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+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_User_Group » EUG_38.ADF
Filename: +P1
Read OK: ✔
File size: 1550 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-
that's 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.


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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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00000b90  61 6e 20 74 72 61 6e 73  66 65 72 20 74 68 65 20  |an transfer the |
00000ba0  65 6e 74 69 72 65 0d 63  6f 6e 74 65 6e 74 73 20  |entire.contents |
00000bb0  6f 66 20 74 68 65 20 73  63 72 65 65 6e 20 6d 65  |of the screen me|
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00000bd0  70 75 74 65 72 20 74 6f  20 66 6c 6f 70 70 79 20  |puter to floppy |
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00000bf0  74 68 65 20 66 69 6e 61  6c 20 70 72 6f 67 72 61  |the final progra|
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