S P E C TR UM T URT L E GR A P H IC S
by
Eri c Dee son and Kei th Jammer
A guide to the AVC program
'turtle'
for the Sinclair ZX Spectrum micro
(c) July 1982
Revised Edition October 1982
AVC SOFTWARE
PO BOX 415
HARBORNE
BIRMINGHAM 617 9TT
" t u r t l e " a n d " t u r t l e 2 "
These notes apply to low resolution program "turtle". If you have “turtle 2”, please refer also
to the insert sheet.
If you have "turtle" you may obtain "turtle 2" on cassette with the insert sheet for £2.50
(overseas £3.00), a reduction of £1.00 on the standard price.
S P E C T R U M T U R T L E G R A P H I C S
by
E r i c D e e s o n a n d K e i t h J a m m e r
A gu i d e t o th e AVC p r o g ram
' tu r tl e '
for the Sinclair ZX Spectrum micro
CONTENTS
Introduction Page 2
The structure of the system 3
Command tables
Sample routine
The main menu 4
The commands
The demonstration routine 8
NOTE
The cassette program, the sample 'turtle' program, and the contents of this booklet are all
copyright. The user may not allow the duplication in any form of any part of this package,
amended or otherwise, for distribution outside his/her home or place of work.
1
INTRODUCTION
AVC's 'turtle' for the 16K ZX Spectrum, although fairly versatile, does not claim to be a full
implementation of LOGO. In being less than complete, however, it is in a number of ways better
constructed than true LOGO and is very easy to use in a wide variety of contexts.
There are two main ways in which this version is below par.
1. It generally uses low resolution graphics. This is partly due to the Spectrum's limitations on
mixing PRINT and PLOT, and partly due to the need to keep it of a length suitable for the 16K
micro.
2. It may not be fully crash-proof, though we hope to have made it so. Again memory restrictions
are relevant.
We are, however, working on a more sophisticated version for the 48K machine.
With all their learning software, it is AVC's policy to make the material as user-friendly as possible.
LOGO and 'turtle' are systems rather than casual programs, however, and need teacher guidance
for most effective development. This booklet is a guide for the teacher or parent rather than for
the young explorer.
AVC's user-friendliness policy also implies that the actual user should have only minimal
knowledge of computers and computing. He or she will need only to know the following:
1. How to use DELETE to correct entries prompted by "L".
2. To use ENTER (E) after such responses.
3. How to obtain decimal point and the symbol '*' used with procedures.
4. How to react to a Spectrum crash if obtained -
GO TO 50 (E) for a complete re-start;
GO TO 70 (E) for return to main menu.
5. How to use tape-recorder and printer if necessary.
Also he should never enter upper case mode and must distinguish between '0' and 'O' on
keyboard and screen. (You may need upper case and other symbols when recording.) All other
requirements should be clear in context with the indirect help of these notes.
Children (and adults) of any age, ability and background will be able to learn a great deal with
unguided use of 'turtle'. (All that is required is the concept of the written word and some sense of
order.) However teachers and parents hoping for full benefit are strongly recommended to read
about LOGO. Articles appear occasionally in the Times' Educational Supplement and such
professional magazines as Computers in Schools and Microscope. The concept is discussed
discursively in Seymour Papert's book Mindstorms, while the Atari Pilot manual Picture this! by
David Thornburg provides a wealth of practical ideas.
2
THE STRUCTURE OF THE SYSTEM
This version of turtle graphics LOGO offers a main menu with ten options, and 'turtle' routines
with these features:
* up to sixty statements ('lines").
* up to five procedures ("sub-routines").
* loops.
* in all, sixteen statement types.
Turtle routines may be edited at any stage, there being special error messages as well as the
Sinclair ones (which should be rare). At any stage, too, the current routine may be saved on
cassette and/or printed out, in the latter case with a copy of the screen display it produces.
The syntax of 'turtle' is fairly straightforward. Almost all statements consist of a four-letter
command word in lower case followed by up to three decimal digits.
The following tables list the commands and summarise their syntax; more detailed notes appear
later. Numbers ('n') refer to the low resolution 22 x 32 screen where applicable.
TA B L E 1 T H E N I N E C L A S S E S O F C O M M A N D
Feature Command(s)
colour penc
control rept use*
draw, low resolution draw wipe
draw, high resolution bend
label *
move draw jump move wipe
sound hoot (and all active commands)
termination endp endr end* stop
turn left turn
TA B L E 2 T H E S I X T E E N C O M M A N D S
bendn A circle of radius n is drawn centred on the turtle
drawn Turtle moves in direction faced a distance of n steps, or to edge of screen, leaving a
trail
endx End of 'turtle' routine section, namely
endp end of main routine
endr end of repeat loop
end*n end of procedure n
hoot n Produces a set of n random notes
jumpn Turtle ('frog'?) jumps in direction faced a distance of n steps, or to edge of screen
left n Turtle turns n degrees anti-clockwise (90, 180, 270 only)
moven Turtle moves in direction faced a distance of n steps, or to edge of screen, leaving
no trail
pencn Specifies the colour (n) of the next 'bend' or 'draw' action
rept n The following statements, up to 'endr', will be executed n times
stop Last significant line of whole program
turn n Turtle turns clockwise by n degrees (90, 180, 270)
use*n Calls the procedure n (1 to 5)
wipen Turtle moves in direction faced a distance of n steps, or as far as screen edge, leaving
no trail and clearing its path
*n Start of procedure n
3
The following screen display was obtained using the 'turtle' routine listed below it and option 8 on
the main menu. You may also care to consider the demonstration program on the cassette; see
later for details about this.
THE MAIN MENU
This appears
(a) when 'turtle' is first loaded (following a two-second delay);
(b) following ENTER after any occasion when 'stop' is encountered;
(c) following ENTER after a 'turtle' error report, except when in practice mode;
(d) following ENTER after 'finish' is entered from the menu;
(e) following GO TO 50 (E) or GO TO 70 (E) after BREAK or a crash.
4
It looks like this:
Select the option desired by pressing the corresponding number key. ENTER is not required.
PRACTICE This is the immediate mode (mode 0). Starting at the centre of the screen, the
turtle will follow each valid entered statement, printing that statement at the top of the screen.
An invalid command leads to an error message and/or it will be ignored. Entries may be
corrected and each requires (E) for execution.
It is not meaningful to enter the following while in mode 0: '*', 'reptn' or any 'end'. Entry of
'use*n' will work if that procedure already resides in 'turtle' memory. The entry of 'stop' (or 's')
will lead to exit from mode 0, and return to menu follows a further (E).
Instruction sets may be tested at any time in immediate mode. However its use will displace
your first line; restore it with 'edit'.
LOAD This allows the line-by-line entry of 'turtle' routines. Line numbers are
presented automatically; inputs are confined to seven characters but are not checked other
than for 'stop'.
Because there is no re-number facility, experienced users follow each 'end' with a few dummy
lines to allow later insertion of extra commands. When 'stop' is met, line entry ceases, and (E)
causes return to menu. Note that use of the LOAD option clears the 'turtle' memory.
LIST The list option gives a line-by-line display of the 'turtle' routine currently in
memory, up to and including the first 'stop'. Commands after the first 'stop' (if there is more
than one) stay in memory but are not displayed. To return to the menu, press (E).
EDIT The current lines are presented one by one with in each case a message
requiring a 'y' or 'n' response (with (E)). If 'n' is given, the new command may then be
entered
As usual, 'stop', whether met and agreed or newly entered, will end this procedure.
STEP Using this, you may step line by line and action by action through the current
'turtle' routine, until 'stop' is met. (E) initiates each next step and also returns one to the menu
at the end.
STEP & LINE This is the same except that the command currently being executed appears at
top left of the display.
5
RUN The complete current routine is executed, from line 1 to 'stop' or 'endp'.
Increase execution speed by holding down the ENTER key.
RUN & LINE This is the same except that the command currently being executed appears at
top left of the display.
COPY or SAVE With this option, one may record the current routine for later use.
Instructions are included. However if the printer is to be used, it must be connected before
'turtle' is first loaded.
If 'paper' (= copy) output is chosen, the printer will give title, name, date, screen display at
end of 'turtle' routine, and the corresponding 'turtle' listing.
With 'tape' (= SAVE), the system will dump to cassette the 'turtle' routine lines, under a valid
title/file-name, specified by the user. It is essential to keep an adequate record of these titles.
There is no VERIFY option incorporated in 'turtle' as a second SAVE is simpler. If you wish to
verify, use:
1. BREAK
2. VERIFY "title" DATA p$(). See Manual, Chapter 20.
3. Return with GO TO 50 (E) or GO TO 70 (E).
The saved routine may later be recovered as follows:
1. Load 'turtle' in the usual way.
2. BREAK.
3. Use LOAD "title" DATA p$(), followed by GO TO 50 or GO TO 70. Any 'turtle' routine in
memory already will be lost.
FINISH Use this when you wish to stop and allow someone else to work on their own
material. (E) after it returns one to the main menu with loss of stored routine.
THE COMMANDS (remember to use lower case throughout)
BENDn A circle of radius n is drawn centred on the turtle's current position. Any value of
n (up to three characters) is allowed — negative values have the same effect as positive ones;
0 has no effect; decimal fractions are accepted. As any attempt to plot outside the display area
leads to a BASIC error, values of n that would do this are ignored.
The colour of the circle is blue unless 'bend n' is preceded by a 'penc n' command, with n ≠ 1.
DRAWn The turtle moves in the direction faced (up, right, down or left) a distance of n
steps, or to the screen edge, leaving a straight line trail. The trail will erase anything met to a
distance of half a character width. Negative values of n are ignored, as is 0; decimal ones are
INTed.
The colour of the trail is blue unless the command is preceded by 'pencn' with n ≠ 1.
ENDX This denotes the end of a 'turtle' routine section, as follows:
endp End of main routine ('e' will suffice)
endr End of repeat loop (see REPT)
end* End of procedure (n, 1 to 5, is optional but advisable)
When 'endp' is met, the turtle will halt as if it had met 'stop'. Use this instead of 'stop' when
there are any procedures, otherwise these will not appear in the listings.
'endr' is equivalent to BASIC 'next', causing no visible change but affecting control.
'end*' is equivalent to BASIC 'return' or 'endproc' at the end of a closed sub-routine or
procedure. It too gives no visible effect.
6
HOOTn The 'hoot' command causes the turtle to produce a set of n notes, randomly
selected within a range that depends on progress through the routine. Here n should be a
whole number in the range 1 to 999 — non-positive values and decimal parts are ignored.
Note that the turtle also bleeps as it moves around.
JUMPn This causes the turtle to jump a distance of n units in the direction faced (up,
right, down, left) or to the screen edge. Intervening screen material is not affected. Negative
and zero values of n have no effect; decimal ones are INTed.
LEFTn Here n can be only 90, 180 or 270, with the turtle rotating that number of
degrees anti-clockwise. Use 'turn' for clockwise movement.
MOVEn Using this one commands the turtle to move n steps in the direction faced, or to
the display area edge. This version of LOGO has no 'back' command, and negative (and
zero) values of n are ignored. Use 'left 180' or 'turn 180' before 'moven' to get that effect,
(The effect of 'back' sometimes arises if the turtle is asked to move forward when close to
the edge of the window. Only sophisticated users should attempt to employ this.) Decimal
parts of n are INTed.
The 'moven' command causes a one-unit wide blank trail to be left; use 'jump n' if you don't
want intervening material erased.
PENCn Short for 'pen colour n', this controls the 'ink' used in the next 'bend' or
'draw' action. Valid values of n are 0 to 7 giving the colours marked by those keys. Note that
n = 5 gives a trace in background colour, which will not be visible.
TURNn As 'left n' but giving clockwise rotation; n can be only 90, 180 or 270.
REPTn (= repeat n times) This sets up a loop action, executing the material between it
and 'endr' n times. Here n should be a positive whole number between 1 and 999. Values
below 1 give an error report; decimal parts are ignored. A loop can be of any length (up to 58
lines) but cannot include a second loop. To get that nesting effect, use procedures (see USE*).
STOP (Sophisticated operators may prefer to use 's' alone.) This is used to denote the
last line of the complete 'turtle' routine. As such it can appear instead of 'endp', but this is not
advisable except where there are no procedures. The only occasion when you may be happy to
omit this command is at the end of a sixty-line routine — such a program has an assumed
final 'stop'.
Not only does 'stop' prevent error reports in an otherwise correct 'turtle' routine, but it signals
the routine's end during a number of menu operations. As in BASIC, 'stop' also has use during
development.
USE*n With this, one calls for procedure n to be executed. Here n may take only
whole number values from 1 to 5. A value of n outside this range gives an error report, while
decimal values and those referring to non-existent procedures are ignored. When the
procedure reaches 'end', control will return to the next command after 'use*' unless procedure
nesting has been attempted. See the entry for *n for more details of 'turtle' procedures.
WIPEn in effect this is exactly the same as 'moven' (and has the same syntax).
However its name explicitly indicates its erasing function.
*n With n taking any whole number value from 1 to 5, this denotes the first line of
a procedure (closed sub-routine), equivalent to the BASIC 'def proc'. When the procedure is
called, with 'use*n', control passes to the next line after *n and continues to 'end' or 'end* n'.
Procedures must appear after the 'endr' that terminates the main section and in numerical
order.
Any allowed 'turtle' command may appear within a procedure except of course 'end' (and it is
pointless to use 'endp' or 'stop'). However, while a procedure may call on another, or indeed
itself, this normally leads to an infinite loop.
7
THE DEMONSTRATION ROUTINE
Note that we have used the word 'routine' throughout to make clear the difference between the
'turtle' program bought on cassette and the user's current set of 'turtle' instructions (commands).
As purchased, the 'turtle' program includes within it a demonstration routine.
To see this in operation, press 7 at main menu stage. At the end of the' run, you get this screen
display:
As you will see when you list with key 2 at main menu, the routine comprises the following lines:
Use this routine in turtle memory to see the effect of the other menu options, except key 3. Only
use this EDIT option when you have a good idea of what is happening and how — and then use it to
modify the various individual statements to explore to the full all the aspects of this version of
turtle graphics.
Only then, really, can you develop your usage of the program with youngsters. When you do that,
introduce them gently step by step to the commands and let them find out the syntax. At all
times, remember the cardinal rule of turtling: WHEN IN DOUBT, PRETEND YOU'RE THE TURTLE!