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PT3/+P1

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/+P1
Read OK: ✔
File size: 1B5F bytes
Load address: 2B204556
Exec address: D3150
Duplicates

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

File contents
Programming Technique
Part 3: Graphics - Screen Memory

Written by Dominic Ford

At the end of the last article, you could program graphics in colour using
the built in MOVE and DRAW commands in BASIC, and could program fast,
detailed graphics in black and white using user defined characters. But
there was a problem if we wanted graphics in both colour and detail, as
by both methods the production of such graphics would be incredibly slow.

There is a solution to this problem, which involves taking advantage of the
way in which the computer's video system operates. The colour of each pixel
on your monitor is determined the data sent to the monitor by your computer -
thats fairly obvious. But if you have drawn a very complex set of shapes on
the computer screen, perhaps using a BASIC program which took several hours
to calculate and draw the result you see on the screen, how does the 
computer remember the colour of every pixel on the screen, and know what
data to send to your monitor? The answer is simple: within your computer's
memory there is an area set aside to store the colour of every pixel on the
screen, and when the monitor requests the colour of a particular pixel, the
computer simply reads the colour of the pixel from this area of memory.
This area of memory is called "Screen Memory". It can be considered as a
sort of giant table, holding data for the colours of all of the pixels in
a huge matrix of memory locations.

But of what use is this to the programmer you might be wondering. The
answer is that when you use user defined characters, MOVE and DRAW commands,
or any other method of putting anything on the screen, the computer simply
alters a few values in this giant table. But the computer takes time to
process your MOVE and DRAW commands to work out which values in the table
to alter. If you could bypass this stage, and set the values in this giant
table yourself without using the computer's general purpose graphics
commands, you can get exactly the result that you want, and much faster
than you would achieve if you were doing the operation through the
computer's commands. These have to process the parameters which you give,
and then calculate where abouts in the giant table you want to alter,
before they can actually do any useful work. Thus a lot of computer time
is wasted in performing calculations, which can actually be skipped if the
programmer is careful.

There are two possible ways of achieving this idea, both of which are useful
in different situations:

1. Save screen memory to disc.

This idea is fairly simple to understand. You, as the programmer, write a
program to plot the artwork for your program onto the screen, taking
however many hours it may take to complete all of the calculations
required to plot the graphics. You run this program once on your computer,
so that your masterpiece is stored in the screen memory of your computer
(ie. is on the monitor of your computer). You than transfer the entire
contents of the screen memory on your computer to floppy disc.

Then, in the final program which you want the user to operate, you include
a command to load this data back from floppy disc into the screen memory
of their computer. In a matter of a couple of seconds, graphics which took
possible hours to calculate will appear on their screen. This is a
technique which I have used in several of my programs. The main menu of
Shipwrecked II: Jupiter 3, where you select Play, Instructions, Game
Complete or Cheat with the starfield backdrop was done in this way - the
screen took nearly an hour for my computer to calculate, but I was able
to make it appear in seconds on your screen. I should also mention the title
screen to Shipwreck with the animated sea, not to mention the endless EUG
title screens I have done in this way - the Christmas EUG 35 screen with
the swirly blue background (credited erroneously to Robert Sprowson) took
two whole days to plot!

In order to achieve this, you simply need to understand two BASIC commands:

*SAVE Filename nnnn mmmm
*LOAD Filename nnnn

These load and save areas of the computer's memory to floppy disc or cassette
with the filename givan in place of the work Filename, starting at memory
location nnnn, and continuing until memory location mmmm (where nnnn and mmmm
are both in hexidecimal - base 16). Don't worry if you don't entirely
understand this - it is not necessary to entirely understand the workings
of these commands, or of hexidecimal to use them to store and retrieve
screen memory. All you need to do is substitute nnnn and mmmm for the
addresses of the start and finish of screen memory. The location of these
within your computer varies depeding upon which graphics mode you are using
(which you selected using the MODE command - see previous tutorial if you
are unsure of what this is), but is always constant for any particular
mode. the values for the different modes are given below:

Mode            nnnn    mmmm   NOTES:
0               3000    7FFF
1               3000    7FFF   - nnnn and mmmm values given to the left are
2               3000    7FFF   all in hexidecimal. Therefore they can be
3               4000    7FFF   substituted directly into the *LOAD and *SAVE
4               5800    7FFF   commands with no conversion required.
5               5800    7FFF
6               6000    7FFF   - Values are not valid for the BBC model A in
7               7C00    7FFF   modes 4,5,6 or 7.

Examples:

Suppose you are in mode 2, and want to store the screen to a floppy. From
the table above, nnnn is 3000 and mmmm is 7FFF for mode 2. Therefore, to
save the screen under the filename "Screen", you would type:

*SAVE Screen 3000 7FFF

Now, if you want to restore the screen again, and put your graphics back
on the screen again, you would use *LOAD. Notice that this time only a
value for nnnn is required, and not for mmmm:

*LOAD Screen 3000

You can try this out now, if you switch on your Electron, and insert a work
disc into your floppy drive. If you use ADFS, you may need to use *MOUNT
to select the floppy disc, and be able to write to it. Now enter mode 2
by typing:

MODE 2

Now type some text on the screen which will represent the graphics which
you want to save. When you have finished, save the contents of screen memory
to floppy disk as shown above, by typing:

*SAVE Screen 3000 7FFF

When the disk drive has finished, clear the text off the screen by typing:

CLS

The text which you typed onto the screen has now been wiped from the
computer's memory, but should still be stored on the floppy disc. We can
show that this is so by typing:

*LOAD Screen 3000

Your text should now reappear on the screen. You can repeat this with any
kind of complex graphics on screen, and it will still work. The same can
be done in any other graphics mode, simply by changing the values of nnnn
and mmmm to those given in the table above. For example in mode 5 you
would use:

*SAVE Screen 5800 7FFF

followed by:

*LOAD Screen 5800 7FFF

00000000  50 72 6f 67 72 61 6d 6d  69 6e 67 20 54 65 63 68  |Programming Tech|
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00000030  4d 65 6d 6f 72 79 0d 0d  57 72 69 74 74 65 6e 20  |Memory..Written |
00000040  62 79 20 44 6f 6d 69 6e  69 63 20 46 6f 72 64 0d  |by Dominic Ford.|
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00001a20  59 6f 75 20 63 61 6e 20  72 65 70 65 61 74 20 74  |You can repeat t|
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00001a40  64 20 6f 66 20 63 6f 6d  70 6c 65 78 20 67 72 61  |d of complex gra|
00001a50  70 68 69 63 73 20 6f 6e  20 73 63 72 65 65 6e 2c  |phics on screen,|
00001a60  20 61 6e 64 20 69 74 20  77 69 6c 6c 20 73 74 69  | and it will sti|
00001a70  6c 6c 20 77 6f 72 6b 2e  20 54 68 65 20 73 61 6d  |ll work. The sam|
00001a80  65 20 63 61 6e 0d 62 65  20 64 6f 6e 65 20 69 6e  |e can.be done in|
00001a90  20 61 6e 79 20 6f 74 68  65 72 20 67 72 61 70 68  | any other graph|
00001aa0  69 63 73 20 6d 6f 64 65  2c 20 73 69 6d 70 6c 79  |ics mode, simply|
00001ab0  20 62 79 20 63 68 61 6e  67 69 6e 67 20 74 68 65  | by changing the|
00001ac0  20 76 61 6c 75 65 73 20  6f 66 20 6e 6e 6e 6e 0d  | values of nnnn.|
00001ad0  61 6e 64 20 6d 6d 6d 6d  20 74 6f 20 74 68 6f 73  |and mmmm to thos|
00001ae0  65 20 67 69 76 65 6e 20  69 6e 20 74 68 65 20 74  |e given in the t|
00001af0  61 62 6c 65 20 61 62 6f  76 65 2e 20 46 6f 72 20  |able above. For |
00001b00  65 78 61 6d 70 6c 65 20  69 6e 20 6d 6f 64 65 20  |example in mode |
00001b10  35 20 79 6f 75 0d 77 6f  75 6c 64 20 75 73 65 3a  |5 you.would use:|
00001b20  0d 0d 2a 53 41 56 45 20  53 63 72 65 65 6e 20 35  |..*SAVE Screen 5|
00001b30  38 30 30 20 37 46 46 46  0d 0d 66 6f 6c 6c 6f 77  |800 7FFF..follow|
00001b40  65 64 20 62 79 3a 0d 0d  2a 4c 4f 41 44 20 53 63  |ed by:..*LOAD Sc|
00001b50  72 65 65 6e 20 35 38 30  30 20 37 46 46 46 0d     |reen 5800 7FFF.|
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