Digital PDFs

DEC-08-AJAA-D

March 1988

40 pages

Original

2.3MB

Document:	An Introduction to FOCAL Preliminary Programming Manual for PDP-8 FPODP-8/% PDP-8/1 March 1968
Order Number:	DEC-08-AJAA-D
Revision:
Pages:	40
Original Filename:	http://bitsavers.org/pdf/dec/pdp8/focal/DEC-08-AJAA-D_An_Introduction_to_FOCAL_196803.pdf

OCR Text

An Introduction to FOCAL
Preliminary Programming Manual
for
PDP-8
PDP-8/ S
PDP-8/ I
March 1968

Order No. DEC-08-AJAA-D from Program Library, Maynard, Mass.
Direct comments concerning this manual to Software Quality Control, Maynard, Mass.

DIGITAL EQUIPMENT CORPORATION• MAYNARD, MASSACHUSETTS

CONTENTS

CHAPTER l
AN INTRODUCTION TO FOCAL

l. l

General

1-1

l. 2

Equi Rment Requirements

1-1

1.3

Loading Procedure

1-1

1.4

Restart Procedure

1-2

l. 5

Saving FOCAL Programs

1-3
CHAPTER 2
FOCAL LANGUAGE

2.1

Arithmetic Operations

2-1

2.2

Outppt Formats

2-2

2.3

I den ti fi ers

2-3

2.4

Subscripted Variables

2-4

2.5

Indirect Commands

2-4

2.6

HandPng Text

2-5

2.7

Error Detection

2-6

2.8

Corrections

2-7
CHAPTER 3
FOCAL COMMANDS

3.1

TYPE

3-1

3.2

ASK

3-2

3.3

WRITE

3-2

3.4

SET

3-3

3.5

ERASE

3-3

3.6

3-3

3.7

GOTO

3-4

3.8

3-4

3.9

3-5

3.10

RETURN

3-6

3.11

QUIT

3-6

3. 12

COMMENT

3-6

3.13

FOR

3-6

iii

CONTENTS (cont)
3.14

MODIFY

3-8

3. 15

Using the Trace Feature

3-9

3.16

Functions

3-9
APPENDIX A
FOCAL COMMAND SUMMARY

A-1

APPENDIX B
ASK/TYPE CONTROL CHARACTERS

B-1

APPENDIX C
ERROR DIAGNOSTICS

C-1

APPENDIX D
ESTIMATING THE LENGTH OF USER'S PROGRAM

D-1

APPENDIX E
LIMITATIONS AND RESTRICTIONS OF PRELIMINARY VERSION

E-1

ILLUSTRATIONS

1-1

1-2

FOCAL Loading Procedure

CHAPTER l
AN INTRODUCTION TO FOCAL

l. l

GENERAL
FOCAL (for FOrmula CALculator) is an on-line, conversational, service program for the PDP-8

family of computers, designed to help scientists, engineers, and students solve numerical problems. The
language consists of short imperative English statements which are relatively easy to learn. Mathematical
expressions are typed, for the most part, in standard notation.

No previous programming experience is

needed either to understand this manual or to use FOCAL at the Teletype console.

However, the best

way to learn the FOCAL language is to sit at the Teletype and try the commands, starting with the examples given in this manual.

l. 2

EQUIPMENT REQUIREMENTS

4K PDP-8/1, PDP-8, or PDP-8/S computer with ASR 33 Teletype.

l .3

LOADING PROCEDURE
The Binary Loader is used to load FOCAL. Check to see if the Binary Loader is in core. If

not, refer to the Binary Loader manual (DEC-08-LBAA-D), and if needed, the RIM Loader manual
(DEC-08-LRAA-D).
The procedure for loading FOCAL is itemized be low.

Place the FOCAL binary tape in the tape reader.

Put 7777 (the starting address of the Binary Loader) in the SWITCH REGISTER.

Press the LOAD AD Dress key.

To use the high-speed tape reader, set bit 0 of the SWITCH REGISTER to 0.

Press the ST ART key.

5. The tape will stop loading at about midpoint because the program is loaded in two sections
for additional checksum protection. After the halt, the contents of the accumulator (AC) should be 0,
otherwise re load the previous section of tape. If the AC is 0, press the CONTINUE key and the tape
wi II continue loading.
6. Place 200 (the starting address of FOCAL) in the SWITCH REGISTER when the tape is
completely loaded.

Press the LOAD ADDress key.

Press the ST ART key.

9. FOCAL is correctly loaded and ready for user input when it types *?00. 00 followed by
an asterisk on the next line. If, FOCAL is incorrectly loaded, then *?02.16 is typed out. Reload the
FOCAL tape starting with step l above.
The FOCAL loading procedure is illustrated in the flowchart (Figure 1-1 ).

1-1

START

CHECK FOR RIM
BINARY LOADER IN
MEMORY

LOAD FOCAL T A P E - - - - - - - - - - - ,
IN READER

LOAD 7777 IN AC

YES

TELETYPE TO

SET BIT 0 IN SR TO 0

a TAPE READER

LINE
TO

START

'--------

PRESS START TO
READ IN TAPE

PRESS CONTINUE

LOAD 200 IN SR

PRESS START

FOCAL IS
FOR USER

READY
INPUT

Figure 1-1

1 .4

CHECKSUM

ERROR

FOCAL Loading Procedure

RESTART PROCEDURE
The two methods of restarting the system are outlined be low.
1.

Type the CTRL/C (control and C) keys simultaneously at any time.

Put 200 in the SWITCH REGISTER

Press the LOAD AD Dress key

Press the ST ART key

1-2

d. The program will then type *?00.00 indicating a manual restart, and an asterisk
on the next line indicating it is ready for user input.

1.5

SAVING FOCAL PROGRAMS
To save a FOCAL program, the user should
1.

Respond to * by typing WRITE A LL

Turn on tape punch

Type severa I @_;signs to get leader tape

Press RETURN (carriage reJ·urn) key

When the user's program has been typed and punched out
5.

Type several@_; signs to get trailer tape

Turn off tape punch

The user may now continue with another FOCAL program.
computer and waiting to operate on user input.

1-3

The FOCAL program is sti 11 in the

CHAPTER 2
FOCAL LANGUAGE

After being loaded, FOCAL types
*$?00. 00

*
The user may then type commands. Commands may be either direct or indirect.

Direct com-

mands sue h as
*TYPE 2-+4.
=+6*
where

/J indicates a carriage return and * indicates that FOCAL is ready to accept another command from

the user, are performed by FOCAL immediately. FOCAL types an equal sign, fol lowed by the result.
The number of digits in the result can be specified by the user. Scientific or floating-point
output may also be specified. FOCAL calculates to six significant digits.
300
+619. For example, to compute 9,999,999
the user would type

Exponents are computed to

*TYPE 9999999 t 300 ~
and FOCAL types
=+0.9571895E+567*
Several statements may be typed on a single line, separated by semicolons, like this:
*TYPE A+B+C; SET D=5; DO l . 21,

2. l

ARITHMETIC OPERATIONS
FOCAL performs the usual arithmetic operations of addition, subtraction, multiplication,

division, and exponentiation. These ore written by using the fol lowing symbols:
Moth Notation

FOCAL

t Exponentiation

3t3

*Multiplication

3.3

3*3

/Division

3+3

3/3

+Addition

3+3

- Subtraction

3-3

(Power must be positive
integer)

These operations may be combined into expressions. When FOCAL evaluates an expression,
which may include several arithmetic operations, the order of precedence is the same as in the list above.
That is, exponentiation is done first followed by multiplication and division followed by addition and·
subtraction. Expressions with the some precedence ore evaluated from left to right.

2-l

A+B*C+D is A+(B;'C)+D not (A+B)*(C+D) nor (A+B)*C+D
A*B+C*D is (A*B)+(C*D) not A*(B+C)*D nor (A*B+C)*D

X/2*Y is
2Y
Expressions are combinations of arithmetic operations or functions which may be reduced by
FOCAL to a single number.

Expressions may be enclosed in properly paired parentheses, square brackets,

and angle brackets (use the enc
enclosures).

of your c

FOCAL is impartial and treats them all merely as

For example,

*TYPE (A+B)* IS+QJ *<E+F>)
=+.157600E+20
Expressions may be nested.

FOCAL computes the value of nested expressions by doing the

innermost first and then working outward.

*TYPE <2+[)-(l*l)+~ +2>)
=+11*

2.2

OUTPUT FORMATS

One of the results shown above was typed by FOCAL in floating-point format.

This is one

format in which FOCAL produces output, but the format may be changed if the user types

TYPE %x.yy
where x is the total number of digits to be output, and yy is the number of digits to the right of the
decimal point. x and y are positive ini·egers, and x cannot exceed 19 digits.

For example, if the de-

sired output format is
mm.nn
the user may type

*TYPE %4.02, 12.22*2.37 ~
and FOCAL wi II type
=+28. 96*
Notice t·hat the format operator must be fo I lowed by a comma.
In the fol lowing examples, t·he number 67823 is typed out in several different formats.
Format Command

Result

%6.01

=+67823.0

=+67823

%8.03

= +67823. 000

2-2

To revert to floating-point format, the user types TYPE %, as shown be low.
*TYPE%, 67823'
= +o. 6782300E +05*
If the specified output format is too small to contain the number, FOCAL types Xs, as shown
below.
*TYPE %3, 67823~
=+XXX*

If the specified format is larger than the number, FOCAL inserts leading spaces.
*TYPE %7, 67823~
=+uu67823*
where the symbols WL-J indicate spaces.

2.3

IDENTIFIERS
Symbolic names consisting of one or two alphanumeric characters may be used in FOCAL com-

mands. The first character must be a letter; however, the first character must not be the letter F.
Identifiers may be defined by using the SET command.
*SET P1=3.14159; SET E=2.71828; SET Al=7~

*
These values are stored by FOCAL in its internal symbol table, and may then be used in expressions, as
shown be low .
*TYPE %4, Pl*21. l,2, 3071.2*E'
=+l 398=+8348*
The value of an identifier can be changed by retyping the identifier and giving it a new value.
*SET E=4.78, SET Al=3t2'1
*SET E=3. 37 ~

*
The user may request FOCAL to type out all of the identifiers he has defined, in the order of
definition, by typing
*TYPE

If FOCAL's symbol table contains the above defined symbols, FOCAL types
Pl (00)= +O. 314 l 590E+Ol
E@(00)=+0.3370000E+Ol

(E was redefined)

Al (00 )= +O. 9000000E +O l

2-3

If an identifier consists of only one

an @is inserted as a second character in the symbol table

printout, as shown in the second line

. An identifier moy

the first two wi II be

2.4

by FOCAL and

stored in

longer than two characters, but only

le.

symbol

PTED VARIA.BLES
FOCAL also a I lows i den ti fie rs, or variable symbo Is, l"cl be further identified by subscripts in

the range 0 through

which are enc

SET statement. A

ript may a

immediately following the i den ti fier in the

which must be reducible by FOCAL to two digits.

*SET A 1(i +3* J )'"2 .71; SET X ·1 (5+3* J )=2. 79
is especially useful in generating arrays for complex programming problems.

2.5

INDIRECT COM/\1\ANDS

If a Teletype ilne is prefixed by a line number, that line is not executed immediately, instead,
l for later execul"ion, usually as

it is stored by

must be in the range l. 01 to 15. 99.

of a sequence of commands.

The number to the lefl- of l·he

number to the right is called the

Line numbers

is called the group number; the

For example,

'"1. 01 SET A=l~

* l . 03 SET B=2"'
*1 .05 TYPE A+B;_

*
ng G

Indirect commands are execu'red

GOTO, or DO commands, which may be

direct or indirect.
The GO command causes

program.

L to go to ihe lowest numbernd line to begin executing the

If the user types a direct GO

aHer the indirect commands above, FOCAL will start

executing at line l. 01 .
command causes FOCAL to si·art the program by executing the command at a
specified line number.

user

"'GOTO l. 03 ~

*
FOCAL wi !I s1·art the program ot the second command in the example above.
DO command is used to tmnsfer control to a specified step, or group of steps, and then
return autornorl ca I ly to the forme sequence.

2-4

*1. 1 SET A=l; SET B=2JI.
*2. 1 TYPE A+B~
*2.2 SET C=3; SET B=4~
*3. 1 DO 2. 1~
*3.2 TYPE A-B~

*GOJJ
=+3=+5*
When the DO command in this sequence is reached, the command TYPE A+B is performed and then the
program returns to line 3.2.
The DO command can also cause FOCAL to jump to a group of commands and then return automatically to the normal sequence.

In the previous example, it the user changed line 3.1 to

*3. l DO 2.0
FOCAL will execute the commands at all lines numbered 2.n (i.e., lines 2.1 and 2.2) and then return
automatically to the command following the DO.
An indirect command can be inserted in a program by using the proper sequential line number.
For example,
*2. l SET A=l; SET B=21Ji
*3. l TYPE B/CJ/.
*2.2 SET C=l .31* .29#

*G01i
=+5*
where line number 2. 2 has been added and wi II be executed following line 2.1.

2.6

HANDLING TEXT
Text strings ore enclosed in quotation marks(".,.") and may include most Teletype printing

characters and spaces. The carriage return, line feed, and leader-trailer characters are not allowed in
text strings, but the exc lama ti on mark (!)causes a carriage return-line feed and the number sign (#)
causes a carriage return to be inserted as text.
Text strings may be included in TYPE and SET commands, as shown in the following examples.
*l .21 TYPE "VALUE IS ZERO'~
*l .22 SET A=l251Ji
*l .23 DO 1.21#
*l .24 TYPE

AiJ

*GOJJ
VALUE IS ZERO VALUE IS ZERO=+l 25*

2-5

When FOCAL encounters steps 1.21 and 1.23, VALUE IS ZERO is typed out. At step 1.24, = +125
is typed out. A carriage return may be used to terminate a text string; therefore, the fol lowing example
is equivalent to line 1 . 21 in the example above.

*l. 21 TYPE "VALUE IS ZEROJi
The following example shows how the exclamation mark is used to generate carriage return-line
feed sequences within a text string.

*l .2 TYPE "ALPHA! BETA! GAMMA!JJ
*DO 1

.2JJ

(FOCAL types)
ALPHA
BETA
GAMMA

*
To generate a carriage return within a text string, but without a line feed, the user inserts the
number sign (#). These operations are useful for formatting output.

2.7

ERROR DETECTION
FOCAL checks for a variety of errors, and if an error is detected, types a question mark fol-

lowed by an error code. A complete list of these error codes is shown in Appendix C. The group number
of an error message indicates the class or general type:

? 00

Manua I restart from console

?01

Interrupt from keyboard viatc.

?02

Excessive number or illegal mathematical operation.

?03

Miscellaneous

?04

Format errors

?05

Function not loaded

The WRITE command, without an argument, can be used to cause FOCAL to print out the entire
indirect program so the user can visually check it for errors.
The trace feature is also useful in program checking. Any part of an indirect statement or
program may be enclosed in question marks, and when that part of the program is executed that portion
surrounded by question marks will be printed out. If only one question mark is inserted, the trace feature
becomes operative, and the program is printed out from that point unti I completion, or unti I another
question mark i~. encountered.

2-6

2.8

CORRECTIONS
If the user types the wrong character, or several wrong characters, he can use the RUBOUT

key, which echoes a backslash(\) for each RUBOUT typed, to erase the wrong character(s). For
example,
*l .01 P\ TYPE X-Yi*l.02 SET $=13\\\ \

X=l3~

*
The left arrow (4-) erases everything which appears to its left, including the line number.
*l. l TYPE X-Y4A line can be overwritten by repeating the same line number and typing the new command.
*14.99 SET C9(N+3)=15;_
is replaced by typing
*14. 99 TYPE C9/Z5-2;_
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~

To delete all of the lines in group 2, the user types
*ERASE 2.0;
Used alone, without an argument, the ERASE command causes FOCAL to erase the user's entire symbol table. Since FOCAL does not zero memory when loaded, it is good practice to ERASE before
defining symbols. The command ERASE ALL erases al I user input.

2-7

CHAPTER 3
FOCAL COMMANDS

3.1

TYPE
The TYPE command is used to request that FOCAL compute and type out the result of an ex-

pression, the value of an identifier, or a text string.

The result of a TYPE command is numeric, in

floating-point format, or another format specified by the user.
*4.14 TYPE 8.1+3.2-(29.3*5}/2.5+7AJl
*4. 15 TYPE (2. 2+3. 5)*(7. 2/3)/59. l +3_,l

*
Several expressions may be computed in a single TYPE command, with commas separating each
expression.
*9.19 TYPE %3.3, Al*2, E+2+5, 2.51*81. l,,l
The output format may be included in the TYPE statement as shown in the example above and as explained
in Chapter 2.
The user may request a typeout of all identifiers which he has defined by typing
TYPE$
This causes FOCAL to type out the identifiers with their values, in the order in which they were defined.
The $may follow other statements in a TYPE command, but must be the last operation on the line.
*SET L=33;SET B=87;SET Y=55;SET C9=91~

*TYPE$~
L@(OO}== +O. 3300000E +02
B@(OO}== +O. 8700001 E+02
Y@(OO)= +O. 5499999E+02
C9( 00)== +O. 91 OOOOOE +02

*
A text string, enclosed in quotation marks, may be included in a TYPE command. A carriage
return may replace the terminating quotation mark.
*l .02 TYPE "X SQUARED==;
A text string or any FOCAL command or group of commands may not exceed the capacity of
a Teletype line, which is 72 characters on the ASR 33 Teletype. A line may not be continued on the
following line. To print out a longer text, each line must start with a TYPE command.
FOCAL does not automatically perform a carriage return after executing a TYPE command.
The user may insert a carriage return-line feed by typing an exclamation mark ( ! }. To insert a carriage

3-1

return without a line feed, the user types a number sign (#). Spaces may be inserted by enc losing
them in quotation marks. These operations are useful in formatting output.

3. 2

ASK
The ASK command is normally used in indirect commands to allow the user to input data at

specific points during the execution of his program. The ASK command is written in the general form,
* 11 . 99 ASK

x, y z
I

When step 11. 99 is encountered by FOCAL, it types a colon; the user then types a value or expression for
the first identifier, followed by a comma or a space; FOCAL then types another colon and the user types
a value for the second identifier. This continues until all the identifiers or variables in the ASK statement have been given values.
*11.99 ASK X, Y,X~
*DO 11 . 99/).

:5, :4, :3

*
where the user typed 5, 4, 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.
*l .10 ASK "HOW MANY APPLES DO YOU HAVE?" APPLESJ}.
*DO 1.10)
HOW MANY APPLES DO YOU HAVE?:25#

(user typed 25)

*
The identifier AP now has the value 25.

3.3

WRITE
The WRITE command, without an argument, causes FOCAL to write out all indirect statements

which the user has typed.

Indirect commands are those preceded by a line number.

A group of line numbers, or a specific line, may be typed out with the WRITE command using
arguments, as shown be low.
*7.97 WRITE 2.0~

(FOCAL types all group 2 lines)

*7. 98 WRITE 2. l~

(FOCAL types line 2. l)

*7. 99 WRITE"

(FOCAL types all numbered lines)

3-2

3 .4

SET
The SET command is used to define identifiers. When FOCAL executes a SET command, the

identifier and its value is stored in the user's symbol table, and that value will be substititued for the
identifier when the identifier is encountered in the program.
*3.4 SET A=2.55; SET B=8.05*
*3. 5 TYPE A+B).

*GO)
= +O. l 060000E +02*
An identifier may be set equal to previously defined identifiers, which may be used in arithmetic expressions.
*3 .7 SET G=(A+B)*2. 2+5*

3.5

ERASE
The ERASE command, without an argument, is used to delete all identifiers, with their values,

from the symbo I table.

If the ERASE command is followed by a group number or a specific line number, a group of
lines

or a specific line is deleted from the program.
*ERASE 2.0).

(deletes all group 2 lines)

*ERASE7.ll*

{deletes line7.ll)

The ERASE ALL command erases all the user's input
In the following example, an ERASE command is used to delete line 1.50.
*l. 20 SET B=2

Ji,

*l .30 SET C=4i),
* l .40 TYPE B+e;_
*l .50 TYPE B+C~
*ERASE l.50JJ
*WRITE ALLJ}
01. 20 SET B=2
01 .30 SET C=4
01 .40 TYPE B+C

*
3.6

GO
The GO command requests that FOCAL execute the program which starts with the lowest num-

bered line.

The remainder of the program wi 11 be executed in line number sequence.

be in the range 1.01 to 15.99.
3-3

Line numbers must

3.7

GOTO
The GOTO command causes FOCAL to transfer coniTOI to a specific line in the indirect pro-

gram.

It must be followed by a specific line number. After executing the command at the specified

line, FOCAL continues to the next higher line number, executing the program sequentially.
RETURN is encountered, control is returned to the statement fol lowing the GOTO.

If a

(See FOR and DO

for other GOTO operations.)
*GOTO 1.01

3.8

DO
The DO command transfers control to a single line, a group of lines, or the entire indirect

program.

If transfer is made to a single line, the statements on that line are executed, and then control

is automatically transferred back to the statement fol lowing the DO command, as shown in this example.
*l. l TYPE "X''AJ

*1.2 DO 2.3; TYPE "Y"~
*1 .3 TYPE "Z'~
*2.3 TYPE "A'~

*GOJ)
XAYZA

*
If a DO command transfers control to a group of lines, FOCAL executes the group sequentially,
and then returns control to the statement fol lowing the DO command.
If the DO command is written without an argument, FOCAL executes the entire indirect program, iust as it would for a GO command.
DO commands cause specified portions of the indirect program to be executed as closed subroutines.

These subroutines may be terminated by a RETURN statement.
When a GOTO or IF command is executed within a DO subroutine, control is transferred to the

line specified by the GOTO or IF command and that line is executed.

If the line is in the group specified

in the DO command, any fol lowing lines in that group are executed and then control is returned to the
statement following the DO.

If, however, the GOTO command within a DO subroutine transfers control

to a line outside the DO subroutine, FOCAL executes only the line specified by the GOTO command
and returns control to the statement fo II owing the DO.
In the following example, the DO statement in line 1.20 makes a subroutine out of line l .10,
so that ANOTHER LOOP is printed unti I A is greater than 5.

3-4

*l. 10 TYPE "ANOTHER LOOP"!)
*l .20 FOR A=l, 1,5; DO 1.10,l.
*DO 1.20~
ANOTHER LOOP
ANOTHER LOOP
ANOTHER LOOP
ANOTHER LOOP
ANOTHER LOOP
ANOTHER LOOP

*
A simple example of how FOCAL executes a DO command follows.
*l .10 TYPE %5.2, 5/2+6*3+2,l.
*DO 1. 1 O,l.
=+ ...... 56.49

*
3.9

J.E..
In order to provide for transfer of control after a comparison, we have adopted from FORTRAN

the IF statement format. The normal form of the IF statement consists of the word IF, a space, a parenthesized expression, and three line numbers separated by commas, in order. The expression is evaluated;
then 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 program below transfers control to line number 2.10, 2.30, or 2.50, according to the
value of the expression in the IF statement.
*2.10 TYPE "LESS THAN ZERO"; QUITI,
*2.30 TYPE "EQUAL TO ZERO"; QUIT,,
*2.50 TYPE "GREATER THAN ZERO"; QUIT*
*IF (25-25)2.10,2.30,2.50~
EQUAL TO ZERO

*
The IF statement may be shortened by terminating with a semicolon or carriage return after the
first or second line number.

If a semicolon follows the first line number, the expression is tested and con-

trol 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"
3-5

In this example, when line 2.20 is executed, if Xis iess than zero, control is transferred to
line 1.8. If not, Q is typed ouL
3. 19 ! F (B) l . 8, 1 . 9

3. 20 TYPE B
In this example, if Bis less than zero, control goes to line 1.8. If Bis equal to zero, control
goes to line 1.9.

If Bis greater than zero, control goes to the next stat·ement, which in this case is line

3.20, and the value of Bis typed.
3.10

RETURN
The RETURN command is used to exit from a DO, GOTO, or GO subrout-ine. When control

is under a group subroutine and a RETURN command is encountered, the program exits from its subroutine

status and returns to the line number fol lowing the DO or GOTO command that initiated the subroutine
status.

3.11

QUIT
A QUIT command causes the program to halt and return control to the user. FOCAL types an

asterisk and the user may type another command.

3. 12

COMMENT
Beginning a command string with the leHer C wi 11 cause the remainder of i·hat line to be ignored

so that comments may be inserted into the program.

Such lines wi 11 be skipped over when the program is

executed, but wi 11 be typed out by a WRITE command.

3.13

FOR
This command is used for convenience in setting up program loops and iterations.

The general

format is

FOR A=B, C, D; (command)
The identifier A is initialized to the value B, then the command string following the semicolon is executed.
When the command has been executed, the value of A is incremented by C and compared to the va
D.

If A is less than or equal to D, the command string after the semicolon is executed again. This pro-

cess is repeated unti I A is greater than D, and then FOCAL goes to the next sequential iine.
The identifier A must be a single variable.
iables, or numbers.

B, C, and D may all be either expressions, var-

If comma and the value Care omitted, then it is assumed that the increment is one.

If C, Dis omitted, this is handled iike a SET statement and no iteration is performed.

3-6

The computations involved in the FOR statement are done in floating-point arithmetic, and
it may be necessary, in some circumstances, to account for this type of arithmetic computation.
In the two examples below, the FOR command is combined with a DO command.
Example l:
*1.lOFORX= 1,1,5;D02.0j
*2.lOTYPE!

01o3, 11x II x

*2.20 SET A = X +100.000 ~
*2.30TYPE!
*ERASE

/o5.2

*GO~

x =+
A=+ 100.99

x =+

A=+ 101 .99

x =+

A:;:+ 102.99

x :::+

A=+ 104.00

x =+

A=+ 105.00

x =+

A=+ 105. 99

Example 2:
*l .10 FOR A=lO, 10,80;DO 2'#
*2.lOTYPE ! "IFA"%3A;TYPE%5 "THENASQUARED 11 A+2,
*ERASE'#

*GO~
IF A=+ l 0 THEN A SQUARED=+
IF A=+ 20 THEN A SQUARED=+
IF A=+ 30 THEN A SQUARED=+
IF A=+ 40 THEN A SQUARED=+
IF A=+ 50 THEN A SQUARED=+
IF A=+ 60 THEN A SQUARED=+
IF A=+ 70 THEN A SQUARED=+
IF A:;:+ 80 THEN A SQUARED=+

l 00
400
899
1600
2500
3600
4899
6400

3-7

3.14

MODIFY
Frequently, a few characters in a particular line require changes. To facilitate this job, and

to eliminate the need to replace the entire line, the user may use the MODIFY command. Thus, to
modify characters in a line, one would type MODIFY 5.41 in order to modify the characters of line 5.41.
This command is terminated by a carriage return whereupon the program waits for the user to type that
character at which he wishes to make changes or additions.

This character is not printed. After he has

typed the search character, the program types out the contents of that line until the search character is
typed.
At this point, the user has seven options:
1.

To type in new characters in addition to the ones that have already been typed out.

2. To type a form-feed; this will cause the search to proceed to the next occurrence, if
any, of the search character.
3. To type a CTRL/BELL; this allows the user to change the search character just as he did
when first beginning to use the MODIFY command.
4.

To use the RUBOUT key to delete ane character to the left.

To type a left arrow(....-) to delete the line over to the left margin.

To type a carriage return to terminate the line at that point; the text to the right wi II be

To type a LINE FEED to save the remainder of the line.

removed.

The ERASE ALL and MODIFY commands are generally used only in immediate mode since they
return to command mode upon completion. The reason for this is that internal pointers may be changed
by these commands.
During command input, the left arrow wi II delete the line numbers as we II 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 Bl LL W$NT UP THE HALL,}
*MODIFY 7. 01_,}
JACK AND B\JILL W$\ENT UP THE HA \ILL_,}
*WRITE 7. 01_,}
7. 01 JACK AND JILL WENT UP THE HI LL

*
To modify line 7.01, a B was typed by the user to indicate the character to be changed.

FOCAL

stopped typing when it encountered 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 CTRL/BELL keys followed by the $, the

next character to be changed. The RUBOUT deleted the $ character. Again a search was made for
an A charact€r. This was changed to I the I ine was typed out correctly.

3-8

CAUTION
When the MODIFY command is used to edit any part
of the user's stored program, the user's symbol table
is erased. If the user defines his symbols by means of
indirect commands prior to their use in his program,
this will cause no difficulty, as symbols are entered
in the symbol table when the statements defining them
are executed.
If the user has defined symbols in direct statements,
and then uses a MODIFY command, his symbol table
entries wi 11 be erased, and must be redefined.

3.15

USING THE TRACE FEATURE
As explained in Chapter 2, the trace feature is useful in checking an operating program.

Those parts of the program which the user has enclosed in question marks wi 11 be printed out as they are
executed.
In the following example, parts of 3 lines are printed.

*l. l SET A=l*
*l .2 SET B=5*
*l .3 SET C=3*
*l.4TYPE%1, ?A+B-C?!~

*l .5 TYPE ?B+A/C? !~
*l .6 TYPE ?B-C/A?~

*GOJ}
A+B-C=+3
B+A/C=+5
B-C/A=+2

*
3.16

FUNCTIONS
The functions are provided to give extended arithmetic capabilities and to give the potential

for expansion to additional input/output devices. There are three basic types of functions of which two
types are included in the basic FOCAL program.

The first type contains interger part, sign part, and

absolute value functions.
A function call always consists of four letters beginning with the letter F and followed by a
parenthetical expression:
FSGN(A-B*2)

3-9

The extended arithmetic functions are loaded at the option of the user. They will consume
approximately 800 locations of his program storage area. These arithmetic functions are adapted from the
extended arithmetic functions of the three-word floating-point package and are fully described with their
limitations in the pertinent document.
An unorthodox and greatly skewed distribution is provided in the basic package for a random
number generator (FRAN).

It uses the FOCAL program itself as a table of random numbers. An expanded

version cou Id incorporate the random number generator from the DEC US library.

Fo Ilowing are examples

of the five functions now available.
*TYPE FSGN(4-6),/
=-1

The SGN function outputs the sign part (+or-) of a number
and the integer part becomes a 1.

*TYPE FSGN(4-4)~
=+l
*TYPE FSGN(-7)

=-1

*
*TYPE %.2FSQT(4)~
=+02

The SQT function computes the square root of the expression
within the parentheses.

*TYPE FSQT(9) ~
=+03
*TYPE FSQT(l44)~
=+12

*
*TYPE FABS(-66)~
=+66

The ABS function outputs the absolute or positive value of the
number in parentheses.

*TYPE FABS(-23) ~
=+23
*TYPE FABS(-99)/)
=+99

3-10

*TYPE FITR(5.2)~

The ITR function outputs the integer part of a number.

=+05
*TYPE Fl TR(55. 66 )_,I
=+55
*TYPE Fl TR(77 .434)"
~+77

*TYPE FITR(-4. l )~
+-05

*
*TYPE% FRAN(

)JI

=-0. 2500000E +00

The RAN function generates a random number.
are typed in floating-point format.)

*TYPE FRAN( )~
=-0.6235351E+OO

*
Future versions of FOCAL will also perform the following functions:
FSIN

Sine

FCOS

Cosine

FATN

Arctangent

FLOG
FEXP

Logarithm (log )
e
Exponent

FADC

Analog-to-Digital

FDIS

Displays y coordinate on scope and intensifies the x-y point.

F DXS

Displays x coordi note on scope

FNEW

Available as a user-defined function

3-11

(The examples

APPENDIX A
FOCAL COMMAND SUMMARY

Command

Abbreviation

Example of Form

Explanation

ASK

ASK X, Y,Z

FOCAL types a colon for each variable;
the user types a value to define each
variable.

COMMENT

If a line begins with the letter C, the
remainder of the line wi II be ignored.

CONTINUE

Dummy lines

D04.l

Execute line 4.1; return to former sequence.

D04.0

Execute all group 4 lines; return to former line number sequence, or when a
RETURN is encountered.

ERASE

Erases the symbol table.

ERASE 2. 0

Erases all group 2 lines.

ERASE 2. l

Deletes line 2. l.

ERASE ALL

Deletes all user input.

FOR i=x,y,z;TYPE i

Where i is a symbol defined iteratively
and the command fol lowing is executed
at each new value.

ERASE

FOR

x=initial value of i
y=va lue added to i unti I i is greater
than z.
GO

Starts indirect program at lowest numbered line number.

GOTO

G3.4

Starts indirect program or transfers control to line 3.4. Must have argument.

IF (X)ln, In, In;

Where Xis a defined identifier, a value,
or the result of an expression, fol lowed
by three line numbers.

If X is less than zero, control is transferred to the first line number
If Xis equal to zero, control is to the
second line number.

If Xis greater than zero, control is to
the third line number.
MODIFY

MODIFY 1.15

Enables editing of any character on line
l .15.

QUIT

Returns contro I to the user.

A-1

Command

Abbreviation

Example of Form

Explanation

RETURN

Terminates GOTO subroutines, returning to next line number in the original
sequence.

SET

SET A=5/B*C;

Defines identifiers in the symbol table.

TYPE

TYPE A+B-C;

Evaluates expression and types out =
and result in current output format.

TYPE A-B, C/E;

Computes and types each expression
separated by commas.

TYPE "TEXT STRING"

Types text. May be fo I lowed by ! to
generate carriage return-Ii ne feed, or
# to generate carriage return.

WRITE

FOCAL types out the entire indirect
program.

WRITE 1.0

FOCAL types out all group 1 lines.

WRITE 1.1

FOCAL types out line l. l.

TYPE %x.y

where xis the total number of digits,
and y is the number of digits to the right
of the decimal point.

TYPE %6. 3 1 23. 456

FOCAL types: =+123 .456

TYPE%

Resets output format to floating point.

TYPE $;

Other statements may not follow on this
line

WRITE

FOCAL Operations
To set output format,

To type symbol table,

Modify Operations
After a MODIFY command, the user types a search character, and FOCAL types out the contents of that line unti I the search character is typed. The user may then perform any of the following
optiona I operations.
1.

Type in new characters. FOCAL wi 11 add these to the line at the point of insertion.

Type a FORM FEED. FOCAL will proceed to the next occurrence of the search character.

Type a CTRL/BELL. After this, the user may change the search character.

4. Type RUBOUT. This deletes characters to the left, one character for each time the user
strikes the RUBOUT key.
5. Type 4 - . Deletes the line over to the left margin, but not the line number when 4-is not the last character on the line.

Type carriage return. Terminates the line, deleting characters over to the right margin.

7. Type LINE FEED. This saves the remainder of the line from the point at which LINE FEED
is typed over to the right margin.
A-2

Summary of Functions
Square Root

F SQT(x)

where x is a positive number or expression greater than zero.

Abso Iute Va Iue

FABS(x)

FOCAL ignores the sign of x.

Sign Part

FSGN(x)

FOCAL evaluates the sign part only,
with l. 0000 as integer.

Integer Part

FITR(x)

FOCAL operates on the integer part of
x, ignoring any fractional part.

Random Number
Generator

FRAN

FOCAL generates a random number.

*Exponential Function (ex)

FEXP(x)

FOCAL generates e to the power x.
(2 .7l 828X)

*Sine

FSI N(x)

FOCAL gene rates the sine of x.

*Cosine

FCOS(x)

FOCAL generates the cosine of x.

*Arctangent

FATN(x)

FOCAL generates the arctangent of x.

*Logarithm

F LOG(x)

*Ana log-to-Di gita I

FADC

FOCAL generates log (x).
e
FOCAL reads from an analog-to-digital
channel, the value of the function is that
integer reading.

Scope Functions
*FDIS

Displays y coordinate on scope and
intensifies x-y point.

*FDXS

Displays x coordinate on scope.

*Not avai Iable in pre Ii mi nary version of FOCAL.

A-3

APPENDIX B
ASK/TYPE CONTROL CHARACTERS

Format delimiter

Text de Ii miter
Carriage return and line feed

Carriage return only

Type the symbo I tab le contents

SPACE

Terminator for names
Terminator for expressions
Terminator for commands

"(Carriage Return)

Terminator for lines

B-1

APPENDIX C
ERROR DIAGNOSTICS
~

I
I
[

I
I
I
I
I

I
I
I
I
I
I

ERROR DIAG!'-.lOSTICS OF FOCAL., 1968
*?00.00-MANUAL START FROM CONSOLE
*?01 .00-iNTERRUPT FROM KEYBOARD VIA CONTROL-C
*?02. 28-GROUP NUMBER OR LITERAL TOO LARGE
*?02.;0-I LLEGAL STEP NUMBER
*?02.46-NONEX!STANT LINE REFERENCED BY' DO'
*?02. 61-NONEXISTANT GROUP REFERENCED BY' DO'
*?02.07-BAD LINE NUtv\BER FORMAT
*?02.29-ILLEGAL COMMAND USED
*?02.67-BAD ARGUEMENT FOR 'MODIFY'
*?02.;7-ILLEGAL OR MISSPELLED FUNCTION NAME
*?02.44·-UNE NUMBER LARGER THAN ALLOWED
*?02. 24-KEYBOA.RD-I NPUT BUFFER OVERFLOW
*?02.87-COMMAND/INPUT BUFFER EXCEEDED
*?02.46-iMAGINARY SQUARE ROOTS
*?02.80-DIVISION BY ZERO
*?02.;3-NUMBER TOO LARGE TO BE MADE AN INTEGER
*?03.50-!MPROPER STEP NUMBER
*?03.79-VARIABLE STORAGE EXCEEDED
*?03.10-BAD ARGUEMENT FOR 'ERASE'
*?03.79-EXPONENT NOT A POSITIVE INTEGER
*?03.42-LOG OF ZERO REQUESTED
*?04.93-DOUBLE PERIODS IN AUNE NUMBER
*?04.:2-MULTIPLE PERIODS It'~ A LINE !'-~UMBER
*?04.12-BAD ARGUEMENT IN '!F'COMMAND
*?04.39-ERROR TO LEFT OF EQUALS SIGN
*?04.53-EXCESS RIGHT PARENTHESIS
*?04.61-ILLEGAL CHARACTER IN 'FOR'
*?04.18-BADARG IN 'FOR', 'SET', OR'ASK'
*?04.13-MISS!NG OPERATOR IN AN EXPRESSION
*?04. 33-0PERATOR MISSING BEFORE PARENTHESIS
*?04.;0-FUNCTIOl'-l NOT FOLLOWED IMMEDIATELY BY PARENS
*?04.;9-DOUBLE OPERATORS IN EXPRESSION
*?04.45-PARENTHESES DO NOT MATCH
*?04.13-!LLEGAL E-FORMAT ON INPUT OR LITERAL
*?05.60-ERROR IN 'FOR' COMMAND FORMAT
*?05.11-NO ARGUEMENT IN 'IF' COMMAND
*?05.;6-FUl'-ICTION NOT LOADED INTO CORE
*?05.28-COMMAND NOT AVAILABLE

I
I
I

I
I
I
I

I
I
I

I
I

I
I
I

I
I
I
I

_ _J

CUT ALONG DOTTED LINE AND POST NEAR THE KEYBOARD.

C-1

APPENDIX D
ESTIMATING THE LENGTH OF USER'S PROGRAM

In 4K systems, after FOCAL is loaded, approximately 17008 locations are available for storage
of the user's indirect program and the user's symbol table.
FOCAL requires five words for each identifier stored in the symbol table, and one word for
each two characters of stored program. This may be calculated by
5s + ~
where

· 1 . 01 = length of user's program

s =Number of identifiers defined
c =Number of characters in indirect program
If the total of program area and symbol table area become too large, FOCAL types the error

message
?02.80
When other systems are loaded you may get a ? 03. 80 error message.
FOCAL occupies core locations 1-33008 and 5300 8 -7576 8 . This leaves approximately l 000 10
locations for the user's program (indirect program, identifiers, and push-down list).

D-1

APPENDIX E
ERRATA AND LIMITATIONS

ERRATA
Chapter 1
Page 1-1, section 1 • 3, step 9 shou Id be changed to read:
9. FOCAL is correctly loaded and ready for user input when it types an asterisk.
is incorrectly loaded, reload the FOCAL tape starting with step 1 above.
Page 1-2, Figure 1-1, the last decision symbol shou Id be changed to read:

Chapter 2
Page 2-1, first paragraph should be changed to read:
After being loaded, FOCAL types

*
Page 2-1, third paragraph, fourth sentence should be changed to read:
For example, to compute 300 factorial, the user would type:
*1.1 SET A=li>
* 1.2 FOR 1=1, 300; SET A=A* I~
*1.3 TYPE%, A*
*GO~

and FOCAL types
=+0.306051E+615*
Chapter 3
Page 3-11, tenth line from bottom of page should be changed to read:
FOCAL will also perform the following functions:

Appendix A
Page A-3, the footnote shou Id be changed to read:
*These are called extended functions.
E-1

If FOCAL

Appendix D
Page D-1, third paragraph shou Id be changed to read:
If the total of ••• types the error message
?03.79
The next sentence shou Id be omitted.

LIMITATIONS
1.

Identifiers must not begin with the letter F. Result, error diagnostic code ?04.16.

2. Subscripts over 99 should not be used.

Example:

*SET A(lOl) = 2~
*TYPE

$iJ

A@(:) =+O. 2000000E+O l *
A subscript over 99 will be altered to some random character by the system.
3. When the MODIFY command is used, all identifiers previously set must be reset, as the
command erases the symbol table contents.

E-2