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SINC-LINK, TORONTO, Vol. 1, No. 3
S I N C - LINK
$1 .50 per copy
The Tor or* o Tiaex/Sinclair Users CIii> Newsletter Vol. 1 , No. 3
P.0. Box 7274, Stn. A Toronto M5W 1X9
In this issue;
From the Past President
Multiplication Accuracy with the ZX81
Advanced Machine Code Programming
A Modem For The Time (X-1000 or ZX-81 )
Tricks of the Trade
Froi the Past President;
Well, the Club is a year old now, and we have 100 members from jcross Canada. We also
have a new President, Greg Lloyd. Greg is one of the founding members of the Club and has
been involved with the Club since day one. I'm sure Greg would appreciate any suggestions,
comments, or ideas you can throw at him. I' 11 be hanging on for awhile as Treasurer and
would like to hear from anyone interested in becoming our new Editor for Sine-Link. I
would like to thank everyone for making our first year such a big success.
Yours sincerely,
Ju2Ll2^LL^£l
’ pete Harvey
MULTIPLICATION ACCURACY WITH THE ZX 81 - G.F. Chambers
As the Sinclair manual states, the ZX81 can multiply with an 8 digit accuracy.
The following program will allow you to multiply two integers of any (practical) lenqth
with absolute accuracy.
30 INPUT X$
40 INPUT Y$
50 DIM A(LEN X$ + LEN Y$)
60 FOR M = LEN X$ TO 1 STEP -1
70 FOR N = LEN Y$ TO 1 STEP -1
80 LET C: H + N
90 LET B = VAL X$ (M)
100 LET A(C) = VAL Y$ (N) * B + A (C)
110 LET I = INT ( A(C)/10)
120 LET A (C) = A (C) - (I* 10)
130 LET A (C-1 ) = A(C-1) +1
140 NEXT N
150 NEXT M
160 CLS
170 PRINT X$; ” X n ; Y$; *• =
180 PRINT
190 FOR P - A to (LEN X$ ♦ LEN Y$)
200 PRINT A (P)
210 NEXT P
220 STOP
Newsletter prepared by GRAPHIC ENTERPRISES, WORD PROCESSING CENTRE
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ADVANCED MACHINE CODE PROGRAMING
'lotrovsxi
This article assumes you have some knowledge of Machine Code programming and as such will
not explain the basics of machine code other than where necessary. There are several good
books on the market explaining machine code, such as Toni Baker's book "Mastering Machine
Code on Your Zi-81".
With just a little knowledge of machine code and a few machine code ctmmards, you are
actually capable of programming ard this article shows some of the uses of those commands.
The problem with the books on teaching machine code is that they tell you everything, what
each command can do, but not a step by step detailed explanation and a working program.
Whenever working with machine code, you are well aware there are no Syntax checking for
errors as in Basic, so you may get something that does not work or a crash. If a program
is not doing what it is supposed to do, always remember that it works in LOGIC and logic
only. That means the only alternative is to get out your pencil and paper, make headings
up of addresses used, stacks and your registers then go through each command line by line,
writing down what the computer would be doing, jumping to or Pushing or Poping and erasing
where numbers are to be erased ard see what the end result is. It is the only way I do it
ard everytime something will not work, I check it out on paper going through it logically
ard always find what went wrong.
Whenever you work in machine code where you will be putting in extended programs, work in
stages and try each stage out until you have all the bugs out, then join the stages to¬
gether into a working program. If you disassemble some small programs such as the Scroll
routines in my last Newsletter article, you can find out what the logic is ard how some of
the commands are being used. This is the way I learned some of my machine codes.
I made up headings with the addresses, registers, etc. ard wrote each line out as explain¬
ed previously. Okay, onto some uses of your learned machine code commands.
Every machine code programmer has used Basic and entered arcade-type games with the result
that they are very, very slow. By the end of this article you will be able to move a
graphic character arouid the screen at super high speed and I do mean fast.
The 1st couple of reminders are that as you have read, the HL ard the A registers are the
preferred registers in the Z-80A microprocessor and these will be used quite frequently.
Since we will be dealing with TV display, let's start with the display varisble. In your
manual, you will by now have come across the various variables from 16385 to 16515. There
are two that we will be dealing with in this article and the first one is 16396. There is
a very in-depth article by Harry Doakes in the July/August issue of "Sync" so I will very
briefly describe how we make use of this address. This address holds the location of the
address location where the display file is located since the display file moves around in
memory. In a long program, it could conceivably be located at around address 32000. This
is why if your program is supposed to be 8K long, you will get addresses printed at a
higher address location. We use this address to obtain the jexact location so that we can
print anywhere we want onto the screen. Another reminder is that we all know the screen
(including the bottom 2 lines) holds 24 lines i 32 columns or 768 possible locations for
any one of the 64 Sinclair characters. But things are not that simple in machine code
land. In Basic, we can print at 10, 15 and know that the character will be somewhere near
the centre of the screen. In machine code we have to deal with code 118 which occurs at
the beginning of the display file and after each line, so now we have to remember 32
columns plus 1 (end-of-line code). It makes calculations a bit more difficult. We will
be using two register pairs to display a character. . . .H. and DE (or BC if you like).
42, 12, 64 LD HL, (16396)
17, 76, 0 LD DE, 76
25 ADD HL, OE
54, 128 LD (HL), 128
201 RET
Let's go through each line logically (there's that word again) and in detail.
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Line 1 : we load HL with the contents of the location of the display file. Remember that
brackets are like a box containing something so we say "the contents of" and in this case
we have to use HL & we are loading HL the address location where the display file begins.
Without brackets the line would read to load HL with that decimal number.
Line 2: we load DC with the decimal number 76 where 76 is a location on the screen at
afJout line 1 column 16. I will let you work out the columns and lines and do not forget
the code 118 where required.
Line 3: we add DE to the contents of address HL which all boils down to this. If the
address location of the display file begins at say, 30000, tten from 30001 (remember code
118) to about 30792 are all the possible locations of displayirg a character on the
screen. (again remember code 118 after each line.) We want the location 76 so this is
added to HL.
line 4: we now load the contents of HL or in other words, that address location with the
character of the code 128 which in this case is inverse space. Don't forget the meaning
of the brackets. If they were left out, you would load HL with the decimal number 128 and
cancel out the number it contained after adding DE to HL .
Line 6: returns us to the original call which in this case is Basic. Code 201 can also be
used within a machine code routine such as 16514 - Call 17000. The machine code immed¬
iately jumps to 17000 and begins executing the machine code instructions. It goes on
until it reaches a return instruction such as 201 and immediately jumps back to 16 517.
(16515 and 16516 holds the address location.)
Enter the above program using your loader program or the one from my last newsletter
article. I will assume you know about characters in the first REM statement otherwise
again read about it in my last newsletter article.
Now, RAND USR 16514 and immediately an inverse space will appear.
That was pretty simple so let's get it to move. (I will be explaining only new lines
added and you can go back each time to read the explanation of the old lines.)
42,
12
17,
54
25
34,
61
35
126
254
, o
12,
2
24,
246
201
LD HL, (16396)
LD 0E, 54
ADD HL. DE
LD (HL) 61 : Letter X
INC HL
LD A, (HL)
CP 0
JRN2 +2
JR -10
RET
Line 5; we add 1 to the display file location. So if the display file is as in the ab ve
program at 34 which is line 1 column 1 we now are at number 35.
Line 6 and Line 7: we want to compare something in line 7 and we can only use the A
register to ccxnpare something with so we have line 6. Now the reason we want to compare
is that we don't want to run off the end of the screen and cause a crash. So we Load A
with the contents of HL which could be any character code or the end-of-line character
code. We now test it in line 7. If the character is 0 which you know is a space then
continuing to line 8 .
Line 8: we jump over 2 bytes if the comparison is not 0. Remember when we do a compari-
son, the computer subtracts the comparison with the contents of A. If it is not zero or
space such as code 118 (0 - 118 is definitely not zero). If it is a space (0-0 = 0)
then it skips over the next command where it comes to line 9 which tells it to jump back
10 bytes to LD (HL), 128. By the way, line 9 shows 246 which to the computer means jump
back 10 bytes. If you, for example, POKE 20000 with - 10, the computer will automatically
convert it to the positive decimal number.
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If you enter you will immediately get 1 line full of X's. Machine code is so fast that
you will not see it check ard print each X.
But this is not moving graphics. There is one thing to keep in mind a<d that is moving
graphics is an optical illusion. We print one character then erase the last one. So
let's change the above program to do this. We will also add a delay loop to slow the pro¬
cess down and you can change the delay loop to see how fast machine code is.
The first 4 lines above are the same so continuing with line 5:
229
PUSH HL
1,1,10
LD BC 2561
11
DEC BC
120
LD A,B
177
OR C
12,251
JR -5
15
INC HL
126
LD A, (HL)
254,0
CP 0
12,8
JRNZ +8
54,61
LD (HL) ,61
225
POP HL
54,0
LD (HL) ,0
35
INC HL
24,211
JR -21 :to
225
POP HL
201
RET
BIBLIOTECA PARTICULAR
JUAN JOSE CASTILLOS
1.0 _ UM _
HHiraOl _
2WE02ST TEV I ID
Line_5: we have to store the old address so one way of doing this is to push HL onto the
stack. We could also store this number in HL at an arbitary address, for example, when I
program, I leave the first 20 addresses (16514 to 16514) for this purpose. So instead of
Pushing Hl I would LD (16514), HL. Then whenever I need this number I just call it back
or in the above program, we will POP it off the stack.
Line 6 to 10: this is a machine code delay loop which I used from Toni 8aker's "Breakout"
game! (You see the value in disassembling machine code routines.) To increase speed
simply decrease 10 to a lower number and to slow down, change 10 to a high number.
Li?e 13; note we again print another X before erasing the old one. In machine code
this occurs so fast that we have the illusion of movement. If we erased the old one be¬
fore the new one was printed, then we get a flashing effect which is disturbing to the
eyes. 3
Line_16: here is where we POP the old address of the stack which contains the location of
the first X or previous X that was printed. We could have simply decreased HL (DEC HL)
but in larger programs that old address could have changed by as much as a couple of hun¬
dred locations so get used to storing numbers for this reason.
Line_1_7: no* that we have the old address we can erase "X" so we load the contents of the
display in HL with 0 or space. So at this point in time an X is now printed at the next
location, 16, while the old location, 15 has been erased.
L_ine18: we now increase the HL location again so that when in the next line we jump to
push HL , the new address will now become the old address.
Line' 20: after the X has reached code 118, it JRNZ to this line. What we are doing here
is clearing the stack. If you follow the program through you will find that HL has been
pushed onto the stack and we cannot go all day long pushing without having an even number
of poping since the stack does have a limit of the number it can hold ard the program
would eventually crash. So this line clears it leaving it empty.
continued
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Now you should be capable of moving the X to the left side of the screen. (By charging
INC HL to DEC Hi). By combining both programs your X will move to the right then the left
and on and on. This leaves us with one more step & that is keyboard control of graphics.
Now for the next variable address we will be using is 16421. This with 16422 holds the
codes of the key being presently pressed. I will explain it a bit and give you a BASIC
program to obtain the codes a little later on.
What we will do now is add a few more lines to the previous program and give both right
and left movements. The following is the whole program. There is a slight difference in
that instead of PUSHing and POPing the display file location, we will store those numbers
at specific addresses - 16514 to 16517. So this time, write to address 16518 and remember
when you are finished to RAND USR 16518.
42.12.64 LD HL (16596)
17,76,0 LD DE ,76
25 ADD HL, DE
34.130.64 LD (16514), HL
54.132.64 LD (16516), HL
(The last two lines simply store the value of HL which will be used in the program to
locate the new location and erase the X at the old location.)
54,61
LD (HL) ,61
205,187,2
CALL 699
2 55
EX ,DE ,HL
33,127,253
LD HL,
16f
AND A
237,82
SBC HL ,DE
200
RET Z
1.1,5
LD 8C,12B1
11
DEC B
120
LD A, 8
177
OR C
32,251
JRNZ -5
205,187,2
CALL 699
235
EX DE,HL
33,247,223
LD HL,
167
AM) A
237,82
SBC HL,DE
32,25
JRNZ +25
{keyboard scan routine
with the codes for break or space
:clear the carry flag
{return to basic if you press break
{Change 5 to 0 for fast
code for key ”5’’ or left arrow key
to next K-Scan
(if it is not obvious by now, you can see that 699 address is a ROM routine that fetches
the codes of the key you are pressing and returns the value in HL where we exchange it
into DE).
We then load HL with the value of the key we wish to tell the computer to act on and in
the first case it was the SPACE key while above it was the "5" key. If HL matches DE
(result is zero after subtracting) then the computer will act. If it does not match it
moves on to the next K-Scan (JRNZ +25). In the case of the SPACE key, it returns us to
Basic otherwise you stay forever in machine code until the computer is sfxjt down.)
Continuing on with the program .
42,132,64 LD HL, (16516) {fetch the display location
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43
DEC HL
126
LD A , ( HL )
254,0
CP 0
52,16
JRNZ +16
34,152,64
LD (16516), HL
54,61
LD ( HL ) , 61
42,150,64
LD HL, (16514)
54,0
LD (HL) ,0
42,1 52,64
LD HL, (16516)
34,130,64
LD (16514), HL
Move X to the
r ight :
205,187,2
CALL 699
255
EX DE,HL
33,239,247
LD HL,
167
AND A
237.82
SBC HL ,D£
52,25
JRNZ +25
42,152,64
LD HL, (16516)
55
INC HL
126
LD A,(HL)
254,0
CP 0
32,16
JRNZ +16
54,1 52,64
LD (16516), HL
54,61
LD (HL) ,61
42,130,64
LD H., (16514)
54,0
LD (HL) ,0
42,132,64
LD HL, (16516)
34,130,64
LD (16514), HL
195,149,64
JP 16533
tdecreasing HL allows us to move left
:is it a space?
sjump to next K-Scan if no space
: store new display location
sprint an X at the new place
:get the old location
serase the old X
:get new location
: store it as an old location
Codes for key "8'’ or right arrow
:to the end of program
: to end
sjump to first K-Scan
Ensure the numbers are correct if it does not work. This program has been tried by me so
in printing the newsletter there may be errors. If you checked 4 it still does not work,
check the mnemonics in your manual for the correct codes.
Before we get into the keyboard scan routines, did you notice how we used addresses to
store the display location of HL instead of PUSHing them? The reason behind this is for
you to get used to storing addresses and when you experiment in moving vertically you will
appreciate this since instead of just INC or DEC HL, you must LD DE ,3 3( remember code 118)
and ADD HL,DE. This location would then be immediately above or below the old location.
(Don’t forget AND A to clear the carry flag before SBC DE,DHL when moving up.)
If you wish to obtain the codes of the keys in case you would rather have some other char¬
acter on the keyboard to be used for movement such as "A" left and "L” right use the fol¬
lowing program to obtain them: (# means space)
10 IF INKEY$=,,,,THEN GOTO 10
20 LET K$=INKEY$
50 LET A = PEEK 16421
40 LET B = PEEK 16422
50 SCROLL
60 PRINT "CODE F0R#”;K$;',#is#,,;A:,V,,,;B
70 GOTO 10
Copy the codes down so you can refer to them when you need them.
With a couple more commands of the over 500 commands available in machine code, you should
be able to write a complete arcade-type game. I did so I’m sure you can.
A M00EM FOR THE TIME (X-1000 or ZX-B1
by Iranz Hrazdira
After looking at many advertisements in various magazines, I found an offer which I was
willing to explore further. Having been reassured on the phone that the modem kit would
require nothing more to function satisfactorily than about (4) hours of my time to put it
together, I ordered it right then. This was back in April.
continued. . .
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When I received the package from Byte-Back, I was impressed - not so much with what I had
paid after the exchange rate, duty and tax was added, as with the quality of the parts and
documentation. Two separate manuals, covering the MD-1 modem and the RS-232 board in
great detail, were supplied.
The BAUD RATE, which is the speed at which the RS-232 board sends and gets data, determin¬
ed by (1) the frequency of the transmit and receive clock and (2) the divide rate of the
INTEL 8251-A USART IC chip (Universal Synchronous/Asynchronous Receiver Transmitter), can
be set to either 9600, 4800, 2400, 1200 or any of these baud rates divided by 16 or 64 -
the most common being 500, which is 4800/16. The schematic makes available, in addition
to the RS-232 level signals, also the TTl level signals for interface to any printer or
other peripheral.
The MD-1 uses the new Motorola MC 6860 modem chip vbich MODulates and DEmodulates your
signal so you can communicate with any other computer over the telephone. To do this, you
first load and run a terminal program which comes on tape as part of the deal and takes up
about 1.5K of your memory. Then, assuming everything was assembled correctly aid the
modem is connected to the phone, you can just dial up any of the following numbers on this
list.
423-3265
ETI between
5PM
and
9 AM
366-2069
CFTR
6PM
9AM
978-6893
MEDICAL
7PM
9AM
499-7023
I8MPC
24
Hours
868-4000
DATAPAC
365-9621
COMPUSERVE
424-1895 for further details or a demo
Normally, you will be connected after a few ring tones. The other computer starts out
with a 2000 Hz answer tone from its modem to which yours sends a 1000 Hz originate tone in
reply. You are now ready to communicate by entering NEWLI^C, and away you go .
A new terminal program which is now available lets you send and receive all kinds of data,
including program in BASIC and Machine Code, while doing an automatic syntactic sum check
for error free transfers. After receiving, you can then review, print or save the data on
tape for later use.
This program also has an automatic print mode which, if selected, will send the other com¬
puter a command to wait. After everything was printed, it will then ask for more data to
be sent, to receive a new screen full and so on, until you change the mode.
Let me tell you also about a service you can subscribe to by phone, which will give your
little computer the power you have only dreamt about until now. It is called CompuServe.
For $5.00 US an hour, you can send and receive Electronic Mail from among 70,000 other
subscribers in Canada and the US, chat on a multi-channel C8 simulator with someone in
Alaska. 8.C., or California, post notices on a number of Special Interest Group (SIG) bul¬
letin boards, play multi-user games, read about commodity futures or search a database on
just about any topic.
I am offering starter kits to CompuServe for $60.00 CAN. They include a very basic manual,
a three ring binder, a user I.D., a password, a subscription to the TODAY magazine, reg¬
ular update bulletins and (5) five hours of free connect time to the service. The modem
will cost you $198.00 CAN in kit form or $246.00 assembled and tested, including the basic
software.
For more information, you can call me at the last number on the list.
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TRICKS Of THE TRADE
By J.'J. C as til los
After d few years of working with the ZX81 , it is only natural that we learned ways to
protect our programs frum being appropriated or altered by other people.
The tips that follow will not stop a determined and knowledgeable person from replacing
your name or copyright notice with his or her own or prevent everyone from peeking and
using your machine code routines, without proper acknowledgement, but they will make it
very difficult for the average ZX81 owner to do these things.
Suppose you put in your program a line such as the following: 1 REM PACMAN XVII - JEAN
SMITH, 1983. It is very easy to delete it and replace it with another, but if you enter
as a command POKE 16510,0/Newline, you will see that your line 1 has became line 0 which
cannot be deleted or edited out.
Now, if your aspiring pirate knows this trick, he will just have to POKE 16510, 1 /New line
aid the line will be again at his mercy for quick disposal.
However, there is a way to make it very hard, if not impossible, for him to do this. Just
begin your program with your REM statement as line 1 and then POKE 16509,62/Newline. Your
line 1 will become line F8 75 arid if you start to add other program lines, you will see
that your REM goes to the bottom of the listing, safe from 99% of other users, undeletable
and uneditable. In order to get to your REM line, people would have to either delete the
whole program to get it back to the top of the screen and reverse the process or disas¬
semble the program to locate the address of the REM line number.
On the other hand, if you have machine code that you want to protect from predatory eyes,
you can put it into a REM statement at the beginning of your program and then make it in¬
visible by entering POKE 1651 A, 1 18/Newline. Users trying to list the program will only
see the line number but nothing else. You can even do better by entering instead of the
above, POKE 16509,120/Newline. This will make the whole line invisible as if it didn't
exist, users will naturally assume that the program begins elsewhere (i.e., the next BASIC
line you enter which will be visible).
An alternative which is just as effective, is to put your machine code as an array which
you dimension, define and after the computer has accepted it, then you delete the BASIC
lines (OIM, LET, etc.) that defined it. Thus, the array with your machine code is in
memory to be transferred wherever you want it to go but to the uninitiated it just doesn't
exist. If he tries to run the program, it will be wiped out.
But let us now reverse the situation and suppose that with all good intentions it is you
who want to alter or see what there is in a program as part of your learning process or to
make improvements. You can see the invisible machine code in the first line by identify¬
ing (use PRINT PEEK command), which of the first few bytes of the line (16514,16515, etc;,
contain a number 118 and then POKE there any number such as 0 or 20. Try to list the
program and if you eliminated all the 118's you will see it all displayed on the screen.
If address 16509 is not 0, make it 0, using a POKE command.
If the programmer used some of the more elaborate tricks such as arrays, you can PEEK
blocks of memory of say, 200 bytes each, until you find the machine code you are looking
for. For the purpose, run a simple program such as the following:
5 FAST
10 REM ENTER STARTING ADDRESS FOR THE SEARCH
20 I»fUT A
25 PRINT A;M:
30 FOR 8=0 TO 200
40 PRINT PEEK (A+B);"-";
50 NEXT B
60 STOP
continued. . .
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Start if you wish your search at location 20000, after the first run try 20200, etc. For
most of the memory you will probably only get strings of 0’s, but wherever you see numbers
instead, it could be the machine code you are looking for. The above is, in fact, a short
and simple disassembler program.
finally, if you wish to make a back-up copy of one of those "unlistable" and "unsaveable"
machine code prugrams in the market, you can easily do it by not using the normal LOAD
command. Set you* computer first in FAST mode, enter then RAND USR 837 Newline ard start
your tape recorder in play. You will see that the program loads but instead of starting
to run and lock itself in machine code, it stops with an error code. Make your back-up
copy using SALE & run by entering GOTO (whichever line number that has a USR Command)/
Newline.
So far, we don't know of any ZX81 program that has been successfully protected from being
copied or peeked in the ways described above, which is no doubt a corstant worry to soft¬
ware manufacturers but a blessing to us users, who most of the time just want to learn
from more knowledgeable people, or be protected from accidentally erasing the only copy of
a valuable program.
HARDWARE AM) SOFTWARE REVIEWS
by J . J . Last ll los
There are so many new add-ons and programs for the TItEX 1000/ZX81 that we could not pos¬
sibly review them all in detail in this Newsletter. With this in mind, I thought it would
be more useful to Club members, to run from time to time brief reviews of many items we
have bought and tested, so that people can have at least some guidelines to help them pick
the right product from among the bewildering array of ads in the computer magazines.
-tjNTER.B.QARDS T.hese men,ory boards provide up to 3K of RAM or EPROM (2K or 4K) memory in
the 8-16K area which is not normally used in tne ZX81. The RAM memory boards include Bat¬
tery back-up, which allows you to keep programs in them for up to 20 years per battery
even when the computer is shut down. These boards are sold as kits which are easy to as¬
semble by anyone with a minimum degree of electronic expertise. The instructions are very
well written and printed and have many useful tips to use and even improve your boards. A
fine product indeed sold at a very reasonable price of US$ 29.95.
MEMOTECH'S MEMOCALC - This spreadsheet is supplied as an EPROM in another of ttcse ele-
gantly designed Memotech add-ons and with a 64K RAM pack, it allows you to have as many as
7000 numbers simultaneously on the grid. It has a switch to connect you to BASIC or
Memocalc directly. Vou see at any one time 3 columns and 17 lines but your screen display
works as a window which can be moved up or down, left or right at will, to anywhere you
want. You can save, print out modify or re-start grids at any time. This is an attrac¬
tive product selling for only US$ 49.95 but frankly, if you have the latest, machine code
version of VU-CALC, you don't really gain so much as to justify the extra expense.
- This program is a compiler, ie., it translates your. BASIC programs into machine
code programs which run up to 20 times faster. This canpiler is somewhat limited in the
sense that it allows you only one single dimension numerical array, it works with integers
only, no multiple conditional statements are allowed etc., but it only takes 2K of memory,
it is not infuriatingly frustrating as other compilers (ZXPRESS for example) and if you
are resourceful enough to adapt your programs to meet its requirements, it canpiles very
easily (you only have to press a key!) and points to any error you may have made so that
you can correct it and go on. The instructions are concise but adequate. MCoder has
received somewhat negative reviews in most magazines but my experience with it has been
positive and I find it very useful.
EVOLUTION - We have all probably heard of those British Columbia kids who wrote a game
based on the theory of evolution for an APPLE computer and made a bundle with it. This
English program for the ZX81 lacks the graphics, colour, sound and speed capabilities of
the APPLE version but it is reasonably fast and it is also an excellent didactic exper¬
ience to familiarize people with the main facts of evolution, at the same time that it
offers a real challenge to your intelligence. It is one of my favourite games and I re¬
commend it to anyone who wants to go beyond the dozens of zap'em & blow-them-up games
currently available.
