated with new index value Otherwise loop is terminated The step may be positive or negative By using a negative step the loop will count down However the Parameters must be placed in 20 reverse order i e lO rather than 19 For example to print the numbers 10 to 1 the following Forth word may be defined TENT01 10 DOCR 1 LOOP ENTER OK In this case LOOP checks if the loop index is greater than the terminating value and repeats if it is true If you need to leave a DO LOOP construct before the loop has finished For example Ifa certain condition is met then the command LEAVE will cause the loop to terminat
19. Position printer cursor on screen to Line nl Column n2 n This constant leaves the number of blocks per editing screen By convention an editing screen is 1024 bytes organised as 16 lines of 64 characters each 31 BEGIN BL BLANKS BLK BLOCK BORDER BRANCH BRIGHT BYE L Calculate the backward branch offset from HERE to addr and compile into the next available dictionary memory address addr A user variable containing the current number base used for input and output conversion nl 02 Beep Duration 1 Pitch n2 U LO Occurs in a colon definition in form BEGIN UNTIL BEGIN AGAIN BEGIN WHILE REPEAT At run time BEGIN marks the start of a sequence that may be repetitively executed It serves as a return point from the corresponding UNTIL AGAIN or REPEAT When executing UNTIL a return to BEGIN will occur if the top of the stack is false for AGAIN and REPEAT a return to BEGIN always occurs A constant that leaves the ASCII value for blank addr count Fill an area of memory beginning at addr with blanks addr U LO A user variable containing the block number being interpreted If zero input is being taken from the terminal input buffer n addr LO Leave the memory address of the block buffer containing block n If the block is not already in memory it is transferred from tape n S
20. TER you may change your commands by using the DELETE to delete any unwanted characters pressing it once for any character to be deleted then re type the remainder of the line The simplest command that you can give to Spectrum Forth is an empty line If you now type ENTER your Spectrum Forth should respond with OK as it has seen there is nothing to do finished the line and is waiting for another command You should try this as it will show that your Spectrum Forth is alive and welt and listening to you 1 11 SCREEN EDITOR Spectrum Forth has been given a Screen Editor to help you re define any words you may have made an error with The Editor provides a copy cursor which may be moved with the Cursor Keys CAPS SHIFT 5 8 The Editor is invoked by pressing any of the cursor keys Position this cursor to the line you wish to Edit then press EDIT key CAPS SHIFT 1 This will copy the character to the Forth Cursor and move both cursors across one character As there is an auto repeat on all keys hold the key down and you can copy a complete line EDITOR COMMANDS CAPS SHIFT 5 Cursor left 6 Cursor down P 7 Cursor up 8 Cursor right 1 Copy letter and move cursor across one character 1 2 WORDS The basic command unit of Forth is called a word word consists of a string of characters delimited by spaces The only restrictions on words are that no word may contain a space an ENTER or a Graphics character The word may b
21. TH 78 L1 Level One definition of FORTH 78 P Has precedence bit set Will execute even when compiling U A user variable Unless otherwise noted all references to numbers are for 16 bit signed integers For 32 bit signed double numbers the most significant part with the sign is on top of the stack All arithmetic is implicitly 16 bit signed integer math with error and under flow indication unspecified n addd LO Store 1 6 bits of n at address Pronounced store ICSP Save the stack position in CSP Used as part of the compiler security 41 d2 L0 Generate double number 41 the next ASCII character which is placed in an output string Result d2 is the quotient after division by BASE and is maintained for further processing Used between lt and gt See S gt d addr count LO Terminates numeric output conversion by dropping d leaving the text address and character count suitable for TYPE 26 5 CODE LOOP ABORT DO FIND LINE t LOOP dl d2 LO Generates ASCII text in the text output buffer by the use of until a zero double number n2 results Used between lt and gt addr Used in the form nnnn Leaves the parameter field address of dictionary word nnnn As a compiler directive executes in a colon definition to compile the address as a literal Used in the form ccecce Ignore a comment that will be delimit
22. WORD replaces the address on the stack by the contents of the 2 bytes at that address For example To place your score on the stack you use SCORES lt ENTER gt OK Sometimes you need to examine the contents of a variable The Forth word outputs the value of the variable whose address is on the top of the stack For example SCORE lt gt O OK The word is used to store a 16 bit value into a location uses the value which is the 2 item on the stack and stores it into the address which is on the stack For example To set the score to 100 100 SCORE lt ENTER gt OK The word adds a value to a variable location For example To increase your score by 100 100 SCORE ENTER OK NB Since the Parameter is used to store intermediate values the need for temporary variables is eliminated 2 3 ALTERING NUMBER BASES Forth has a user variable called base which stores the current number base You may alter this variable to any value between 2 and 36 to select bases other than decimal and hex For example suppose you wish to work in binary then you may do this by 2 BASE lt ENTER gt OK then all the following numbers will be printed in binary Remember input must also be in binary 2 4 ARRAYS Arrays of data are important in many applications For example instead of having 10 variables TO T2 etc it would be better to use 10 successive data elements TEMP Through suitable addr
23. ample 8000 64 BLANKS lt EIMTER gt OK Puts 64 32 s from 8000 onward TABLE 4 MEMORY OPERATIONS Word Description Before After Fetch contents of item whose address is at top of stack 100 236 Stores the 2 item on stack into location whose address is on top of stack 3 20000 empty Fetches and prints contents of item whose address is on top of stack 100 empty Increments the location whose address is on top of stack 2 item on stack 701 2000 empty C Fetch a byte whose address at top of stack 100 20 c Stores a byte 2 5 into the location at top of stack 254 2000 CMOVE FILL ERASE As 2 4 BLANK Double Precision see glossary EXERCISES l Define EXCHANGE to exchange the contents of 2 variables that is 1f A and B are variables then the result of the command A B Exchange should be to place the value of A in B and value of B in A 2 Define TRANSFER to move data between two arrays of the same length 11 empty 3 0 INPUT AND OUTPUT In order to perform any function it is necessary To input data into the computer and obtain the results Forth has various ways of doing this 3 1 CHARACTER SET The User Definable Graphics 144 164 may be defined using the word DEF To define a character put 8 bytes on the stack representing bit patterns of the bottom row of the character to the top row then put the character code you wish to change then the word DEF This may seem complex but here s how to define aman e
24. are generally used to maintain discipline in the stack when it contains Parameters Practising with these words will make them useful to you quickly When practising keep in mind 2 elementary rules 1 Keep parity everything on must come off 2 Never remove more items than you have placed on the stack After you have become familiar with both Arithmetic Operators and Stack Manipulators you will want to create your own words For example To square a number use SQUARE DUP ENTER OK 1 7 DEFINITIONS Part of Forth s power lies in the ability to allow you to define your own words For example it might be that we often have to calculate the cube of a number It is easy to define a new word to do the job CUBE DUP DUP lt ENTER gt OK Here is what each component does Begins a definition CUBE The name of the word to be added to the Dictionary DUP DUP The Forth words defining what to do Ends a definition After making this definition we can calculate and print a cube whenever we want For example 4 CUBE lt ENTER gt 64 OK 3 CUBE lt ENTER gt 27 OK What would have happened if you had used the word CUBE before it was defined Forth would not have allowed it It would print CUBE MSG undefined word error Fortunately for the novice programmer Forth has a rich vocabulary of pre defined words For example which prints the contents of the memory locations addressed by the top of t
25. called the RETURN STACK which is used by the interpreter for storage of return addresses Any error message empties both STACKS 1 5 ARITHMETIC Forth has a pro defined set of Arithmetic operators See Table 1 Since Forth uses a Push Down Stack and reverse polish notation the Parameters must be on the stack before the operation can be performed thus to add two numbers together and display the results type in 527 ENTER 32 OK Breaking this line down into its constituent parts you will find that 5 Pushes the value 5 onto the stack 27 Pushes the value 27 onto the stack SE Removes the top two items from the stack adds them together and places the sum back onto the stack NOTE The stack has a net loss of 1 item Removes the top item from the stack and displays it 32 Thus you leave the stack just as it was before you started Processing of Comparisons may also be unfamiliar Forth assumes the conventions of positive logic Truth Value False True The Forth relation words such as lt gt etc may be remembered as written between the second stack entry on the left and the top stack item on the right Thus A B lt will test for A gt B and leave only a truth value on the stack since both A and B have been removed 1 6 STACK MANIPULATIONS Other frequently performed operations are classified as Stack Manipulations for which Spectrum Forth provides a few simple words These words described In Table 3
26. computation stack and place as the most accessible on the return stack Use should be balanced with R gt in the same definition ADDR LO Print the value contained at the address in free format according to the current base Issue error message if not compiling Issue error message if stack position differs from value in CSP fn Issue an error message number n if the boolean flag is true Issue an error message if not executing Issue an error message if not loading nl n2 Issue an error message if nl does not equal n2 The message indicates that compiled conditionals do not match Issue an error message if The stack is out of bounds This definition may be installation dependent addr n LO Leave the 16 bit contents of address Clear the stacks and enter the execution state Return control to the operators terminal printing a message appropriate to the installation n u LO Leave the absolute value of n as u Used in a colon definition in the form BEGIN AGAIN Atrun time AGAIN forces execution to return to corresponding BEGIN There is no effect on the stack Execution cannot leave this loop unless R gt DROP is executed one level below n LO Add the signed number to the dictionary pointer DP May be used to reserve dictionary space or re origin memory n is with regard to computer address type byte or word nl n2 n3 LO Leave the bit wise logical and of nl and n2 as n3 nl n2
27. d DUP DUP 2 What is the difference between OVER SWAP and SWAP OVER TABLE 1 ARITHMETIC OPERATORS Word Description Example Example Normal of Stack of Stack 16 Bit Before After Top Top Adds 96 2 9 8 Subtracts 96 2 9 4 2 Multiply signed 96 2 9 12 Divides signed 96 2 93 1 Add 1 96 2 963 2 Add 2 96 2 964 ABS Leaves absolute value 9 6 2 9 6 2 MAX Leaves largest of top two entries 96 2 9 6 MIN Leaves smallest of top two entries 962 92 MINUS Unary minus 2 s compliment 96 2 9 6 2 MOD Leaves Modulus division remainder 96 2 9 0 Multiplies 2 amp 39 and divides by first 962 27 MOD As above but leaves remainder 962 270 Apply sign of 1 to 2 96 2 9 6 MOD Divide 2 by 1 leaving remainder and quotient 962 9 0 3 AND Leave bitwise logical 9 6 3 9 2 OR Bitwise logical OR 963 9 7 XOR Bitwise exclusive OR 963 9 5 Double precision and Mixed operator commands also exist see glossary They are proceeded until either D M or U Word Word DROP DUP DUP OVER ROT SWAP TABLE 2 COMPARISON OPERATORS Description Before Compares leaves 1 if 2 less After 9 91 960 960 960 After 11 11 320 3212 4213 312 12 322 3 2 3 2 than 1 otherwise 9 6 Compares leaves 1 if 2 greater than 1 otherwise 9 62 Tests for zero leaves 1 if top entry is less than zero otherwise 962 Tests for negative leaves 1 if top entry is less than zero otherwi
28. e any length with the first 31 characters being significant which allows the use of meaningful words Words may be entered in upper or lower case After terminating a line of text with ENTER the FORTH TEXT INTERPRETER scans the input breaking it up into words which will be executed in the order of entry Each Word in Forth has a name the way you refer to it and a definition the meaning i e what it does To execute a word the interpreter searches the Dictionary to determine the definition of the word If the word is found the definition is interpreted If it is not found the interpreter attempts to convert the word to a 1 6 bit integer If the word is not a valid number in the current base an error message 0 is given The system returns with cursor for new input This Dictionary may be extended by adding new words which call upon existing words See Section 1 7 1 3 NUMBERS Numbers can be expressed in any base from 2 36 The system defaults to decimal at power up However at any time you can use the commands words DECIMAL or HEX or you can define another base This establishes the number base to treat succeeding numbers both for input and output In general you should stick to one base throughout all your definitions to avoid confliction in interpretation Numbers may be typed in as positive or negative integers Positive unsigned integers 0 65535 are accepted or signed integers 32768 32767 as well Y
29. e at the next LOOP or LOOP Example EX3 10 0 DO I DUP 6 IF LEAVE ELSE THEN LOOP lt gt is a rather clumsy way to print the numbers 5 but illustrates the use of this word Exercise 1 Define POWER so that POWER computes the n th power of m for non negative n 4 5 NESTING STRUCTURES DO LOOP and IF THEN sequence may contain either such sequence but only if they are properly nested that is one entire DO LOOP may be inside another but they may not overlap I For example RIGHT 100 DO LOOP THEN 100 DO THEN LOOP Exercise 1 How would you define MAX MIN and ABS supplied SPECTRUM Forth 2 Define FACTORIAL to compute the factorial of a number TABLE 6 AND OR XOR 10 10 10 1110 1111 1101 0 00 0 10 0 10 21 5 0 TAPE STORAGE Forth normally works interactively and once a definition has been typed in there is no way of changing it without re typing it Forth also provides a method of storing the code on a numbered screen A screen consists of 16 lines of 64 characters Programs are stored on a screen using the Editor see Editor Manual 5 1 SAVING PROGRAMS Once the program has been written on the screen it is possible to save it onto cassette To do this you must set up your computer for saving see relevant chapter In computer manual The command FLUSH tells SPECTRUM Forth to save the current screen onto cassette If you type FLUSH l
30. eaching LOOP the index is incremented by one Until the new index equals or exceeds the limit execution Loops back to just after DO otherwise the loop parameters are discarded and execution continues ahead Both n1 and n2 are determined at run time and may be the result of other operations Within a loop T will copy the current value of the index to the stack See I LOOP L00L LEAVE LO A word which defines the run time action within a high level defining word DOES gt alters the code field and first parameter of the new word to execute the sequence of compiled word addresses following DOES gt Used in combination with BUI LDS When the DOES part executes it begins with the address of the first parameter of the new word on the stack This allows interpretation using this area or its contents Typical uses include the Forth assembler mulit dimensial arrays and compiler generation addr U L A user variable the dictionary pointer which contains the address of the next free memory above the dictionary The value may be read by HERE and altered by ALLOT addr U LO A user variable containing the number of digits to the right of the decimal on double integer input It may also be used to hold output column location of a decimal point in user generated formatting The default value on single number input is 1 nl n2 Draw a line from current plot position n1 in x direction n2 in Y direction n LO Drop the number from
31. ed by a right parenthesis on the same line May occur during execution or in a colon definition A blank after the leading parenthesis is required Ct The run time procedure compiled by which transmits the following in line text to the selected output device See The run time procedure compiled by CODE that rewrites the code field of the most recently defined word to point to the following machine code sequence See CODE n C2 The run time procedure compiled by LOOP which increments the loop index by n and tests for loop completion See LOOP Executes after an error when WARNING is 1 This word normally executes ABORT but may be altered with care to a user s alternative procedure The run time procedure compiled by DO which moves the loop control parameters to the return stack See DO addr1 addr2 pfa b tf ok LI addr2 ff bad Searches the dictionary starting at the name field address addr2 matching to the text at addr1 Return parameter field address length byte of name field and boolean true for a good match If no match is found only a boolean false is left nl n2 addr count Convert the line number n1 and the screen n2 to the tape buffer address containing the data A count of 64 indicates he full line text length C2 The run time procedure compiled by LOOP which increments the loop index and tests for loop completion See P LO P LO 27
32. ess executed REPEAT returns to begin continue if truth value false zero then continue is executed Example to search through memory as before SEARCH 1 0 DUP WHILE 1 REPEAT ENTER OK try and work this out for yourself The final kind of indefinite loop is of the form EXZ BEGIN process AGAIN ENTER which is repeated indefinitely This is an Infinite loop and can only be terminated by the break key 19 4 3 RETURN STACK Forth uses two stacks the Parameter Stack and the Return Stack This is because otherwise Parameters and Return addresses may get confused There are several commands to transfer Parameters from one stack to another gt removes a number from the Parameter stack and places it on the Return stack R gt removes a number from the Return stack and places it on the Parameter stack NB Ifused in a definition both must be used to compliment each other R or I make a copy of the number or the top of the Return stack onto the Parameter stack The return stack is not altered In order to try to use these commands try the following Define a Forth word 2SWAP to swap the first 2 numbers of the stack with the third and forth that 1 after 123452SWAP the stack should contain 14523 with 3 on the top 4 4 CONTROLLED LOOPS A controlled loop is one which is repeated a certain number of times Forth provides a DO LOOP structure for this This takes the form TEN TIMES 10 DO process LOOP lt
33. essing arithmetic you may compute the required elements address This is more flexible to program as well as more economical of Dictionary space To set aside space in the Dictionary for arrays you use the Forth word ALLOT In the case of temp above you write 0 VARIABLE TEMP 18 ALLOT lt ENTER gt OK Where 0 VARIABLE TEMP defines a variable 2 bytes wide named TEMP 18 puts 18 on stack ALLOT allocates a further 18 bytes for Temp To assess the n th element place n on top of the stack and follow it with 2 TEMP lt ENTER gt OK NOTE Elements numbered 0 9 NOT 1 10 values for n outside this range will give unpredictable results To initialise the n th element type value n 2 TEMP ENTER OK 2 5 OTHER MEMORY OPERATIONS There are 4 words which can be used to manipulate memory locations 1 Move the specified quantity of bytes 1 on stack from address 3 on stack to address 2 on stack Contents of lowest address moved first Example 16396 8000 64 CMOVE ENTER OK In theory moves 64 bytes from 16396 to 8000 2 FILL Fills memory at address 3 on stack with quantity 2 on stack of bytes 1 on stack Example 8000 64 0 FILL ENTER OK Puts 64 bytes of 0 from 8000 onwards 3 ERASE Fills a block of memory with zero s equivalent to 0 FILL Example 8000 64 ERASE ENTER OK Erases 64 bytes starting from 8000 4 BLANKS Fills a block with spaces 732 FILL ASCII See Chapter 3 10 Ex
34. et P are executed rather than being compiled PCLO Terminate colon definition and stop further compilation Compiles the run time S Used with Forth assembler Stop interpretation of a screen S is also the run time word compiled at the end of a colon definition which returns execution to the calling procedure 01 n2 f LO Leave a true flag if n1 is less than n2 otherwise leave a false flag Set up for pictured numeric output formatting using the words lt S SIGN gt The conversion is done on a double number producing ext at PAD Used within a colon definition C LO ccce lt BUILDS DOES Each time is executed lt BUILDS defines a new word with a high level execution procedure Executing ccecce in the form ccce nnnn uses lt BUILDS to create a dictionary entry for nnnn with a call to the DOES gt part for nnnn When nnnn is later executed it has the address of its parameter area on the stack and executes the words after DOES gt in cccc lt BUI LDS and DOES gt allow run time procedures to be written in high level rather than in assembler code nl n2 f LO Leave a true flag if nl n2 otherwise leave a false flag nl n2 f LO Leave a true flag if nl is greater than n2 otherwise a false flag 30 gt CSP ERROR LOADING PAIRS STACK ABORT ABS AGAIN ALLOT AND AT B SCR n c LO Remove a number from the
35. et border colour to n C2 LO The run time procedure to unconditionally branch An in line offset is added to the interpretive pointer If to branch ahead or back BRANCH 1 compiled by ELSE AGAIN REPEAT n Brightness of Characters 0 normal 1 bright Return to basic b addr Store 8 bits at address On word addressing computers further specification is necessary regarding byte addressing 32 CC CFA CIRCLE CLS CMOVE COLD COMPILE b Store 8 bits of b into the next available dictionary byte advancing the dictionary pointer addr b Leave the 8 bit contents of memory address pfa cfa Convert the parameter field address of a definition to its code field address nl 02 n3 Draw a circle of radius n1 at co ordinates n2 n3 Clears the screen from to count Move the specified quantity of bytes beginning at address from to address to The contents of address from is moved first proceeding toward high memory The cold start procedure to adjust the dictionary pointer to the minimum standard and restart via ABORT May be called from the terminal to remove application programs and restart C2 When the word containing COMPILE executes the execution address of the word following COMPILE is copied compiled into the dictionary This allows specific compilation situations to be handled in addition to simply compiling an execution address which the interpreter already d
36. g lt gt 81 81 66 3C FF 7E 18 18 lt EIMTER gt DECIMAL lt ENTER gt 144 DEF lt ENTER gt OK The character 144 is now a little man 3 2 INPUT OF INFORMATION Forth has no real input commands as such because the numbers of Parameters for each command is stored on the Parameter stack This is usually placed on the stack preceding the execution of the command For example suppose we wish to calculate 4x 3x 2 for any value x Then it is easy to define a command which finds x and then one to calculate the rest of the cubic Example CUBE DUP DUP CUBIC DUP CUBE 4 SWAP3 2 ENTER OK Define a function CUBIC to calculate the cubic The value of the Parameter x to be calculated is placed on the stack before the command is used This is done as follows 8 CUBIC lt gt 2026 OK Thus any parameter may be passed into a command and no input command is needed However Forth has got a command to accept a key from the key board This Forth word is KEY This is similar to the INKEYS in Basic However KEY waits for key to be pressed whereas INKEY does not Suppose during the execution of a program you need some input from the keyboard For example DO YOU WANT INSTRUCTIONS Y OR N KEY then places the ASCII value of the key pressed into the Parameter stack where it may be examined In Forth there is no routine to input a number more than 1 digit from the keyboard However there
37. he stack has a simple definition 2 gt Another simple combination that is pre defined is 1 which adds 1 to the item at the top of the stack 1 has the definition 1 1 1 8 MODES The Forth text interpreter operates in two modes Immediate Execution and Compilation In Immediate Execution Mode each word of the input string is looked up in the Dictionary and executed immediately During Compilation however most word are not executed Instead a reference to them is compiled in the Dictionary The word places the interpretation in Compile Mode whereas returns it to Immediate Execution The compiled form of the definition consists of pointers to the addresses of routines that will be executed by the inner interpreter when the definition is executed This form of interpretation is extremely fast To distinguish between the modes of immediate execution and compilation try the following examples 905 lt ENTER gt 905 OK executes immediately note the interaction SHOW 905 lt ENTER gt OK compiles nothing happens yet SHOW lt gt 905 OK executes the compiled routine to produce the desired result To learn quickly you must practice with the basic Forth words and words you evolve out of experiments Develop a kind of notation which will leave you with a sketch of what you have done to help you avoid making the same mistakes twice EXERCISES 1 What Is the difference between DUP DUP an
38. ining n is left on the stack so that a fetch or store may access this location addr U A user variable containing the address of a field in the definition of the most recently created vocabulary All vocabulary names are linked by these fields to allow control for FORGETTING thru multiple vocabularies VOCABULARY E Lu A defining word used in the form VOCABULARY To create a vocabulary definition cccc Subsequent use of cccc will make it the CONTEXT vocabulary which is searched first by INTERPRET The sequence DEFINITIONS will also make cece the CURRENT vocabulary into which new definitions are placed In SPECTRUM Forth cccc will be so chained as to include all definitions of the vocabulary in which cccc is itself defined AII 43 VLIST WARNING WHILE WIDTH WORD XOR COMPILE vocabularies ultimately chain to Forth By con vention vocabulary names are to be declared IMMEDIATE See VOC LINK List the names of the definitions in the context vocabulary Break will terminate the listing Must be 0 for no disc installation f run time 2 Occurs in colon definition in the form BEGIN WHILE REPEAT At run time WHILE selects conditional execution based on boolean flag f If fis true non zero WHILE continues execution of the true part thru to REPEAT which then branches back to BEGIN If f is false zero execution skips to just after REPEAT exiting the st
39. n is suspended for the compile time calculation of a value compilation is resumed and LITERAL compile this value n LO Begin interpretation of screen n Loading will terminate at S See Sand gt Occurs in a colon definition in form DO LOOP At run time LOOP selectively controls branching back to the corresponding DO based on the loop index and limit The loop index is incremented by one and compared to the limit The branch back to DO occurs until the index equals or exceeds the limit at that time the parameters are discarded and execution continues ahead 39 M M MOD MAX MESSAGE MIN MINUS MOD NEXT NFA NUMBER OR OUT OVER PAD PAPER PERM nl n2 d A mixed magnitude math operation which leaves the double number signed product of two signed numbers dnl n2 n3 A mixed magnitude math operator which leaves the signed remainder n2 and signed quotient n3 from a double number dividend and devisor nl The remainder takes its sign from the dividend udl u2 u3 ud4 An unsigned mixed magnitude math operation which leaves a double quotient ud4 and remainder u3 from a double dividend ud1 and single divisor u2 nl n2 max LO Leave the greater of two numbers n Print MSG Zn nl n2 min LO Leave the smaller of two numbers nl n2 LO Leave the two s complement of a number nl n2 mod LO Leave the remainder of n1 n2 with the same sign as nl Used with FORTH assembler
40. oes CONSTANT LO CONTEXT COPY COUNT CR CREATE A defining word used in the form n CONSTANT to create word cccc with its parameter field containing n When cccc is later executed it will push the value of n to the stack addr U LO A user variable containing a pointer to the vocabulary within which dictionary searches will first begin Send a copy of screen to the printer addrl addr2 and byte LO Leave the byte address addr2 and byte count n of a message text beginning at address addr1 It is pre summed that the first byte at addrl contains the text byte count and the actual text starts with the second byte Typically COUNT Is followed by TYPE LO Transmit a carriage return and line feed to the selected output device A defining word used in the form CREATE by Such words as CODE and CONSTANT to create a dictionary header for a Forth definition The code field contains the address of the words parameter field The new word is created in the CURRENT vocabulary 33 C5L D R DABS DECIMAL DEF addr U A user variable temporarily storing the stack pointer position for compilation error checking dl 42 dsum Leave the double number sum of two double numbers din d2 Apply the sign of n to the double number d1 leaving It as d2 d L1 Print a signed double number from a 32 bit two s complement value The high order 16 bits are most accessible on the
41. ou may also use double precision numbers which are signed integers 2147483648 21 47483647 These 32 bit double precision numbers must be prefixed by a dot Since all the numbers are stored in binary form you can take advantage of Numeric base selection to perform number conversions to convert a decimal number to a Hexadecimal number For example type DECIMAL 258 HEX lt gt and you will receive the response 102 OK but remember you are now in HEXADECIMAL 1 4 THE PARAMETER STACK All computer programs manipulate data by using an established set of Parameters In Forth most of the Parameters are maintained on a PUSH DOWN stack called the Parameter Stack A Push Down stack is a particular arrangement of memory storage Forth Words which refer to the Parameter Stack only do so by accessing the topmost items LAST IN FIRST OUT To place a number on the stack you can type it as part of your input command The Forth Word dot removes the top number off the stack and prints it in the current base on the screen For example To place numbers on the stack 2 4 6 8 lt gt The stack now looks like this 8 6 4 2 If you now type lt ENTER gt the output will be 8 OK The stack now looks like this 6 4 2 Now type ENTER 6 4 2 OK The stack is now empty Suppose you type lt gt the computer responds MSG 1 stack underflow errors Forth also has another stack which is
42. r etc S puts the remaining digits into the output buffer if non zero HOLD used as 46 HOLD which puts the character whose character code is 46 into the next part of the buffer gt terminate pictured output leaves the address and length of the output buffer on the stack Hence the string may be outputted using the TYPE command Example Suppose you define a new command PIC to produce pictured output PIC lt 46 HOLDZ ZS gt ENTER gt OK This will print any double precision numbers as X x xX Example 11473 PIC TYPE lt ENTER gt 11 4 73 OK 0 PIC TYPE lt ENTER gt 0 00 OK This needs practice to master the art of using numeric pictured output Double precision commands exist See Glossary 3 5 OTHER PRINTING OPERATIONS ZX Forth has four further commands to aid formatting on the screen This word as it suggests prints a space on the screen Equivalent to 32 EMIT SPACES This word Is used to print a given number of spaces specified by the number on the top of the stack For example 5 SPACES lt gt OK prints 5 spaces on the screen HOME This word moves the print position to the top left hand corner of the screen All further output starts at the top of the screen CLS This word clears the screen and moves the print position to the top left hand corner of the screen 14 3 6 COLOUR HI RESOLUTION AND SOUND Most of the Colour and Hi Resolution graphic
43. r n addresses line in blk Execute error notification and re start of system WARNING is first examined If 1 the text of line n relative to screen 4 of drive 0 is printed This line number may be positive or negative and beyond Just screen 4 If WARNING O n is just printed as a message number non disc installation If WARNING is 1 the definition ABORT is executed which executes the system ABORT The user may cautiously modify this execution by altering ABORT ZX FORTH saves the contents of IN and B LK to assist in determining the location of the error Final action is execution of QUIT addr Execute the definition whose code field address is on the stack The code field address is also called the compilation address addr count LO Transfer characters from the terminal to address until a return or the count of characters have been received One or more nulls are added at the end of the text 36 FILL FIRST FLASH FORGET FORTH GOVER HERE HEX HLD HOLD HOME addr U A user variable containing an address below which FORGETTING is trapped To forget below this point the user must alter the contents of FENCE addr quan b Fill memory at the address with the specified quantity of byte b n A constant that leaves the address of the block buffer nl Defines whether character is flashing or steady 0 Steady 1 Flashing E LO Executed in the form FORGET
44. ructure Maximum length of word name 31 in ZX Forth C Readthe next text characters from the input stream being interpreted until a delimiter c is found storing the packed character string beginning at the dictionary buffer HERE WORD leaves the character count in the first two or more blanks Leading occurrences of c are ignored If BLK is zero text is taken from the terminal input buffer otherwise from the block stored in BLK See BLK IN nl n2 xor L1 Leave the bitwise logical Exclusive Or of two values P LI Used in a colon definition in form XXX words more Suspend complication The words after are executed not compiled This allows calculation or compilation exceptions before resuming compilation with I See LITERAL P C Used in a colon definition in form xxx COMPILE FORTH COMPILE will force the compilation of an 44 45
45. s commands are available in Standard Spectrum Forth The colour codes are the same as Basic 1 Red etc The only difference using these commands in Forth is that the parameters are proceeding the command rather than following the command i e to change the ink colour to red BASIC FORTH INK 1 lt ENTER gt 1 INK lt ENTER gt N B In Forth these colours are only temporary and revert back to the power up colours when a CLS is executed To make these colours permanent type the word PERM e g Blue paper White ink 2 Paper 7 Ink PERM lt ENTER gt Colour Sound and Hi Resolution Commands Basic Forth INK X X INK PAPER X X PAPER FLASH X X FLASH BRIGHT X X BRIGHT INVERSE X X INV OVER X X GOVER BORDER X X BORDER PLOT X Y X Y PLOT CIRCLE X Y radius radius X Y CIRCLE DRAW X Y X Y DRAW BEEP X Y X Y BEEP 15 14 31 TABLE 5 CHARACTER CODES CONTROL CHARACTERS DELETE CAPS shift 0 54 6 CONTROL CHARACTERS 55 7 ENTER 56 8 57 9 58 Symbol shift 59 Symbol shift 60 Symbol shift 61 Symbol shift NOT USED 62 gt 63 64 65 66 67 C 68 D SPACE 69 E Symbol shift 70 F Symbol shift P 71 G Symbol shift 3 72 H Symbol shift 5 73 I 74 J Symbol shift 6 75 K Symbol shift 7 76 L Symbol shift 8 77 M Symbol shift 9 78 N Symbol shift 79 0 Symbol shift K 80 Symbol shift N 81 Q Symbol shift I 82 R Symbol shift M 83 S Symbol
46. se 9 6 2 Tests for number equals leave 1 if top two equal otherwise 962 TABLE 3 STACK MANIPULATION OPERATORS Description Before Prints item on top of stack 123 Discard top entry 321 Duplicates top entry 321 Duplicates top entry 321 or if it is non zero 3 2 Copies 2 entry over top entry 321 Rotates top 3 entries 432 Swaps top 2 entries 321 Prints 2 item right Justified in a field of 1 entry 3 24 Copy top of Return stack ret ret to computation stack 29 32 20 32 123 123 38 2 0 DATA DECLARATION Forth allows you to set aside memory for constants variable and arrays 2 1 CONSTANTS To assign names to constants you use the word CONSTANT These are used as they are easier to recall than the number or are used often For example 5280 CONSTANT FT MILE lt ENTER gt OK creates a new word FT MILE and assigns it a value 5280 After FT MILE has been defined you can use it as you would 5280 to place value on the STACK e g 3 FT MILE computes the number of FT in three miles NB Once a value has been defined as a constant its binary value is independent of the current number base 2 2 VARIABLES The Forth word VARIABLE names a location whose value is likely to change Suppose we wish to keep the score of a game of Space Invaders then we can declare a variable as follows 0 VARIABLE SCORE lt ENTER gt OK 1 Initial value When you invoke a variable by name its address is placed on the stack The FORTH
47. t ENTER gt SPECTRUM Forth will respond READY CASSETTE Now press record on your cassette recorder and press lt gt One screen takes 30 seconds to save or LUSH If you type any character other than lt ENTER gt then the command is aborted and the cursor re appears 5 2 LOADING PROGRAMS SPECTRUM FORTH gives you two words to load a program from tape LIST is used to list a screen in the form n LIST where n 19 the screen number If screen n is already in memory it is listed onto the screen If it is not SPECTRUM Forth will try to load it from cassette Connect your computer for loading see manual and position the tape to the silent part immediately preceding the screen you want to load Press lt ENTER gt and then play on the cassette recorder If the screen loads correctly it will be listed on the screen If however it does not or you attempted to load the wrong screen then the READY CASSETTE will be repeated Try again use lt gt or any other key to abort as with FLUSH The variable FIRST contains the screen number of any screen currently in memory This can be examined by FIRST lt gt If you wish to stop loading press space key LOAD is used to compile the definition in a screen This must be preceded by the screen number as with LIST If the screen is not already in memory it is treated as 1f it had been typed from the keyboard 22 5 3 SCREEN FORMAT Each screen has a
48. t of commands either until a given condition is satisfied or not or a set number of times In this section we will look at the former The first type is EXAMPLE BEGIN process condition UNTI L continue lt ENTER gt OK where EXAMPLE begins definition BEGIN marks the beginning of an indefinite loop process defines the action to be executed condition leaves truth value on the stack UNTIL pops the value off the stack and returns to BEGIN if the condition 1 zero false continue continue execution when value is true non zero NB END and UNTIL may be used interchangeably e g Suppose you wish to search through memory for a given 16 bit number and print the address of the number we can define a word SEARCH to do this for us Suppose we are searching for the first occurrence of zero from the beginning of memory SEARCH BEGIN DUP SWAP 1 SWAP UNTIL 1 ENTER OK The code between BEGIN and UNTIL is repeatedly executed until the value found is The before the loop is initialisation in the case of the start address of the search The code after UNTIL prints the address of the occurrence of A second form of indefinite looping is EX1 BEGIN condition WHILE process REPEAT continue ENTER OK where EXI begins definition BEGIN marks the beginning of an indefinite loop condition leaves a truth value on the stack WHILE if the truth value is true non zero process then proc
49. th name field addr1 is the address of either the length byte or the last letter If n 1 the motion is toward hi memory if n 1 the motion is toward low memory The addr2 resulting is address of the other end of the name 42 U U UNTIL USER VARIABLE VOC LINK addr count LO Transmit count characters from addr to the selected output device ul u2 ud Leave the unsigned double number product of two unsigned numbers ud ul u2 u3 Leave the unsigned remainder u2 and unsigned quotient u3 from the unsigned ed double dividend ud and unsigned divisor ul f run time occurs within a colon definition in the form BEGIN UNTIL At run time UNTIL controls the conditional I branch back to the corresponding BEGIN If f is false execution returns to just after BEGIN if true execution continues ahead n LO A defining word used in the form n USER which creates a user variable cccc The parameter field of cccc contains n as a fixed offset relative to the user pointer register UP for this user variable When cccc is later executed it places the sum of its offset and the user area base address on the stack as the storage address of that particular variable E Lu A defining word used in the form n VARIABLE When VARIABLE is executed it creates the definitions with its parameter field initialized to n When cccc is later executed the address of its parameter field conta
50. the stack n nn LO Duplicate the value on the stack Occurs within a colon definition in the form IF ELSE ENDIF At run time ELSE executes after the true following IP ELSE forces execution to skip over the following false part and resumes execution after the ENDIF It has no stack effect 35 EMIT EMPTY BUFFERS ENCLOSE END ENDIF ERASE ERROR EXECUTE EXPECT LO Transmit ASCII character c to the selected output for each character output Mark all block buffers as empty not necessarily affecting the contents This is also an initialization procedure before first use of the tape addr addrl n1 n2 n3 The text scanning primitive used by WORD From the text address addr1 and an ASCII delimiting character c is determined the byte offset to the first non delimiter character n1 the offset to the first delimiter after the text n2 and the offset to the first character not included This procedure will not process past an ASCII null Treating it as an unconditional delimiter P C2 LO This is an alias or duplicate definition for UNTIL Occurs in a colon definition form IF ENDIF IF ELSE ENDIF At run time ENDIF serves only as the destination of a forward branch from IF or ELSE It marks the conclusion of the conditional structure THEN is another name for ENDIF Both names are supported in ZX FORTH See also IF and ELSE addr n Clear a region of memory to zero from addr ove
51. to the computation stack addr U A user variable which may contain the location of an editing cursor or other file related function addr bl f The fig FORTH standard disc read write linkage addr specifies the source or destination block buffer b1 k is the sequential number of the referenced block and f is a flag for f 0 write and f 1 read R W determines the location on mass storage performs the read write and performs any error checking n LO Remove the top value from the return stack and leave it on the computation stack See gt R and addr U A user variable containing the initial location of the return stack Pronounce R zero See RP Used within a colon definition in the form P C2 BEGIN WHILE REPEAT At run time REPEAT forces an unconditional branch back to just after the corresponding BEGIN Reset smudge bit of the most recent entry in the dictionary nl n2 n3 n2n3 nl LO Rotate the top three values on the stack bringing the third to the top A computer dependent procedure to initialize the return stack pointer from user variable RO n d Sign extend a single number to form a double number addr U A user variable that contains the initial value for the stack pointer Pronounced S zero See SP I 41 SCR SIGN SMUDGE sp SP SPACE SPACES STATE SWAP TASK THEN TIB TOGGLE TRAVERSE addr U A user variable containing the screen number
52. xt to the selected output device If executed outside a definition will immediately print the text until the final nl n2 Print the number n1 right aligned in a field whose width is n2 No following blank is printed nl n2 quot LO Leave the signed quotient of n1 n2 nl n2 rem quot LO Leave the remainder and signed quotient of n1 n2 The remainder has the sign of the dividend n These small numbers are used so often that it is attractive to define them by name in the dictionary as constants n f LO Leave a true flag if the number 1s less than zero negative otherwise leave a false flag n f LO Leave a true flag if the number is equal to zero other wise leave a false flag 29 OBRANCH 1 2 lt BUILDS f C2 The run time procedures to conditionally branch If f is false zero the following in line parameter is added to the interpretive pointer to branch ahead or back Compiled by IF UNTIL and WHILE nl n2 L1 Increment n1 by 1 nl n2 Leave nl incremented by 2 Used in the form called a colon definition P E LO Creates a dictionary entry defining cccc as equivalent to the following sequence of Forth word definitions until the next or CODE The compiling process is done by the text interpreter as long as STATE is non zero Other details are that the CONTEXT vocabulary is set to the CURRENT vocabulary and that words with the precedence bit s
53. y SPECTRUM FORTH INSTALLATION INSTRUCTIONS LOAD the Forth compiler by LOAD CODE CODE extended mode I Stop the tape when it has loaded The Forth Editor is next on the tape To load the Editor type the following 1 LOAD lt ENTER gt The computer will respond with READY CASSETTE Pressing ENTER will ititiate the cassette loading routine Start the tape and then press ENTER The first screen takes a few seconds to load Stop the tape when the characteristic blue and yellow lines finish Wait for READY CASSETTE to appear and then start the tape and press ENTER There are a total of 3 screens to be loaded If the program does not load properly re wind tape to the beginning of the screen After the third screen has loaded OK will appear N B Don t be alarmed by an error MSG 4 Your Editor is now loaded Now start programming in Forth HAPPY PROGRAMMING amp GOOD LUCK 1 0 BASIC OPERATIONS The easiest way to learn FORTH is to use it As Forth Is an inter active language you can sit down and experiment In this Users Manual there are many examples to illustrate the capabilities of Forth We suggest you try them for yourself 1 1 GETTING STARTED The Forth will announce itself and tell you how much free memory you have The graphic m is your cursor and will appear when the system is ready for input from the keyboard You are now ready to type a command terminated by ENTER Until you have actually typed EN
Download Pdf Manuals
Related Search