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P/+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_39.ADF
Filename: P/+P1
Read OK: ✔
File size: 0CBF bytes
Load address: D6576
Exec address: 0000
Duplicates

There are 2 duplicate copies of this file in the archive:

File contents
Programming Technique
Part 4: Graphics - Screen Memory access from BASIC

Written by Dominic Ford

In the last EUG issue I described how screen memory is laid out, and how
this knowledge can be of use to the programmer. The problem is that this
is all of little use in BASIC - your program would crawl along if it tried
to display graphics by directly accessing screen memory from BASIC. The
only technique which is of use if you don't know machine code is that of
using *LOAD and *SAVE to reload a previous screen memory display.

This doesn't mean, however, that it is impossible for BASIC programmers to
make use of screen memory techniques. The only part of your program which
has to be an machine code is the part which accesses screen memory - the
rest can be in BASIC. Let's say, for example, that you are writing a game
which requires a sprite to move across the screen. The part of the program
which physically plots the sprite into screen memory must be in machine
code to be able to achieve reasonable speed. But the part of the program
which manipulates the sprite, keeps track of your score, checks if you've
walking into an enemy sprite, etc. can all be done in BASIC.

This means that if BASIC programmers could have a module of machine code
which does the screen memory access for them, and which they can call
every time the want to display a sprite on the screen, they too could make
use of this powerful technique. That is exactly what I am providing in
this issue of EUG - a small machine code module which BASIC programmers
can use to plot multi-coloured sprites onto the screen. There are even a
couple of sample demonstrations to show how the module can be put to use.

Obviously such a system does have some limitations - whereas machine
code programmers can devise a specialist module for the particular needs
of their program, this module does force the user into following certain
rules in their program. The main limitations are:

- Your program must operate in MODE 2 when the module is being used to
produce the graphics.
- Your sprites must consist of blocks of 8x8 pixels (one character),
although larger sprites can be used simply by tiling several one character
sprites together, side by side.
- Your program must not make use of memory locations above &2C00. This
does put quite a severe size limit on your program on an ADFS system. If
you recieve "No Room" error messages when you try to use your program,
you've hit that limit!
- You must not use conventional VDU23 style user-defined characters in
your program, because this module uses the same workspace as these.

Before you can use the module, you must design some sprites for it to
display. This is done using the program 'Editor' which is available at the
end of this article, or can be found in the 'P' directory of this EUG
disc. If you don't want to get artistic at this stage of trying out the
module, there are a couple of files of sample sprites included with the
editor: 'B+B' are those used by the bat and ball demonstration and
'WalkSpr' are those used by the 'Walking' demonstration. For the moment I
will assume that you are using these sample sprite files - more details on
how to create your own sprite files using the editor program will be given
later.

00000000  50 72 6f 67 72 61 6d 6d  69 6e 67 20 54 65 63 68  |Programming Tech|
00000010  6e 69 71 75 65 0d 50 61  72 74 20 34 3a 20 47 72  |nique.Part 4: Gr|
00000020  61 70 68 69 63 73 20 2d  20 53 63 72 65 65 6e 20  |aphics - Screen |
00000030  4d 65 6d 6f 72 79 20 61  63 63 65 73 73 20 66 72  |Memory access fr|
00000040  6f 6d 20 42 41 53 49 43  0d 0d 57 72 69 74 74 65  |om BASIC..Writte|
00000050  6e 20 62 79 20 44 6f 6d  69 6e 69 63 20 46 6f 72  |n by Dominic For|
00000060  64 0d 0d 49 6e 20 74 68  65 20 6c 61 73 74 20 45  |d..In the last E|
00000070  55 47 20 69 73 73 75 65  20 49 20 64 65 73 63 72  |UG issue I descr|
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000000b0  20 6b 6e 6f 77 6c 65 64  67 65 20 63 61 6e 20 62  | knowledge can b|
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000001c0  20 63 6f 64 65 20 69 73  20 74 68 61 74 20 6f 66  | code is that of|
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000003d0  6f 64 65 20 74 6f 20 62  65 20 61 62 6c 65 20 74  |ode to be able t|
000003e0  6f 20 61 63 68 69 65 76  65 20 72 65 61 73 6f 6e  |o achieve reason|
000003f0  61 62 6c 65 20 73 70 65  65 64 2e 20 42 75 74 20  |able speed. But |
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00000480  65 74 63 2e 20 63 61 6e  20 61 6c 6c 20 62 65 20  |etc. can all be |
00000490  64 6f 6e 65 20 69 6e 20  42 41 53 49 43 2e 0d 0d  |done in BASIC...|
000004a0  54 68 69 73 20 6d 65 61  6e 73 20 74 68 61 74 20  |This means that |
000004b0  69 66 20 42 41 53 49 43  20 70 72 6f 67 72 61 6d  |if BASIC program|
000004c0  6d 65 72 73 20 63 6f 75  6c 64 20 68 61 76 65 20  |mers could have |
000004d0  61 20 6d 6f 64 75 6c 65  20 6f 66 20 6d 61 63 68  |a module of mach|
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00000540  6e 74 20 74 6f 20 64 69  73 70 6c 61 79 20 61 20  |nt to display a |
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000005c0  0d 74 68 69 73 20 69 73  73 75 65 20 6f 66 20 45  |.this issue of E|
000005d0  55 47 20 2d 20 61 20 73  6d 61 6c 6c 20 6d 61 63  |UG - a small mac|
000005e0  68 69 6e 65 20 63 6f 64  65 20 6d 6f 64 75 6c 65  |hine code module|
000005f0  20 77 68 69 63 68 20 42  41 53 49 43 20 70 72 6f  | which BASIC pro|
00000600  67 72 61 6d 6d 65 72 73  0d 63 61 6e 20 75 73 65  |grammers.can use|
00000610  20 74 6f 20 70 6c 6f 74  20 6d 75 6c 74 69 2d 63  | to plot multi-c|
00000620  6f 6c 6f 75 72 65 64 20  73 70 72 69 74 65 73 20  |oloured sprites |
00000630  6f 6e 74 6f 20 74 68 65  20 73 63 72 65 65 6e 2e  |onto the screen.|
00000640  20 54 68 65 72 65 20 61  72 65 20 65 76 65 6e 20  | There are even |
00000650  61 0d 63 6f 75 70 6c 65  20 6f 66 20 73 61 6d 70  |a.couple of samp|
00000660  6c 65 20 64 65 6d 6f 6e  73 74 72 61 74 69 6f 6e  |le demonstration|
00000670  73 20 74 6f 20 73 68 6f  77 20 68 6f 77 20 74 68  |s to show how th|
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000006b0  73 74 65 6d 20 64 6f 65  73 20 68 61 76 65 20 73  |stem does have s|
000006c0  6f 6d 65 20 6c 69 6d 69  74 61 74 69 6f 6e 73 20  |ome limitations |
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000006e0  65 0d 63 6f 64 65 20 70  72 6f 67 72 61 6d 6d 65  |e.code programme|
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00000750  72 63 65 20 74 68 65 20  75 73 65 72 20 69 6e 74  |rce the user int|
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00000770  61 69 6e 0d 72 75 6c 65  73 20 69 6e 20 74 68 65  |ain.rules in the|
00000780  69 72 20 70 72 6f 67 72  61 6d 2e 20 54 68 65 20  |ir program. The |
00000790  6d 61 69 6e 20 6c 69 6d  69 74 61 74 69 6f 6e 73  |main limitations|
000007a0  20 61 72 65 3a 0d 0d 2d  20 59 6f 75 72 20 70 72  | are:..- Your pr|
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000007d0  6e 20 74 68 65 20 6d 6f  64 75 6c 65 20 69 73 20  |n the module is |
000007e0  62 65 69 6e 67 20 75 73  65 64 20 74 6f 0d 70 72  |being used to.pr|
000007f0  6f 64 75 63 65 20 74 68  65 20 67 72 61 70 68 69  |oduce the graphi|
00000800  63 73 2e 0d 2d 20 59 6f  75 72 20 73 70 72 69 74  |cs..- Your sprit|
00000810  65 73 20 6d 75 73 74 20  63 6f 6e 73 69 73 74 20  |es must consist |
00000820  6f 66 20 62 6c 6f 63 6b  73 20 6f 66 20 38 78 38  |of blocks of 8x8|
00000830  20 70 69 78 65 6c 73 20  28 6f 6e 65 20 63 68 61  | pixels (one cha|
00000840  72 61 63 74 65 72 29 2c  0d 61 6c 74 68 6f 75 67  |racter),.althoug|
00000850  68 20 6c 61 72 67 65 72  20 73 70 72 69 74 65 73  |h larger sprites|
00000860  20 63 61 6e 20 62 65 20  75 73 65 64 20 73 69 6d  | can be used sim|
00000870  70 6c 79 20 62 79 20 74  69 6c 69 6e 67 20 73 65  |ply by tiling se|
00000880  76 65 72 61 6c 20 6f 6e  65 20 63 68 61 72 61 63  |veral one charac|
00000890  74 65 72 0d 73 70 72 69  74 65 73 20 74 6f 67 65  |ter.sprites toge|
000008a0  74 68 65 72 2c 20 73 69  64 65 20 62 79 20 73 69  |ther, side by si|
000008b0  64 65 2e 0d 2d 20 59 6f  75 72 20 70 72 6f 67 72  |de..- Your progr|
000008c0  61 6d 20 6d 75 73 74 20  6e 6f 74 20 6d 61 6b 65  |am must not make|
000008d0  20 75 73 65 20 6f 66 20  6d 65 6d 6f 72 79 20 6c  | use of memory l|
000008e0  6f 63 61 74 69 6f 6e 73  20 61 62 6f 76 65 20 26  |ocations above &|
000008f0  32 43 30 30 2e 20 54 68  69 73 0d 64 6f 65 73 20  |2C00. This.does |
00000900  70 75 74 20 71 75 69 74  65 20 61 20 73 65 76 65  |put quite a seve|
00000910  72 65 20 73 69 7a 65 20  6c 69 6d 69 74 20 6f 6e  |re size limit on|
00000920  20 79 6f 75 72 20 70 72  6f 67 72 61 6d 20 6f 6e  | your program on|
00000930  20 61 6e 20 41 44 46 53  20 73 79 73 74 65 6d 2e  | an ADFS system.|
00000940  20 49 66 0d 79 6f 75 20  72 65 63 69 65 76 65 20  | If.you recieve |
00000950  22 4e 6f 20 52 6f 6f 6d  22 20 65 72 72 6f 72 20  |"No Room" error |
00000960  6d 65 73 73 61 67 65 73  20 77 68 65 6e 20 79 6f  |messages when yo|
00000970  75 20 74 72 79 20 74 6f  20 75 73 65 20 79 6f 75  |u try to use you|
00000980  72 20 70 72 6f 67 72 61  6d 2c 0d 79 6f 75 27 76  |r program,.you'v|
00000990  65 20 68 69 74 20 74 68  61 74 20 6c 69 6d 69 74  |e hit that limit|
000009a0  21 0d 2d 20 59 6f 75 20  6d 75 73 74 20 6e 6f 74  |!.- You must not|
000009b0  20 75 73 65 20 63 6f 6e  76 65 6e 74 69 6f 6e 61  | use conventiona|
000009c0  6c 20 56 44 55 32 33 20  73 74 79 6c 65 20 75 73  |l VDU23 style us|
000009d0  65 72 2d 64 65 66 69 6e  65 64 20 63 68 61 72 61  |er-defined chara|
000009e0  63 74 65 72 73 20 69 6e  0d 79 6f 75 72 20 70 72  |cters in.your pr|
000009f0  6f 67 72 61 6d 2c 20 62  65 63 61 75 73 65 20 74  |ogram, because t|
00000a00  68 69 73 20 6d 6f 64 75  6c 65 20 75 73 65 73 20  |his module uses |
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00000a20  63 65 20 61 73 20 74 68  65 73 65 2e 0d 0d 42 65  |ce as these...Be|
00000a30  66 6f 72 65 20 79 6f 75  20 63 61 6e 20 75 73 65  |fore you can use|
00000a40  20 74 68 65 20 6d 6f 64  75 6c 65 2c 20 79 6f 75  | the module, you|
00000a50  20 6d 75 73 74 20 64 65  73 69 67 6e 20 73 6f 6d  | must design som|
00000a60  65 20 73 70 72 69 74 65  73 20 66 6f 72 20 69 74  |e sprites for it|
00000a70  20 74 6f 0d 64 69 73 70  6c 61 79 2e 20 54 68 69  | to.display. Thi|
00000a80  73 20 69 73 20 64 6f 6e  65 20 75 73 69 6e 67 20  |s is done using |
00000a90  74 68 65 20 70 72 6f 67  72 61 6d 20 27 45 64 69  |the program 'Edi|
00000aa0  74 6f 72 27 20 77 68 69  63 68 20 69 73 20 61 76  |tor' which is av|
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00000ad0  6c 65 2c 20 6f 72 20 63  61 6e 20 62 65 20 66 6f  |le, or can be fo|
00000ae0  75 6e 64 20 69 6e 20 74  68 65 20 27 50 27 20 64  |und in the 'P' d|
00000af0  69 72 65 63 74 6f 72 79  20 6f 66 20 74 68 69 73  |irectory of this|
00000b00  20 45 55 47 0d 64 69 73  63 2e 20 49 66 20 79 6f  | EUG.disc. If yo|
00000b10  75 20 64 6f 6e 27 74 20  77 61 6e 74 20 74 6f 20  |u don't want to |
00000b20  67 65 74 20 61 72 74 69  73 74 69 63 20 61 74 20  |get artistic at |
00000b30  74 68 69 73 20 73 74 61  67 65 20 6f 66 20 74 72  |this stage of tr|
00000b40  79 69 6e 67 20 6f 75 74  20 74 68 65 0d 6d 6f 64  |ying out the.mod|
00000b50  75 6c 65 2c 20 74 68 65  72 65 20 61 72 65 20 61  |ule, there are a|
00000b60  20 63 6f 75 70 6c 65 20  6f 66 20 66 69 6c 65 73  | couple of files|
00000b70  20 6f 66 20 73 61 6d 70  6c 65 20 73 70 72 69 74  | of sample sprit|
00000b80  65 73 20 69 6e 63 6c 75  64 65 64 20 77 69 74 68  |es included with|
00000b90  20 74 68 65 0d 65 64 69  74 6f 72 3a 20 27 42 2b  | the.editor: 'B+|
00000ba0  42 27 20 61 72 65 20 74  68 6f 73 65 20 75 73 65  |B' are those use|
00000bb0  64 20 62 79 20 74 68 65  20 62 61 74 20 61 6e 64  |d by the bat and|
00000bc0  20 62 61 6c 6c 20 64 65  6d 6f 6e 73 74 72 61 74  | ball demonstrat|
00000bd0  69 6f 6e 20 61 6e 64 0d  27 57 61 6c 6b 53 70 72  |ion and.'WalkSpr|
00000be0  27 20 61 72 65 20 74 68  6f 73 65 20 75 73 65 64  |' are those used|
00000bf0  20 62 79 20 74 68 65 20  27 57 61 6c 6b 69 6e 67  | by the 'Walking|
00000c00  27 20 64 65 6d 6f 6e 73  74 72 61 74 69 6f 6e 2e  |' demonstration.|
00000c10  20 46 6f 72 20 74 68 65  20 6d 6f 6d 65 6e 74 20  | For the moment |
00000c20  49 0d 77 69 6c 6c 20 61  73 73 75 6d 65 20 74 68  |I.will assume th|
00000c30  61 74 20 79 6f 75 20 61  72 65 20 75 73 69 6e 67  |at you are using|
00000c40  20 74 68 65 73 65 20 73  61 6d 70 6c 65 20 73 70  | these sample sp|
00000c50  72 69 74 65 20 66 69 6c  65 73 20 2d 20 6d 6f 72  |rite files - mor|
00000c60  65 20 64 65 74 61 69 6c  73 20 6f 6e 0d 68 6f 77  |e details on.how|
00000c70  20 74 6f 20 63 72 65 61  74 65 20 79 6f 75 72 20  | to create your |
00000c80  6f 77 6e 20 73 70 72 69  74 65 20 66 69 6c 65 73  |own sprite files|
00000c90  20 75 73 69 6e 67 20 74  68 65 20 65 64 69 74 6f  | using the edito|
00000ca0  72 20 70 72 6f 67 72 61  6d 20 77 69 6c 6c 20 62  |r program will b|
00000cb0  65 20 67 69 76 65 6e 0d  6c 61 74 65 72 2e 0d     |e given.later..|
00000cbf
P/+P1.m0
P/+P1.m1
P/+P1.m2
P/+P1.m4
P/+P1.m5

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