FOCAL USER'S MANUAL
Contents
1. 185 00 ASK Command The ASK command is normally used in indirect commands to allow the user to input data at specific points during the execu tion of his program The ASK command is written in the form 11 99 ASK X Y Z Y When line 11 99 is encountered by FOCAL the user then types a value in any format for the first identifier followed by a terminator Terminators are space comma alt mode and return keys FOCAL then asks for another value no visible indication on the paper and the user types a value for the second identi fier This continues until all the identifiers or variables in the ACK statement dere been piven values The user must end the last number in an ASK sequence with the return key 1 11 99 ASK X Y Z DO 11 99 5 4 3 RETURN KEY WAS TYPED HERE where the user typed 5 4 and 3 as the values respectively for X Y and Z A text string may be included in an ASK statement by enclosing the string in quotation marks just as in the TYPE command 1 10 ASK HOW MANY APPLES DO YOU HAVE APPLES DO 1 10 HOW MANY APPLES DO YOU HAVE 25 ki The identifier AP FOCAL RECOGNIZES the first two characters only now has the value 25 Error Correction Commands and Procedures sith Yireltton WOmMMmands and Frocedures One of the advantages in the FOCAL language is the ability to find errors and to make the appropriate corrections with a minimum of effort Several commands and procedures are a2. ment is the value carried to the subroutine The variable g replaces the second argument and carries its value 1 1 ASK X TYPE FSBR 99 X 3 Q 99 1 S 61 83S5 amp 2 55 3 000061 99 2 S amp 2 1 631 FABS 52 86 6 83 99 3 S E E 52 2 G 99 2 99 3 R Initialize Input Output Device Function These functions FINI FINO allow input and output devices requiring specialized initialization to be set up through FOCAL The address of the initialization routines are placed in the dis patch tables and the function is called with the device number as an argument S X FINI 2 Cursor Address and TV Memory Functions These functions FCUR FTVM allow direct cursor addressing and direct TV memory examination Currently drivers exist only for the Digital Group system TV board 20 Part Three FOCAL Version 3D FOCAL Version 3D has a number of new features and optimiza tions These features include increased speed string functions more powerful DO and SET commands explicit error messages im proved random number generator and priority interrupt control SET SET now has the ability to evaluate more complex expressions For example it is now possible to execute the following statements SET X Y Z 9 SET X 1 Y 2 SET X Y A B 4 This new feature enables the user to generate much more compact and efficient statements The following examples illustrate the same concepts written in both old and new FOCAL Old S A
3. ASCII equivalent characters and vice versa Here are some examples S X FOUT 65 Ar S X FCHR At T X 65 ANhB In the first example 65 is converted to the character A and printed Any printable character control character or binary value can be printed or sent to the output device In the second example X is set to the decimal value of the character typed or sent to FOCAL Using these two functions the numeric values of characters may be tested and operated upon mathematically Echo Control Function The echo control function FECH allows echo to the console device to be suppressed S X FECH 1 S X FECH B Setting the control to one suppresses the echo setting it to zero restores echo Memory Examine and Deposit Function This function FMEM allows machine level memory locations to be examined or deposited by FOCAL If a memory location is examined the function carries two arguments the decimal equi valent of the high and the low byte of the address If a value is to be deposited a third argument carries the decimal value to be loaded into memory T FMEM 28 81 25 0000 S X FMEM 20 01 00 T FMEM 20 01 D 00000 FOCAL Subroutine Function The subroutine function FSBR allows the user to create functions within a propr m FSRR calls a line or a group of 1 19 FOCAL code and returns a value The first argument in the func tion is the line or group number of the code and the second argu
4. characters will be lost while FOCAL stores the text One solution involves having the mass storage device under start stop control This way FOCAL can read text as it is needed Another solution is to insert delays or fill characters after every carriage return The delay should be between 25M and 5040 msec If this method is used the echo flag must be turned off while the text is loaded The dilemma for us is to find a program save routine that will work on all possible devices For example delays work fine on cassette mass storage but are worthless on paper tape The lowest common denominator is the memory image method since every system can save programs this way 26 0000 8142 BEGIN MAIN MEM FOCAL Memory Map os E eee ee ee n a TE o o o o o u a o o o a M a I I I ZERO PAGE I I I lt D m am me mm an pe Ap ap me np ap an m air ve e e om Mm am ae am ee pp ME am e CE e e A o e a D ae ae e de am b dien ar ao am an qp me An ae me my am ARE r ee ab lm e b ae p ae dem na ae mo S ap np uo qh pn a ye nr ee eo A ow sn am ua we ee me e er en e e a ee ee d n LINE BUFFER STACK PAGE gt lt m as m pe r Ee e e o a e r e ee p e e ye e as e p n e E P un e e e e r AR e ae am eee eee ei MAIN BODY OF FOCAL ese a ee Se eee Ee Ew o EP ee ee EB ee En En eee u r o o eee eee eo m ae eee eee ei eee TEXT AREA HH H HH HH HH HAHAHA A Re HA HA A HH A AH HA HA HUA AA HH IVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
5. learn the other commands and the use of line numbers to write a sequence of FOCAL statements As you learn these techniques you will be advancing rapidly in the art of computer programming Part Teo Sequential Commands Indirect Commands Up to this point ail of our commands have been executed immediately upon hittin the reli tov IF a line is pre fixed by a line number cat Wim 9 9 ovscuted immediately instead it is stored ir the compiter ss rory for later execu tion usually as part a sequence ci conmands Line numbers must be in the range from 1 01 to 99 99 The numbers 1 04 2 08 et are ill sal line numbers they are used to identily the entire group he number to the left of the point is called the group number the number to the right is called the step number 1 1 SET A 1 1 3 SET B 2 1 5 TYPE 1 A B Indirect commands are executed by typing GO GOTO or DO commands which may be direct or indirect GO Command The GO command causes FOCAL to go to the lowest numbered line to begin executing the program U the user types a direct GO command after the indirect commands in the example above FOCAL will carry out the command at line 1 1 and then the others sequentially FGO 3 GOTO Command The GOTO conman causes FOCAL to transfer control to a specific line in the indirect program It must be followed by a specific line number After executing the command at the spe cific line FOCA
6. next higher inte ger TYPE FINR 5 5 5 0000 TYPE FINR 5 6 6 2000 Random Number Geneia it A random number generator FRAN generates repeatable random numbers between and 8 999999 TYPE FRAT 0 607295 TYPE FRAN 0737615 The random number generator must be initialized before it is used aia this is done at the beginning of a program It can be initializes cing the SET command S X FRAN 1 S X FRAN 1 If FRAN ic fp ey 2 positive number the random sequence 1s repeatable If it is set toa negative value the sequence is entirely ramon S X FRAN O OR S X FRANC FRAN with a zero subscript returns with X set to a random value Input and Mita Der Funztions Input device oc ten CFIDV and output device function FODV allow FOCAL to send and receive information through more than one device Pou cxample S X FIDV 1 will set FOCAL s input device to device number one Generally device number is the console device Other devices such as printers or cassette recorders can be added by placing the addresses of the 1 0 drivers in the 1 0 dis patch table at the end of the lisctint tor The symbols INTYM and ODEVM set the maximun tember ci lapat and output devices allowed They gt may be altered by patching their value in the routines labeled CHKODV ane fiction 18 Character Input and Output Functions The functions FOUT and FCHR allow decimal numbers to be converted to
7. 2 71828 SET R 9 12739 Symbols may consist of one or two depends on the imple mentation you are using alphanumeric letters and numbers char acters The first character must be a letter but cannot be the letter F which has special meaning as we will see later Just for practice let s use FOCAL to calculate the volume of a sphere which has a radius of 9 12739 We re going to use two of the symbols we have just defined in the SET commands above The formula is V 4 3 PI R whcih we can type like this T R43 PI 4 3 You might be interested in running a timing test to show how long it takes to do such calculations by hand with a calcu lator and with FOCAL The Talking Typewriter To make the output of your program absolutely clear to other people it is sometimes useful to give FOCAL certain messages or column headings We call these character strings These mes sages are enclosed in quotation marks SET E 2 71828 SET PI 3 14159 TYPE PI TIMES E PI E 3 and FOCAL types out PI TIMES E 8 5397 You are not allowed to use the carriage return line feed or leader trailer characters in these character strings but you can tell FOCAL to do a carriage return line feed by inserting an exclamation mark You can get just a carriage return by in serting a number sign 4 TYPE JOHN BILL FRED SAM i RICHARD SAM RICHARD ve Spaces can be used in character strings as neede
8. 4 S B D 33S G A B New S G A 4 B D 3 The ability to evaluate complex expressions is also available in other FOCAL commands For example IF Y D 3 5 2 1 2 2 2 3 TYPE I X U GOTO Y G 1 DO Z Y R 1 X A B For Xz1 Y 4 R3 T X The SET command can now be used to execute a function without setting a variable For example both of these expressions are correct SET X FECH 1 SET FECH 1 Strings String functions allow up to 258 characters to be stored in a single variable Generally these are ASCII characters but they can also be any 8 bit number 255 The format of string variable names is the same as regular variables except that they are followed by a dollar sign A B5 C45 23 Each position in a string is numbered starting with zero For example B3 23 designates the twenty third character in string B3 Each character in a string can be treated as a numeric value and as such can be operated upon by all normal FOCAL com mands In addition a number of new functions have been created to deal with strings FISL This function sets aside a certain amount of sp ce for a string For example S FISL 108 A sets aside 100 characters for A string Several strings can be initialized at once by including several arguments with the function S FISL 1900 A 259 B7 When a string is initialized it fills with spaces If a string is called by a routine without having been initialized with the FISL f
9. FOCAL USER S MANUAL FOCAL Version 3D Copyright 1977 resco FOCAL MINI MANUAL This manual is designed to be a brief introduction to FOCAL commands functions and the language as a whole FOCAL is a registered trademark of Digital Equipment Corp tNote Before running FOCAL on your system make sure that the zero page memory location PDLIST is set correctly for your memory size It should contain the address of the highest memory byte minus FF Also check the section of the listing called FOCDEF This area contains definitions of all of the special definition used by FOCAL Make sure they match your system configuration Loading FOCAL on your KIM system FOCAL is supplied as an audio cassette containing two blocks The ID of both blocks is 1 One block contains page zero 0000 0100 the other block contains the FOCAL program 2000 36B0 Both must be loaded for FOCAL to operate cor rectly After loading both blocks enter address 2880 and G to GO FOCAL will type 17 and an asterisk You are now in FOCAL command mode If you ordered FOCAL on paper tape load the tape using the L command or whatever command loads KIM TIM format tapes on your particular system Part One Getting Started in Programming FOCAL is a programming language which allows the human user to communicate with a computer in higher level mathematical terms without having to be concerned with electronic particulars of the given compu
10. L continues to the next higher line The GOTO causes a program branch we have jumped from one sequence of lines to another Sometimes we merely jump back and repeat a sequence of commands This techuique of repeating sequences is called iteration and it ls often aced by experienced computer programmers GOTO 1 3 2 Since Line 1 1 has not been executed FOCAL gives the symbol A the value zeros 6 For a line number in the GOTO command FOCAL will allow an arithmetic expression which begins with a variable e g X 2 for example a statement in a program may look like the following 1 7 GOTO X 2 Note that the command GOTO 2 X is illegal since the arithmetic expression begins with a number rather than a variable Other commands in which line number computation is provided for are the DO IF MODIFY and ON commands DO Command The DO command is used to transfer control to a specific step or group of steps and then return automatically to the com mand following the DO command The step or group of steps may be given as an arithmetic expression beginning with a variable as in the GOTO command 1 1 SET A 1 SET B 2 1 2 TYPE STARTING 1 3 DO 3 2 2 1 TYPE FINISHED 3 1 SET A 3 SET B 4 43 2 TYPE 1 A B G0 STARTING 3 FINISHED 7 If a DO command is written without an argument FOCAL exe cutes the entire indirect progran 1 1 SET A 1 1 3 SET B 2 1 5 TYPE 1 AtB DO The following ex
11. VVVVVVVVVI I I I AVAILABLE MEMORY I I I TARA AA RA RARA ARA ARA ARA RA RARA RARA 444 T I 1 I SOFTWARE STACK I I I 5 22222222 2 2 2 S 282z eezmez zm END OF MEMORY 27 HH HH H HHH FH a o o o o HH OH Oo HH ot HH HH HH 2040 Command ASK COMMENT CONTINUE DO ERASE FOR GO IF MODIFY ON QUIT RETURN summary of FOCAL Commands Example ASK X ASK TEXT X 3 C TEST PROGRAM 1 1 C DO 23 73 DO 18 E E 12 1 E A FOR 1 1 10 lt TEXT gt FOR J 1 3 25 lt TEXT gt GO GO 1 23 IF X 1 1 1 2 1 3 IF Y 2 3 5 lt TEXT gt I Z X 3 4 lt TEXT gt MOD 1 53 M 1 34 5 43 ON X 1 2 3 0 Y 2 3 lt TEXT gt R Explanation Input value of variable Type text then input X Ignore rest of line Null target of branch Call line 23 73 as a sub routine Call all of group 10 as a subroutine Erase variables Delete program line 12 18 Erase both text and variables DO 99 I 1 1B lt TEXT gt 99 CONTINUE In Algol for J 1 STEP 3 UNTIL 25 In Fortran Start program Unconditional branch to 1 23 Conditional branch as in Fortran IF If Y gt Y go to 2 3 else DO lt TEXT gt If Z lt X go to 3 4 else DO lt TEXT gt Edit line 1 53 Edit line 5 43 and put the result in line 1 34 Leave 5 43 alone Conditional subroutine call 2 If Y do line 2 3 in any case DO 2 3 Stop program execution Go to an mode Return from subroutine Comman
12. ample causes a programming loop which could be terminated by inserting line 1 5 QUIT see below 1 1 SET A 1 1 1 TYPE A 1 3 DO 2 0 1 4 TYPE FINISHED 2 1 SET A A 1 2 2 TYPE A DO commands cause specified portions of the indirect program to be executed as closed subroutines these subroutines may also be terminated by a return command explained below 7 RETURN Command The RETURN command is used to exit from a DO subroutine When a RETURN command is encountered during execution of a DO subroutine the program exits from its subroutine status and returns to the command following the DO command that initiated the subroutine status 1F Command In order to transfer control after a comparison FOCAL contains a conditional IF statement The normal form the the IF statement consists of the word IF a space a parenthesized ex pression or variable and the three line numbers separated by commas The expression is evaluated and the program transfers control to the first line number if the expression is less than zero to the second line number if the expression has a value of zero or to the third line number if the value of the expression is greater than zero The IF expression or variable must be enclosed in parentheses The line numbers can be arithmetic ex pressions but these expressions must begin with a variable as in the GOTO command The program below transfers control to line number 2 10 2 38 or 2 58 acco
13. another GOTO or IF ERASE ALL 1 1 TYPE A SET X 1 DO 3 1 TYPE D DO 2 1 2 DO 2 2 1 TYPE C 2 2 IF X 2 5 2 6 2 7 2 5 TYPE H 2 6 TYPE I 2 7 TYPE J 2 8 TYPE K i 2 9 TYPE 2 01 X TYPE SET X X 1 3 1 TYPE B GOTO 5 1 TYPE F x 5 1 TYPE c 5 2 TYPE g 5 3 TYPE L sgo FOCAL types the answer ABCDGHIJK 1 GIJK 1 0 BCEL ON Command The ON command is a three branch conditional DO It functions similarly to the IF command actually uses some of IF s coding except that the line number or group is executed as in a DO command rather than as in a GOTO command Example ON X 2 1 5 3 4 D 6 CONTINUE 9 gt In this case If X lt line 2 1 is executed and then group 6 is executed control continues onto the next line If X group 5 is executed then group 6 If X gt line 3 4 is executed then group 6 In the ON command the line number may be an arithmetic expression but the arithmetic expression must begin with a vari able as in the GOTO command More about Symbols The value of a symbolic name or identifier is not changed until the expression to the right of the equal sign is evaluated by FOCAL Therefore before it is evaluated the value of a symbolic name or identifier can be changed by retyping the identifier and giving it a new value l SET Al 342 SET Al Altl TYPE 2 Al 19 Note Symbolic names or identifiers mus
14. d Keeping Track of the Decimal Point FOCAL results are accurate to 9 significant digits depends again on the implementation you are using As we have shown in the examples so far FOCAL assumes at the start that you want to see your results with 4 digits or spaces to the left of the decimal point and 4 digits to the right of the decimal point This is called fixed point notation You can change the output format within a type statement by typing X Y where X is the total number of digits to be output and Y is the number of digits to the right of the decimal point Both X and Y are positive integers equal to or less than 31 If Y is a single digit it must be preceded by For example 6 02 indicates six digits to the left and two to the right of the decimal point Correcting Mistakes If you should strike the wrong key you can delete it by striking the rubout key Each time you strike rubout another previously typed character will be deleted When you strike rubout FOCAL echoes back a backslash MX may vary with imple mentations to inform you that it has deleted the character You may erase everything back to the left margin by strik ing the backarrow key Summary of Part One In this first part you have learned how one FOCAL command type is used to evaluate expressions to type out character strings enclosed in quotes and to use symbols defined in SET commands in expressions In the second part you will
15. d SET TYPE WRITE Example SET X 23 3 2 TYPE X X X X 43 T lt TEXT gt WRITE W 3 29 Explanation Assign the value of 23 3 2 to X Type the values of X Xt2 and X 3 Type lt TEXT gt then carriage return line feed List the program List group 3
16. error message to show you that an illegal command was used An error code table is available at the beginning of each source listing To pinpoint an ur in line 3 2 for example type DO 3 3 and the progran will be traced until the error is found Using the Trace Feature The trace feature is useful in checking a program s opera tion Those parts oi ihe program which the user has enclosed in question marks will be printed out as they are executed In the cilonics examples parts of three lines are printed ERASE ALL 1 1 SET A 1 1 2 SET B 5 1 3 SET C 3 1 4 TYPE 2 A B C 1 5 TYPE B A C 21 6 TYPE 5 72 27 GO AtB C 3 B tA C 5 B C A 2 When only one is inserted the trace feature becomes operative as FOCAL encounters the during execution and the pro gram is printed out from that point until another is encountered The program may loop through the same until an error is en countered exec tlon stops and an error message is typed or until program completion ERASE ALL 1 1 SET A 0 TYPE 233 4 1 2 for B 1 1 4 TYPT E A G0 SET A 0 TYPE 3 A 1 TYPE B A 2 TYPE B A 3 TYPE B A ys In this example CAL encountered the as it entered line 1l 1l and ten cat ipe preenam 16 FOCAL Functions The functions are provided to improve and simplify arith metic capabilities and to give potential for expansion to addi tional input output devices A standard function cal
17. ing up program loops and iterations The general format is SFOR A B C D COMMAND S The identifier A is initialized to the value B then the commands following the semicolon are executed When the commands have been executed the value of A is incremented by C and con pared to the value of D If Ais less than or equal to D the commands after the semicolon are executed again This process is repeated until A is greater than D at which time FOCAL goes to the next sequential line The identifier A must be a single variable B C and D may be either expressions variables or numbers If the comma and the value C are omitted it is assumed that the increment is one If C D is omitted it is handled like a SET statement and no iteration is performed The computations involved in the FOR statement are done in floating point arithmetic and it may be necessary in some circum stances to account for this type of arithmetic computation Example 1 below is a simple example of how FOCAL executes a FOR command Example 2 shows the FOR command combined with a DO command 1 Example 1 ERASE ALL 1 1 SET A 189 1 2 FOR B 1 1 5 TYPE 5 82 N IS B A GO N 1S 191 098 IS 182 88 IS 103 00 15 194 00 1S 105 00 ZZZZ Example 2 1 1 FOR X 1 1 5 DO 2 6 1 2 GOTO 3 1 2 1 TYPE 1 SZ X X 2 2 SET A X 100 820 22 3 TYPE 95 92 ANA O E gt 101 00 102 08 103 29 y 104 90 dede e e PX
18. l consists of four or fewer letters beginning with the letter F and fol lowed by a parenthetical expression FABS A B 2 There are basic mathematical functions provided as part of the FOCAL system and extended functions which allow the user to do special operations like reading data from external storage devices Also FOCAL provides the mechanism for user written functions programmed in either FOCAL or machine language Some FOCAL functions are specific to the machine you are using or the implementation of the FOCAL system on that machine FOCAL functions are executed using the SET or the TYPE command T FINT Y S X FODV 1 A11 FOCAL functions return a value and the value can either be placed in a variable or typed out If the function is a mathe matic operation the value returned is the result of the operation With some non mathematic function such as FODV the value returned is superfluous and can be ignored oO Absolute Value The absolute value function FABS outputs the absolute or positive value of the number in parentheses TYPE FABS 66 66 TYPE FABS 23 23 TYPE FABS 99 99 Integer Integer part function FINT outputs the integer part of a number TYPE FINT 5 2 5 TYPE FINT 55 66 55 TYPE FINT 77 434 77 TYPE FINT 4 1 z y Integer Rounding The interer ro anaing function FINR keeps the integer part and round fv oa d 5 or larger to the
19. lash 1 for each rubout typed to erase one character to the left each time the rubout key is de pressed For example ERASE ALL Erases program text and variables 1 1 P TYPE X Y 21 2 SET S 13 X 13 WRITE 01 09 TYPE X Y 01 20 SET X 13 A line can be corrected by retyping the line number and typing the new command 14 99 SET C9 N 3 15 ve IS REPLACED BY TYPING 14 99 TYPE C9 Z5 2 WRITE 14 99 14 99 TYPE C9 25 2 ds MODIFY Command Frequently only a lew characters in a particular line re quire changing To facilitate this job and to eliminate the need to retype the entire line the FOCAL programmer may use the MODIFY command Thus in order to modify the characters in line 5 41 the 14 user types MODIFY 5 41 This command is terminated by a car riage return whereupon the program waits for the user to type an altmode character and then a search character FOCAL then proceeds to the first occurrence of that character Characters can now be inserted or deleted from that point If the user wishes to search for another character in another position in the line he types another altmode and the search character If the user types a line feed the remainder of the line will be accepted as is If the user types a carriage return all of the text from the current position to the end of the line will be deleted The ERASE ALL and MODIFY commands are generally used only in immediate mode because they
20. ng with the text 3 4 and then loads them into variables X and Y Notice that the input and output devices must be restored to the console once the operation is complete This is accomplished by using the R 0 and R I commands The former restores the out put device and the latter restores the input device One powerful and unusual feature available in FOCAL is the ability to execute FOCAL code contained within a string If the text contained within a string is a series of FOCAL commands the text can be executed using the DO command FOR Loops FOCAL now has the ability to exit from a FOR loop before the loop is completed This is accomplished using the RETURN command For example i 1 1 F X 1 18 1 X 6 1 3 1 2 G 6 1 1 3 T DONE R The FOR loop will execute until the IF statement sees that X has exceeded 6 The program then goes to line 1 3 where it encounters the RETURN The RETURN forces the FOR loop to finish and sends the program to line 1 2 FOR loops can now be stepped in both a positive and nega tive direction For example 1 1 F X 10 1 15 T X 18 bb hn Y E Mn Io wo Exponents FOCAL now understands negative exponents For example AT 34 674 12 24 Priority Interrupts FOCAL now has the ability to deal with hardware interrupts through the FPIC function The interrupting external device can be given a priority from 1 to 7 with 7 having the highest pri ority The priority levels c
21. orrespond to bits in a special byte the most significant bit corresponds with the lowest level of priority The user must provide machine language routines to set the bits that correspond with a specific external device FOCAL deals with interrupts by executing a DO of a FOCAL line number that is assigned to the interrupting device FOCAL will ignore the interrupt bits unless the specific bits are en abled with the FPIC function Example S FPIC 1 99 3 22 1 7 38 The arguments are arranged in pairs with the first argument belng the device priority bit and the second argument being the line to be executed if that device interrupts FOCAL looks for interrupts before each command FETCH so that in some routines the interrupts would not get serviced very quickly Whenever FOCAL returns from program execution to command mode all interrupts are disabled and must be re enabled during program execution Accuracy FOCAL now calculates floating point numbers to about nine digits of accuracy This means that FOCAL can deal with numbers as large as 999999999 Saving FOCAL Programs There are several methods of saving programs generated in FOCAL for later use Since they depend heavily on the type of mass storage device available the exact routines have been left to the user 1 Complete Memory Image The simplest method of saving a program is to copy the com plete core image of FOCAL and the text area onto the mass storage device If
22. r it outputs a prompting character usually an asterisk High Speed Calculations Using the TYPE Command You only need to learn one FOCAL command TYPE abbrevi ated T in order to do calculations such as the following 10 x 3 19 21 2 To do this with FOCAL you type T 1043 3 10 21 2 And after you press the return key 319 6000 FOCAL computes this result Every line must end with a return This example shows the arithmetic operations performed by FOCAL These are done from left to right except that exponentia tion 4 is done first then multiplication then division then addition or subtraction This means that T 6 642 is 5000 and not 2 8828 because multiplication is done before division in FOCAL Enclosures To make sure that the computer performs these operations in the order you want you can place them inside parenthesis marks When the computer sees an expression enclosed in paren theses it does that first If the statement includes parentheses within parentheses nesting it computes the innermost first 7 6 3 5 2 3 In this example the computer first computes the values of the expressions enclosed in parentheses 6 3 is 2 and 5 2 is 25 Then 7 2 25 3 66 6000 Another Command SET This useful command tells FOCAL Store this symbol and its numerical value When I use this symbol in an expression insert the numerical value RSET PIs3 141593 SET E
23. rding to the value of the expression in the 1F statement 2 1 TYPE LESS THAN ZERO QUIT 2 3 TYPE EQUAL TO ZERO QUIT 2 5 TYPE GREATER THAN ZERO QUIT RIF 25 25 2 1 2 3 2 5 EQUAL TO ZERO The IF statement may be shortened by terminating it with a semicolon or carriage return after the first or second line number If a semicolon follows the first line number the ex pression is tested and control is transferred to that line if the expression is less than zero If the expression is not less than zero the program continues with the next statement 2 20 IF X 1 8 TYPE Q in the above example when line 2 20 is executed if X is less than zero control is transferred to line 1 8 If not o is typed out 3 19 IF B 1 8 1 9 3 28 TYPE B In this example if B is less than zero control goes to line 1 8 If B is equal to zero control goes to line 1 9 IfB is greater than zero control goes to the next statement which in this case is line 3 28 and the value of B is typed out If a GOTO or an IF command is executed within a DO subroutine two actions are possible 1 If a GOTO or IF command transfers to a line inside the DO group the remaining command in that group will be executed as in any subroutine before returning to the command following the DO 2 If transfer is to a line outside the DO group that line is executed and control is returned to the command following the DO unless that line contains
24. return to command mode upon completion During command input the left arrow will delete the line numbers as well as the text if the left arrow is the rightmost character on the line Notice the errors in line 7 01 below 7 01 JACK AND BILL WNT UP THE HALL MODIFY 7 61 JACK AND B JILL WSXENT UP THE HA ILL WRITE 7 01 07 61 JACK AND JILL WENT UP THE HILL To modify line 7 01 a B was typed by the user to indicate the character to be changed FOCAL stopped typing when it en countered the search character B The user typed the rubout key to delete the B and then typed the correct letter J He then typed the alt mode keys followed by the the next character to be changed The rubout deleted the character and the user typed an E Again a search was made for an A character This was changed to I A line feed was typed to save the remainder of the line Error Detection in Indirect Statements When an error occurs in executing an indirect statement the error message is typed out when the statement is encountered during program execution In addition to the error code FOCAL types the line number containing the error as shown in the follow ing example 1 10 SET A 2 TYPE A A 1 20 SET B 4 TYPE B B 1 39 GOTO 1 81 21 40 TYPE A B A B GO A 2 0000 15 4 0900 2 3 1 30 vt ip FOCAL executes lines 1 1 and 1 2 and then recognizes that line 1 3 is an illegal command Therefore it issued the
25. send char acters until either the 100th character is sent or a carriage return is encountered in the string FSLK Allows all or part of one string to be compared with all or part of another string The function requires four arguments to define the start and end of the first string and the start and end of the second string S Z PSLK AS 1 A 5 BS 0 BS 50 In this example FOCAL will try to find the characters of the first string in the second string If a match is found Z will be set to the position number where the match began If no match is found Z is set to 1 One of the most powerful string functions in FOCAL allows strings to be set as input and output devices This enables text or data generated during program execution to be stored directly into a string It also enables data or text to be read directly into an executing program We ll go through an example step by step S FODV AS This sets A string as the output device Now all information that would normally go to the CRT or teletype is loaded into A string T HELLO Executing a TYPE command loads the text into the string This is the usual technique used to pre load a string S FSTO B Executing this function copies B string into A string Similar kinds of operations can be performed by setting a string as the input device For example 23 1 1 S FODV AS T 3 4 5R 0 41 2 S FIDV A A X Y R I This routine loads the stri
26. t not begin with the letter F The user may request FOCAL to type out all of the defined symbolic identifiers by typing a dollar sign in a TYPE command TYPE 6 05 The user s symbol table is typed out like this 1111 11000 3060851 301 00000 oon A 8 B 88 c 0 Subscripted Variables FOCAL allows variables to be further identified by sub scripts max range may depend on implementation positive and negative which are enclosed in parentheses immediately following the variable name A saleseript may also be an expression SET A1 1 3 5 22 713 CET 510K 3 J 2 79 10 The ability of FOCAL to compute subscripts is especially useful in generating arrays for complex programming problems An important difference between the FOCAL language and most other languages is that only the elements in an array that are actually used by the program are set aside in the computer s memory Thus no definition of array size is needed QUIT Command A QUIT command causes the program to halt and return con trol to the user FOCAL types an asterisk and the user may type another command COMMENT Command Beginning a command string with the letter C will cause the remainder of that line to be ignored so that comments may be inserted into the program Such lines will be skipped over when the program is executed but will be typed out by a WRITE command FOR Command This command is used for convenience in sett
27. ter FOCAL is an interactive language in that you communicate directly with it through the use of a keyboard input system and it reacts immediately to the commands you give it This allows for quick and easy creation testing and execu tion of programs This reduces the amount of human effort in volved in programming which in many cases represents the major expenditure in solving problems with computers Once the FOCAL system has been loaded and initialized on the computer that you are using procedures may vary with computer type then you are ready to interact with the computer through commands expressed in the FOCAL language Much of the rest of this document will be concerned with the structure and rules of the FOCAL language Some Fundamental Ideas The user communicates with the computer through the use of commands which are recognized by the FOCAL system and cause it to perform certain specific actions A FOCAL command must always begin with a command name which may be abbreviated The command name is then followed by other parameters upon which the command will operate Several commands may be placed on one line by separating them with the semicolon character The entire line is ended with the RETURN character CR on some keyboards The general format may be illustrated as COMMAND PARAMETER1 COMMAND2 PARAMETER COMMAND3 PARAMETER3 RETURN When the FOCAL system requires a line to be input by the use
28. the mass storage device is fast this is probably the easiest way to store a program If the mass storage device is slow this method is tedious and inefficient A slightly more efficient method involves saving a memory image of the text and the text pointers only Before the image is 25 saved the variable list should be deleted using the ERASE com mand The text pointers are on zero page All of the memory image between the labels PC and VAREND should be saved The text area follows the FOCAL interpreter The pointer on zero page labeled VARBEG indicates the end of the current FOCAL pro gram The text is stored between the label PBEG at the end of the interpreter and the address pointed to by the label VARBEG One disadvantage of these two methods is that a memory image cannot be loaded into a different version of FOCAL or even into the same version of FOCAL assembled at different addresses 2 Listing Save The most obvious method of saving a program is to have FOCAL send a listing to the mass storage device FOCAL can then read the text back in as though it were being typed into the key board This method has the advantage of being able to transfer the program between assemblies and versions of FOCAL The problem with this method is that after FOCAL sees the carriage return at the end of the text line it must do quite a bit of processing to store the line in the text area If one line of text immediately follows another
29. unction the default string length sets the string to 72 characters Once a string has been initialized its length cannot be changed FSTI This function allows characters read from the input device to be inserted into a string The function requires three argu ments the number of characters to be loaded the name of the string and string position and a terminating character S FSTI 1898 A 4 13 This statement instructs FOCAL to read characters into A string starting at character position number 4 The function will read characters into the string until either 100 characters are loaded or the character represented by the decimal ASCII value 13 is encountered In this case 13 is the decimal value for a carriage return If the keyboard is the input device the user could type characters into A string until either 100 characters are loaded or a carriage return is typed Since FOCAL can deal with strings either as characters or as numeric values a simple method has been provided to convert characters into their numeric values Preceding a character with a single quote returns the decimal value of that character 2 eT A 265 0060 S FSTI 109 A AF X 0 25 5 Y Atx FSTO This function allows part or all of a string to be sent to the output device It has the same format as the FSTI function For example S FSTO 180 A 4 13 will send characters from A string starting with position 4 It will continue to
30. vailable to the programmer for quick and easy debugging WRITE Command The WRITE command without an argument can be used to cause FOCAL to print out the entire indirect program allowing the user to visually check it for errors A group of line numbers or a specific line may be typed out with the WRITE command using arguments as shown below WRITE 2 08 FOCAL types all group 2 lines WRITE 2 1 FOCAL types line 2 1 WRITE FOCAL types all numbered lines ERASE Command A line or group of lines may be deleted by using the ERASE command with an argument For example to delete line 2 21 the user types ERASE 2 21 13 To delete all of the lines in group 2 the user types ERASE 2 9 Used alone without an argument the ERASE command causes FOCAL to earase the user s symbols and control remains in the same mode as the one in which the command was issued either the stored mode or the immediate mode In the stored mode the ERASE command must be the last command in the line Note that the command ERASE with an argument causes control to go to the im mediate mode Typing WRITE after making corrections causes FOCAL to print the indirect program as altered This is useful for check ing commands and for obtaining a clear program printout Corrections If the user types the wrong character or several wrong characters he can use the rubout key as we explained in Part One which echoes a backs
Download Pdf Manuals
Related Search