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PT3/+P2

This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.

Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.

Tape/disk: Home » Personal collection » Acorn ADFS disks » Electron » EUG_submission.ADF
Filename: PT3/+P2
Read OK: ✔
File size: 1CBC bytes
Load address: 2B204556
Exec address: D3250
Duplicates

There is 1 duplicate copy of this file in the archive:

File contents
There are, however, a few precautionary points which should be mentioned
about this technique of saving screen memory:

- As you probably realise, in four colour modes, the four colours you
can use can be changed from the default black, red, yellow and white by
using VDU 19 to change the palette. The colours in the palette are not
saved when this technique is used, and must be restored by repeating the
VDU 19 commands again. If you define the palette to, for example, black,
blue, cyan and white, and create a picture which looks good in this colour
scheme, you must remember that unless you define the palette to these four
colours before performing *LOAD to restore the picture, it may appear
rather odd when reloaded, as it will appear in black, red, yellow and
white - the default colour scheme.

- The exact reason why this happens is rather complicated, but if you allow
the cursor to go down off the bottom line of the screen, and allow the screen
to scroll up one line, this technique stops working until you perform a
MODE command, or clear the screen using CLS. Basically what happens is that
the computer does not physically move every line of text on the screen up
one line, but instead makes a note that every line in screen memory is stored
one line lower than it should be. When you reload from disk, this note is
lost, and the computer draws every line lower than it should be. Each time
that CLS or MODE is executed, screen memory is cleared and reinitiated, so
that every line is stored in memory where you would expect it to be. Simply
remember not to allow the screen to scroll between clearing it, and saving
the contents of screen memory to disc.

- The files created when you save the contents of screen memory to disk
are fairly large - so large, in fact, that you can often only fit around
15 such files onto each disk that you use (the exact number depends upon
what modes the images are in). Therefore this technique is not feasable
for a large number of images. Each screen in a game like Shipwrecked cannot
be stored in this way - if I had stored every screen of the game like this,
it would have required six floppy disks to store the data for the game!
This means that saved screens must be reserved for the purposes of title
screens only.

Earlier in the article, I mentioned several examples of screens which I have
done for EUG using saved screens using this technique. These included the
title screens for both Shipwrecked games, and also several EUG title
screens. You may have noticed that these were actually animated (see the
starfield in Jupiter 3 and the EUG 35 title). This was achieved by palette
switching as used in the previous article. If you look again at the
programming technique demonstration from this article (on EUG 36), and
select option 5, the same effect will appear on your screen. The same
principal was applied in both animated screens mentioned above - the only
difference is that in these title screens the pattern on screen which is
being palette switched is rather more complex. Shipwreck's title screen
(with the animated water scene) was also created in this way, except that
for this screen there was also a machine code program altering the contents
of screen memory as well, to enhance the effect.

So, we have a technique which allows us to store a small number of very
detailed screens on floppy, and recall then at very great speed. But what if
we want to use this method for a large number of screens? And what if we
want a situation like a game, where the user can move sprites on the screen
around. This can also be done by altering the values in screen memory
directly, but in a different way...

2. Editing screen memory

The basic idea is this: in any program, you only usually require a certain,
small number of little 'blocks' to make up a whole picture. In a word
processor, for example, you only need the letters of the alphabet to appear
on the screen. These letters are small blocks of graphics which appear on
your screen (called characters). Because only these 26 blocks are required,
the word processor does not need to store every letter as all of the pixels
which make it up (ie. as a bitmap), but just as a code for that particular
letter. When the word processor wants to display a letter on the screen, it
looks up the letter code, and then copies the graphic data for what that
letter looks like onto the screen. Therefore, the graphic data for what
colour all of the pixels should be for that particular letter (which might
take several bytes) need only be stored once, and that letter can be stored
in your script in just one byte, for the letter code. Thus, the memory
taken up by your script is reduced dramatically.

The same principal can be used in graphics. Look at two brick walls in
an arcade adventure such as Shipwrecked - can you see the difference
between them? If they are the same, surely you only need to store the
graphic data for a brick wall once, and then copy it onto the screen
whenever you want a brick wall. In Shipwrecked, for example, the graphic
data to describe what a brick wall looks like took 128 bytes. Yet every
time I wanted that brick wall (which was over a hundred times) I could
simply use one byte to say "Put a brick wall here!".

The easiest way to use this block idea, as you may already have realised,
is using user defined characters. You redefine the letters of the alphabet
as different pictures, and then use the standard BASIC PRINT command to
output these pictures to the screen, as and when they are required. But,
as we already realised in the last article, this only allows you to use
one colour in each character. Even so, this is a reasonable technique if
you don't mind the lack of colour. Many commercial games actually never
addressed this problem, and had graphics which lacked colour somewhat.
Even Dizzy (for those who have never heard of it, a great arcade adventure
of the 1980s, which caused quite a cult following at the time on the
Spectrum and C64, but never quite reached the Elk) did not have more than
one colour (other than black) per block. Different blocks were, however,
different colours - trees were green and the ground was red - but there
was never any overlap between the two colour areas.

There is, however, a way of creating an effect just like user defined
characters, but in full colour, and the principal is very similar to
that used before in saving screen memory. Basically, you design you small
block on the screen, using whatever technique you like to draw it, and then
save the small part of screen memory which contains your design. This time,
however, you will not save it to floppy disc, but to another area of memory.
You can then copy this block back to screen memory whenever and wherever you
want it. If you want it, say, in the fifth row down, and twelve characters
along, you can calculate where abouts this corresponds to in screen memory,
and copy the block to this location. As the block is stored in your
computer's RAM, as opposed to on floppy, this transfer can be done by
machine code at very great speed (taking only a small fraction of a second).
For those programmers who are not familiar with machine code, a module
is included with this tutorial allowing you to do this. The program code
for this module is actually taken directly from Shipwrecked.

00000000  54 68 65 72 65 20 61 72  65 2c 20 68 6f 77 65 76  |There are, howev|
00000010  65 72 2c 20 61 20 66 65  77 20 70 72 65 63 61 75  |er, a few precau|
00000020  74 69 6f 6e 61 72 79 20  70 6f 69 6e 74 73 20 77  |tionary points w|
00000030  68 69 63 68 20 73 68 6f  75 6c 64 20 62 65 20 6d  |hich should be m|
00000040  65 6e 74 69 6f 6e 65 64  0d 61 62 6f 75 74 20 74  |entioned.about t|
00000050  68 69 73 20 74 65 63 68  6e 69 71 75 65 20 6f 66  |his technique of|
00000060  20 73 61 76 69 6e 67 20  73 63 72 65 65 6e 20 6d  | saving screen m|
00000070  65 6d 6f 72 79 3a 0d 0d  2d 20 41 73 20 79 6f 75  |emory:..- As you|
00000080  20 70 72 6f 62 61 62 6c  79 20 72 65 61 6c 69 73  | probably realis|
00000090  65 2c 20 69 6e 20 66 6f  75 72 20 63 6f 6c 6f 75  |e, in four colou|
000000a0  72 20 6d 6f 64 65 73 2c  20 74 68 65 20 66 6f 75  |r modes, the fou|
000000b0  72 20 63 6f 6c 6f 75 72  73 20 79 6f 75 0d 63 61  |r colours you.ca|
000000c0  6e 20 75 73 65 20 63 61  6e 20 62 65 20 63 68 61  |n use can be cha|
000000d0  6e 67 65 64 20 66 72 6f  6d 20 74 68 65 20 64 65  |nged from the de|
000000e0  66 61 75 6c 74 20 62 6c  61 63 6b 2c 20 72 65 64  |fault black, red|
000000f0  2c 20 79 65 6c 6c 6f 77  20 61 6e 64 20 77 68 69  |, yellow and whi|
00000100  74 65 20 62 79 0d 75 73  69 6e 67 20 56 44 55 20  |te by.using VDU |
00000110  31 39 20 74 6f 20 63 68  61 6e 67 65 20 74 68 65  |19 to change the|
00000120  20 70 61 6c 65 74 74 65  2e 20 54 68 65 20 63 6f  | palette. The co|
00000130  6c 6f 75 72 73 20 69 6e  20 74 68 65 20 70 61 6c  |lours in the pal|
00000140  65 74 74 65 20 61 72 65  20 6e 6f 74 0d 73 61 76  |ette are not.sav|
00000150  65 64 20 77 68 65 6e 20  74 68 69 73 20 74 65 63  |ed when this tec|
00000160  68 6e 69 71 75 65 20 69  73 20 75 73 65 64 2c 20  |hnique is used, |
00000170  61 6e 64 20 6d 75 73 74  20 62 65 20 72 65 73 74  |and must be rest|
00000180  6f 72 65 64 20 62 79 20  72 65 70 65 61 74 69 6e  |ored by repeatin|
00000190  67 20 74 68 65 0d 56 44  55 20 31 39 20 63 6f 6d  |g the.VDU 19 com|
000001a0  6d 61 6e 64 73 20 61 67  61 69 6e 2e 20 49 66 20  |mands again. If |
000001b0  79 6f 75 20 64 65 66 69  6e 65 20 74 68 65 20 70  |you define the p|
000001c0  61 6c 65 74 74 65 20 74  6f 2c 20 66 6f 72 20 65  |alette to, for e|
000001d0  78 61 6d 70 6c 65 2c 20  62 6c 61 63 6b 2c 0d 62  |xample, black,.b|
000001e0  6c 75 65 2c 20 63 79 61  6e 20 61 6e 64 20 77 68  |lue, cyan and wh|
000001f0  69 74 65 2c 20 61 6e 64  20 63 72 65 61 74 65 20  |ite, and create |
00000200  61 20 70 69 63 74 75 72  65 20 77 68 69 63 68 20  |a picture which |
00000210  6c 6f 6f 6b 73 20 67 6f  6f 64 20 69 6e 20 74 68  |looks good in th|
00000220  69 73 20 63 6f 6c 6f 75  72 0d 73 63 68 65 6d 65  |is colour.scheme|
00000230  2c 20 79 6f 75 20 6d 75  73 74 20 72 65 6d 65 6d  |, you must remem|
00000240  62 65 72 20 74 68 61 74  20 75 6e 6c 65 73 73 20  |ber that unless |
00000250  79 6f 75 20 64 65 66 69  6e 65 20 74 68 65 20 70  |you define the p|
00000260  61 6c 65 74 74 65 20 74  6f 20 74 68 65 73 65 20  |alette to these |
00000270  66 6f 75 72 0d 63 6f 6c  6f 75 72 73 20 62 65 66  |four.colours bef|
00000280  6f 72 65 20 70 65 72 66  6f 72 6d 69 6e 67 20 2a  |ore performing *|
00000290  4c 4f 41 44 20 74 6f 20  72 65 73 74 6f 72 65 20  |LOAD to restore |
000002a0  74 68 65 20 70 69 63 74  75 72 65 2c 20 69 74 20  |the picture, it |
000002b0  6d 61 79 20 61 70 70 65  61 72 0d 72 61 74 68 65  |may appear.rathe|
000002c0  72 20 6f 64 64 20 77 68  65 6e 20 72 65 6c 6f 61  |r odd when reloa|
000002d0  64 65 64 2c 20 61 73 20  69 74 20 77 69 6c 6c 20  |ded, as it will |
000002e0  61 70 70 65 61 72 20 69  6e 20 62 6c 61 63 6b 2c  |appear in black,|
000002f0  20 72 65 64 2c 20 79 65  6c 6c 6f 77 20 61 6e 64  | red, yellow and|
00000300  0d 77 68 69 74 65 20 2d  20 74 68 65 20 64 65 66  |.white - the def|
00000310  61 75 6c 74 20 63 6f 6c  6f 75 72 20 73 63 68 65  |ault colour sche|
00000320  6d 65 2e 0d 0d 2d 20 54  68 65 20 65 78 61 63 74  |me...- The exact|
00000330  20 72 65 61 73 6f 6e 20  77 68 79 20 74 68 69 73  | reason why this|
00000340  20 68 61 70 70 65 6e 73  20 69 73 20 72 61 74 68  | happens is rath|
00000350  65 72 20 63 6f 6d 70 6c  69 63 61 74 65 64 2c 20  |er complicated, |
00000360  62 75 74 20 69 66 20 79  6f 75 20 61 6c 6c 6f 77  |but if you allow|
00000370  0d 74 68 65 20 63 75 72  73 6f 72 20 74 6f 20 67  |.the cursor to g|
00000380  6f 20 64 6f 77 6e 20 6f  66 66 20 74 68 65 20 62  |o down off the b|
00000390  6f 74 74 6f 6d 20 6c 69  6e 65 20 6f 66 20 74 68  |ottom line of th|
000003a0  65 20 73 63 72 65 65 6e  2c 20 61 6e 64 20 61 6c  |e screen, and al|
000003b0  6c 6f 77 20 74 68 65 20  73 63 72 65 65 6e 0d 74  |low the screen.t|
000003c0  6f 20 73 63 72 6f 6c 6c  20 75 70 20 6f 6e 65 20  |o scroll up one |
000003d0  6c 69 6e 65 2c 20 74 68  69 73 20 74 65 63 68 6e  |line, this techn|
000003e0  69 71 75 65 20 73 74 6f  70 73 20 77 6f 72 6b 69  |ique stops worki|
000003f0  6e 67 20 75 6e 74 69 6c  20 79 6f 75 20 70 65 72  |ng until you per|
00000400  66 6f 72 6d 20 61 0d 4d  4f 44 45 20 63 6f 6d 6d  |form a.MODE comm|
00000410  61 6e 64 2c 20 6f 72 20  63 6c 65 61 72 20 74 68  |and, or clear th|
00000420  65 20 73 63 72 65 65 6e  20 75 73 69 6e 67 20 43  |e screen using C|
00000430  4c 53 2e 20 42 61 73 69  63 61 6c 6c 79 20 77 68  |LS. Basically wh|
00000440  61 74 20 68 61 70 70 65  6e 73 20 69 73 20 74 68  |at happens is th|
00000450  61 74 0d 74 68 65 20 63  6f 6d 70 75 74 65 72 20  |at.the computer |
00000460  64 6f 65 73 20 6e 6f 74  20 70 68 79 73 69 63 61  |does not physica|
00000470  6c 6c 79 20 6d 6f 76 65  20 65 76 65 72 79 20 6c  |lly move every l|
00000480  69 6e 65 20 6f 66 20 74  65 78 74 20 6f 6e 20 74  |ine of text on t|
00000490  68 65 20 73 63 72 65 65  6e 20 75 70 0d 6f 6e 65  |he screen up.one|
000004a0  20 6c 69 6e 65 2c 20 62  75 74 20 69 6e 73 74 65  | line, but inste|
000004b0  61 64 20 6d 61 6b 65 73  20 61 20 6e 6f 74 65 20  |ad makes a note |
000004c0  74 68 61 74 20 65 76 65  72 79 20 6c 69 6e 65 20  |that every line |
000004d0  69 6e 20 73 63 72 65 65  6e 20 6d 65 6d 6f 72 79  |in screen memory|
000004e0  20 69 73 20 73 74 6f 72  65 64 0d 6f 6e 65 20 6c  | is stored.one l|
000004f0  69 6e 65 20 6c 6f 77 65  72 20 74 68 61 6e 20 69  |ine lower than i|
00000500  74 20 73 68 6f 75 6c 64  20 62 65 2e 20 57 68 65  |t should be. Whe|
00000510  6e 20 79 6f 75 20 72 65  6c 6f 61 64 20 66 72 6f  |n you reload fro|
00000520  6d 20 64 69 73 6b 2c 20  74 68 69 73 20 6e 6f 74  |m disk, this not|
00000530  65 20 69 73 0d 6c 6f 73  74 2c 20 61 6e 64 20 74  |e is.lost, and t|
00000540  68 65 20 63 6f 6d 70 75  74 65 72 20 64 72 61 77  |he computer draw|
00000550  73 20 65 76 65 72 79 20  6c 69 6e 65 20 6c 6f 77  |s every line low|
00000560  65 72 20 74 68 61 6e 20  69 74 20 73 68 6f 75 6c  |er than it shoul|
00000570  64 20 62 65 2e 20 45 61  63 68 20 74 69 6d 65 0d  |d be. Each time.|
00000580  74 68 61 74 20 43 4c 53  20 6f 72 20 4d 4f 44 45  |that CLS or MODE|
00000590  20 69 73 20 65 78 65 63  75 74 65 64 2c 20 73 63  | is executed, sc|
000005a0  72 65 65 6e 20 6d 65 6d  6f 72 79 20 69 73 20 63  |reen memory is c|
000005b0  6c 65 61 72 65 64 20 61  6e 64 20 72 65 69 6e 69  |leared and reini|
000005c0  74 69 61 74 65 64 2c 20  73 6f 0d 74 68 61 74 20  |tiated, so.that |
000005d0  65 76 65 72 79 20 6c 69  6e 65 20 69 73 20 73 74  |every line is st|
000005e0  6f 72 65 64 20 69 6e 20  6d 65 6d 6f 72 79 20 77  |ored in memory w|
000005f0  68 65 72 65 20 79 6f 75  20 77 6f 75 6c 64 20 65  |here you would e|
00000600  78 70 65 63 74 20 69 74  20 74 6f 20 62 65 2e 20  |xpect it to be. |
00000610  53 69 6d 70 6c 79 0d 72  65 6d 65 6d 62 65 72 20  |Simply.remember |
00000620  6e 6f 74 20 74 6f 20 61  6c 6c 6f 77 20 74 68 65  |not to allow the|
00000630  20 73 63 72 65 65 6e 20  74 6f 20 73 63 72 6f 6c  | screen to scrol|
00000640  6c 20 62 65 74 77 65 65  6e 20 63 6c 65 61 72 69  |l between cleari|
00000650  6e 67 20 69 74 2c 20 61  6e 64 20 73 61 76 69 6e  |ng it, and savin|
00000660  67 0d 74 68 65 20 63 6f  6e 74 65 6e 74 73 20 6f  |g.the contents o|
00000670  66 20 73 63 72 65 65 6e  20 6d 65 6d 6f 72 79 20  |f screen memory |
00000680  74 6f 20 64 69 73 63 2e  0d 0d 2d 20 54 68 65 20  |to disc...- The |
00000690  66 69 6c 65 73 20 63 72  65 61 74 65 64 20 77 68  |files created wh|
000006a0  65 6e 20 79 6f 75 20 73  61 76 65 20 74 68 65 20  |en you save the |
000006b0  63 6f 6e 74 65 6e 74 73  20 6f 66 20 73 63 72 65  |contents of scre|
000006c0  65 6e 20 6d 65 6d 6f 72  79 20 74 6f 20 64 69 73  |en memory to dis|
000006d0  6b 0d 61 72 65 20 66 61  69 72 6c 79 20 6c 61 72  |k.are fairly lar|
000006e0  67 65 20 2d 20 73 6f 20  6c 61 72 67 65 2c 20 69  |ge - so large, i|
000006f0  6e 20 66 61 63 74 2c 20  74 68 61 74 20 79 6f 75  |n fact, that you|
00000700  20 63 61 6e 20 6f 66 74  65 6e 20 6f 6e 6c 79 20  | can often only |
00000710  66 69 74 20 61 72 6f 75  6e 64 0d 31 35 20 73 75  |fit around.15 su|
00000720  63 68 20 66 69 6c 65 73  20 6f 6e 74 6f 20 65 61  |ch files onto ea|
00000730  63 68 20 64 69 73 6b 20  74 68 61 74 20 79 6f 75  |ch disk that you|
00000740  20 75 73 65 20 28 74 68  65 20 65 78 61 63 74 20  | use (the exact |
00000750  6e 75 6d 62 65 72 20 64  65 70 65 6e 64 73 20 75  |number depends u|
00000760  70 6f 6e 0d 77 68 61 74  20 6d 6f 64 65 73 20 74  |pon.what modes t|
00000770  68 65 20 69 6d 61 67 65  73 20 61 72 65 20 69 6e  |he images are in|
00000780  29 2e 20 54 68 65 72 65  66 6f 72 65 20 74 68 69  |). Therefore thi|
00000790  73 20 74 65 63 68 6e 69  71 75 65 20 69 73 20 6e  |s technique is n|
000007a0  6f 74 20 66 65 61 73 61  62 6c 65 0d 66 6f 72 20  |ot feasable.for |
000007b0  61 20 6c 61 72 67 65 20  6e 75 6d 62 65 72 20 6f  |a large number o|
000007c0  66 20 69 6d 61 67 65 73  2e 20 45 61 63 68 20 73  |f images. Each s|
000007d0  63 72 65 65 6e 20 69 6e  20 61 20 67 61 6d 65 20  |creen in a game |
000007e0  6c 69 6b 65 20 53 68 69  70 77 72 65 63 6b 65 64  |like Shipwrecked|
000007f0  20 63 61 6e 6e 6f 74 0d  62 65 20 73 74 6f 72 65  | cannot.be store|
00000800  64 20 69 6e 20 74 68 69  73 20 77 61 79 20 2d 20  |d in this way - |
00000810  69 66 20 49 20 68 61 64  20 73 74 6f 72 65 64 20  |if I had stored |
00000820  65 76 65 72 79 20 73 63  72 65 65 6e 20 6f 66 20  |every screen of |
00000830  74 68 65 20 67 61 6d 65  20 6c 69 6b 65 20 74 68  |the game like th|
00000840  69 73 2c 0d 69 74 20 77  6f 75 6c 64 20 68 61 76  |is,.it would hav|
00000850  65 20 72 65 71 75 69 72  65 64 20 73 69 78 20 66  |e required six f|
00000860  6c 6f 70 70 79 20 64 69  73 6b 73 20 74 6f 20 73  |loppy disks to s|
00000870  74 6f 72 65 20 74 68 65  20 64 61 74 61 20 66 6f  |tore the data fo|
00000880  72 20 74 68 65 20 67 61  6d 65 21 0d 54 68 69 73  |r the game!.This|
00000890  20 6d 65 61 6e 73 20 74  68 61 74 20 73 61 76 65  | means that save|
000008a0  64 20 73 63 72 65 65 6e  73 20 6d 75 73 74 20 62  |d screens must b|
000008b0  65 20 72 65 73 65 72 76  65 64 20 66 6f 72 20 74  |e reserved for t|
000008c0  68 65 20 70 75 72 70 6f  73 65 73 20 6f 66 20 74  |he purposes of t|
000008d0  69 74 6c 65 0d 73 63 72  65 65 6e 73 20 6f 6e 6c  |itle.screens onl|
000008e0  79 2e 0d 0d 45 61 72 6c  69 65 72 20 69 6e 20 74  |y...Earlier in t|
000008f0  68 65 20 61 72 74 69 63  6c 65 2c 20 49 20 6d 65  |he article, I me|
00000900  6e 74 69 6f 6e 65 64 20  73 65 76 65 72 61 6c 20  |ntioned several |
00000910  65 78 61 6d 70 6c 65 73  20 6f 66 20 73 63 72 65  |examples of scre|
00000920  65 6e 73 20 77 68 69 63  68 20 49 20 68 61 76 65  |ens which I have|
00000930  0d 64 6f 6e 65 20 66 6f  72 20 45 55 47 20 75 73  |.done for EUG us|
00000940  69 6e 67 20 73 61 76 65  64 20 73 63 72 65 65 6e  |ing saved screen|
00000950  73 20 75 73 69 6e 67 20  74 68 69 73 20 74 65 63  |s using this tec|
00000960  68 6e 69 71 75 65 2e 20  54 68 65 73 65 20 69 6e  |hnique. These in|
00000970  63 6c 75 64 65 64 20 74  68 65 0d 74 69 74 6c 65  |cluded the.title|
00000980  20 73 63 72 65 65 6e 73  20 66 6f 72 20 62 6f 74  | screens for bot|
00000990  68 20 53 68 69 70 77 72  65 63 6b 65 64 20 67 61  |h Shipwrecked ga|
000009a0  6d 65 73 2c 20 61 6e 64  20 61 6c 73 6f 20 73 65  |mes, and also se|
000009b0  76 65 72 61 6c 20 45 55  47 20 74 69 74 6c 65 0d  |veral EUG title.|
000009c0  73 63 72 65 65 6e 73 2e  20 59 6f 75 20 6d 61 79  |screens. You may|
000009d0  20 68 61 76 65 20 6e 6f  74 69 63 65 64 20 74 68  | have noticed th|
000009e0  61 74 20 74 68 65 73 65  20 77 65 72 65 20 61 63  |at these were ac|
000009f0  74 75 61 6c 6c 79 20 61  6e 69 6d 61 74 65 64 20  |tually animated |
00000a00  28 73 65 65 20 74 68 65  0d 73 74 61 72 66 69 65  |(see the.starfie|
00000a10  6c 64 20 69 6e 20 4a 75  70 69 74 65 72 20 33 20  |ld in Jupiter 3 |
00000a20  61 6e 64 20 74 68 65 20  45 55 47 20 33 35 20 74  |and the EUG 35 t|
00000a30  69 74 6c 65 29 2e 20 54  68 69 73 20 77 61 73 20  |itle). This was |
00000a40  61 63 68 69 65 76 65 64  20 62 79 20 70 61 6c 65  |achieved by pale|
00000a50  74 74 65 0d 73 77 69 74  63 68 69 6e 67 20 61 73  |tte.switching as|
00000a60  20 75 73 65 64 20 69 6e  20 74 68 65 20 70 72 65  | used in the pre|
00000a70  76 69 6f 75 73 20 61 72  74 69 63 6c 65 2e 20 49  |vious article. I|
00000a80  66 20 79 6f 75 20 6c 6f  6f 6b 20 61 67 61 69 6e  |f you look again|
00000a90  20 61 74 20 74 68 65 0d  70 72 6f 67 72 61 6d 6d  | at the.programm|
00000aa0  69 6e 67 20 74 65 63 68  6e 69 71 75 65 20 64 65  |ing technique de|
00000ab0  6d 6f 6e 73 74 72 61 74  69 6f 6e 20 66 72 6f 6d  |monstration from|
00000ac0  20 74 68 69 73 20 61 72  74 69 63 6c 65 20 28 6f  | this article (o|
00000ad0  6e 20 45 55 47 20 33 36  29 2c 20 61 6e 64 0d 73  |n EUG 36), and.s|
00000ae0  65 6c 65 63 74 20 6f 70  74 69 6f 6e 20 35 2c 20  |elect option 5, |
00000af0  74 68 65 20 73 61 6d 65  20 65 66 66 65 63 74 20  |the same effect |
00000b00  77 69 6c 6c 20 61 70 70  65 61 72 20 6f 6e 20 79  |will appear on y|
00000b10  6f 75 72 20 73 63 72 65  65 6e 2e 20 54 68 65 20  |our screen. The |
00000b20  73 61 6d 65 0d 70 72 69  6e 63 69 70 61 6c 20 77  |same.principal w|
00000b30  61 73 20 61 70 70 6c 69  65 64 20 69 6e 20 62 6f  |as applied in bo|
00000b40  74 68 20 61 6e 69 6d 61  74 65 64 20 73 63 72 65  |th animated scre|
00000b50  65 6e 73 20 6d 65 6e 74  69 6f 6e 65 64 20 61 62  |ens mentioned ab|
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00001a80  0d 77 61 6e 74 20 69 74  2e 20 49 66 20 79 6f 75  |.want it. If you|
00001a90  20 77 61 6e 74 20 69 74  2c 20 73 61 79 2c 20 69  | want it, say, i|
00001aa0  6e 20 74 68 65 20 66 69  66 74 68 20 72 6f 77 20  |n the fifth row |
00001ab0  64 6f 77 6e 2c 20 61 6e  64 20 74 77 65 6c 76 65  |down, and twelve|
00001ac0  20 63 68 61 72 61 63 74  65 72 73 0d 61 6c 6f 6e  | characters.alon|
00001ad0  67 2c 20 79 6f 75 20 63  61 6e 20 63 61 6c 63 75  |g, you can calcu|
00001ae0  6c 61 74 65 20 77 68 65  72 65 20 61 62 6f 75 74  |late where about|
00001af0  73 20 74 68 69 73 20 63  6f 72 72 65 73 70 6f 6e  |s this correspon|
00001b00  64 73 20 74 6f 20 69 6e  20 73 63 72 65 65 6e 20  |ds to in screen |
00001b10  6d 65 6d 6f 72 79 2c 0d  61 6e 64 20 63 6f 70 79  |memory,.and copy|
00001b20  20 74 68 65 20 62 6c 6f  63 6b 20 74 6f 20 74 68  | the block to th|
00001b30  69 73 20 6c 6f 63 61 74  69 6f 6e 2e 20 41 73 20  |is location. As |
00001b40  74 68 65 20 62 6c 6f 63  6b 20 69 73 20 73 74 6f  |the block is sto|
00001b50  72 65 64 20 69 6e 20 79  6f 75 72 0d 63 6f 6d 70  |red in your.comp|
00001b60  75 74 65 72 27 73 20 52  41 4d 2c 20 61 73 20 6f  |uter's RAM, as o|
00001b70  70 70 6f 73 65 64 20 74  6f 20 6f 6e 20 66 6c 6f  |pposed to on flo|
00001b80  70 70 79 2c 20 74 68 69  73 20 74 72 61 6e 73 66  |ppy, this transf|
00001b90  65 72 20 63 61 6e 20 62  65 20 64 6f 6e 65 20 62  |er can be done b|
00001ba0  79 0d 6d 61 63 68 69 6e  65 20 63 6f 64 65 20 61  |y.machine code a|
00001bb0  74 20 76 65 72 79 20 67  72 65 61 74 20 73 70 65  |t very great spe|
00001bc0  65 64 20 28 74 61 6b 69  6e 67 20 6f 6e 6c 79 20  |ed (taking only |
00001bd0  61 20 73 6d 61 6c 6c 20  66 72 61 63 74 69 6f 6e  |a small fraction|
00001be0  20 6f 66 20 61 20 73 65  63 6f 6e 64 29 2e 0d 46  | of a second)..F|
00001bf0  6f 72 20 74 68 6f 73 65  20 70 72 6f 67 72 61 6d  |or those program|
00001c00  6d 65 72 73 20 77 68 6f  20 61 72 65 20 6e 6f 74  |mers who are not|
00001c10  20 66 61 6d 69 6c 69 61  72 20 77 69 74 68 20 6d  | familiar with m|
00001c20  61 63 68 69 6e 65 20 63  6f 64 65 2c 20 61 20 6d  |achine code, a m|
00001c30  6f 64 75 6c 65 0d 69 73  20 69 6e 63 6c 75 64 65  |odule.is include|
00001c40  64 20 77 69 74 68 20 74  68 69 73 20 74 75 74 6f  |d with this tuto|
00001c50  72 69 61 6c 20 61 6c 6c  6f 77 69 6e 67 20 79 6f  |rial allowing yo|
00001c60  75 20 74 6f 20 64 6f 20  74 68 69 73 2e 20 54 68  |u to do this. Th|
00001c70  65 20 70 72 6f 67 72 61  6d 20 63 6f 64 65 0d 66  |e program code.f|
00001c80  6f 72 20 74 68 69 73 20  6d 6f 64 75 6c 65 20 69  |or this module i|
00001c90  73 20 61 63 74 75 61 6c  6c 79 20 74 61 6b 65 6e  |s actually taken|
00001ca0  20 64 69 72 65 63 74 6c  79 20 66 72 6f 6d 20 53  | directly from S|
00001cb0  68 69 70 77 72 65 63 6b  65 64 2e 0d              |hipwrecked..|
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PT3/+P2.m2
PT3/+P2.m4
PT3/+P2.m5

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