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eug_titles/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 hard disk » misc » misc3 |
| Filename: | eug_titles/PT3/+P2 |
| Read OK: | ✔ |
| File size: | 1CBC bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
Duplicates
There is 1 duplicate copy of this file in the archive:
- Personal collection » Acorn ADFS disks » Electron » EUG_submission.ADF » PT3/+P2
- Personal collection » Acorn hard disk » misc » misc3 » eug_titles/PT3/+P2
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| 00000b60 6f 76 65 20 2d 20 74 68 65 20 6f 6e 6c 79 0d 64 |ove - the only.d| 00000b70 69 66 66 65 72 65 6e 63 65 20 69 73 20 74 68 61 |ifference is tha| 00000b80 74 20 69 6e 20 74 68 65 73 65 20 74 69 74 6c 65 |t in these title| 00000b90 20 73 63 72 65 65 6e 73 20 74 68 65 20 70 61 74 | screens the pat| 00000ba0 74 65 72 6e 20 6f 6e 20 73 63 72 65 65 6e 20 77 |tern on screen w| 00000bb0 68 69 63 68 20 69 73 0d 62 65 69 6e 67 20 70 61 |hich is.being pa| 00000bc0 6c 65 74 74 65 20 73 77 69 74 63 68 65 64 20 69 |lette switched i| 00000bd0 73 20 72 61 74 68 65 72 20 6d 6f 72 65 20 63 6f |s rather more co| 00000be0 6d 70 6c 65 78 2e 20 53 68 69 70 77 72 65 63 6b |mplex. Shipwreck| 00000bf0 27 73 20 74 69 74 6c 65 20 73 63 72 65 65 6e 0d |'s title screen.| 00000c00 28 77 69 74 68 20 74 68 65 20 61 6e 69 6d 61 74 |(with the animat| 00000c10 65 64 20 77 61 74 65 72 20 73 63 65 6e 65 29 20 |ed water scene) | 00000c20 77 61 73 20 61 6c 73 6f 20 63 72 65 61 74 65 64 |was also created| 00000c30 20 69 6e 20 74 68 69 73 20 77 61 79 2c 20 65 78 | in this way, ex| 00000c40 63 65 70 74 20 74 68 61 74 0d 66 6f 72 20 74 68 |cept that.for th| 00000c50 69 73 20 73 63 72 65 65 6e 20 74 68 65 72 65 20 |is screen there | 00000c60 77 61 73 20 61 6c 73 6f 20 61 20 6d 61 63 68 69 |was also a machi| 00000c70 6e 65 20 63 6f 64 65 20 70 72 6f 67 72 61 6d 20 |ne code program | 00000c80 61 6c 74 65 72 69 6e 67 20 74 68 65 20 63 6f 6e |altering the con| 00000c90 74 65 6e 74 73 0d 6f 66 20 73 63 72 65 65 6e 20 |tents.of screen | 00000ca0 6d 65 6d 6f 72 79 20 61 73 20 77 65 6c 6c 2c 20 |memory as well, | 00000cb0 74 6f 20 65 6e 68 61 6e 63 65 20 74 68 65 20 65 |to enhance the e| 00000cc0 66 66 65 63 74 2e 0d 0d 53 6f 2c 20 77 65 20 68 |ffect...So, we h| 00000cd0 61 76 65 20 61 20 74 65 63 68 6e 69 71 75 65 20 |ave a technique | 00000ce0 77 68 69 63 68 20 61 6c 6c 6f 77 73 20 75 73 20 |which allows us | 00000cf0 74 6f 20 73 74 6f 72 65 20 61 20 73 6d 61 6c 6c |to store a small| 00000d00 20 6e 75 6d 62 65 72 20 6f 66 20 76 65 72 79 0d | number of very.| 00000d10 64 65 74 61 69 6c 65 64 20 73 63 72 65 65 6e 73 |detailed screens| 00000d20 20 6f 6e 20 66 6c 6f 70 70 79 2c 20 61 6e 64 20 | on floppy, and | 00000d30 72 65 63 61 6c 6c 20 74 68 65 6e 20 61 74 20 76 |recall then at v| 00000d40 65 72 79 20 67 72 65 61 74 20 73 70 65 65 64 2e |ery great speed.| 00000d50 20 42 75 74 20 77 68 61 74 20 69 66 0d 77 65 20 | But what if.we | 00000d60 77 61 6e 74 20 74 6f 20 75 73 65 20 74 68 69 73 |want to use this| 00000d70 20 6d 65 74 68 6f 64 20 66 6f 72 20 61 20 6c 61 | method for a la| 00000d80 72 67 65 20 6e 75 6d 62 65 72 20 6f 66 20 73 63 |rge number of sc| 00000d90 72 65 65 6e 73 3f 20 41 6e 64 20 77 68 61 74 20 |reens? 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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..| 00001cbc
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