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shipwren/+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 hard disk » misc » misc3 |
| Filename: | shipwren/+P1 |
| Read OK: | ✔ |
| File size: | 1633 bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
Duplicates
There is 1 duplicate copy of this file in the archive:
- Personal collection » Acorn ADFS disks » Archimedes » Unlabelled_disk_2.ADF » +P1
- Personal collection » Acorn hard disk » misc » misc3 » shipwren/+P1
File contents
PROGRAMMING TECHNIQUE
by Dominic Ford
Part 2: Graphics
The most important decision which you must make when you are
writing the graphics routines for a program is which graphics mode
to use. There is seven different display types available on the
electron, and an eighth is available on the BBC. Each display type
has its own characteristics, and you must choose which is most suitable
for your particular program. You select graphics modes by using the
MODE x command. Details of the seven different available modes are
given below:
x Colours Graphics? Res/G Res/T Memory
0 2 Yes 640x256 80x32 20K
1 4 Yes 320x256 40x32 20K
2 16 Yes 160x256 20x32 20K
3 2 No N/A 80x25 16K
4 2 Yes 320x256 40x32 10K
5 4 Yes 160x256 20x32 10K
6 2 No N/A 40x25 8K
7 16 No N/A 40x25 1K <- BBC ONLY!
KEY:
x: MODE number.
Colours: The maximum number of different colours which may be
used on screen at any one time.
Graphics?: Does this graphics mode allow the use of the graphics commands (ie. MOVE, DRAW, etc.)? Modes 3 & 6 do not allow these commands, as there are thin horizontal stripes in between the rows of characters, in which nothing can be displayed. Thus, any graphics drawn in these modes would have black horizontal stripes across them, and would look rather silly! To see what I mean, turn on your computer and type: MODE 6:COLOUR 128:COLOUR 0:CLS
Res/G: The resolution of the screen in this mode (ie. the number of pixels across and down the screen. Generally, the higher the value, the smoother your graphics will look. Note that this is not the same as the parameters taken by the MOVE and DRAW commands - these always scale the number of pixels across the screen to 1280 and down the screen to 1024, so that (1280,1024) will always be the top-right corner of the screen, and (0,0) the bottom left.
Res/T: The number of characters which fit across the screen in this mode when using text.
Memory: The amount of memory used by the graphics mode. If you are writing a long program, this is an important consideration, as you may not have room for the program after subtracting 20K from the computer's 32K RAM for a MODE 2 screen.
Having decided which graphics mode you are going to use, you
must decide how you are going to get your graphics onto the screen.
There are many methods for producing graphics, and which you
choose depends upon what you want the graphics for, and what you
want them to do (and, of course, your programming ability). The
most obvious is the PRINT command. This is a quick, easy way of
putting text onto the screen. When used with the COLOUR command
and the TAB(x,y), many interesting effects can be achieved.
These are, however, greatly limited by the fact that only letters
can be displayed on the screen. This limitation may be overcome using
user defined characters (see the Graphics chapter of the User Manual
for details). Even using these characters, however, there are serious
limitations. They can only use one colour per character, and so even
if you make different characters different colours, your screen will
still appear rather dull. Also, they are difficult to use when
dealing with large areas of the screen.
This is where MOVE, DRAW and PLOT are useful. Using these
commands, lines can be drawn across the screen, and, using PLOT,
triangles can be drawn. These commands are covered in the
Graphics chapter of the user manual. The other major advantage of
these graphics commands is that they can be used not just to set
the colour of an area, but also to invert it. The GCOL command,
which selects the colour in which graphics are displayed, takes
two parameters:
GCOL x,c
c is the new colour in which graphics should be drawn.
x selects how this colour should be applyed:
x=0 means to set every pixel to that colour, regardless of its
previous colour.
x=1 means to mix the light from the pixel already on the screen
with that of the new colour. For example, if a red pixel is set
to blue, the red and blue mix to form purple.
x=2 means that the light from the pixels is filtered with the new
colour. If a red pixel is set to blue now, it is as if a blue
filter has been placed in front of the red light, and the
resultant colour is black (red light contains no blue component).
x=3 means to invert the old colour with the new colour.
x=4 means to set the pixels with the inverse of the new colour
(regardless of the old colour).
At first sight, it may seem that user defined characters serve no purpose once a programmer has learnt about the more flexible graphics commands, but this is not the case. If you try to produce detailed graphics using MOVE and DRAW, you will find that your program crawls along at an unacceptable rate. User defined characters, on the other hand, can produce detail quickly, especially if the pattern repeats, so that one character definition can be used repeatedly in several areas of the screen.
So, it seems that programmers are faced with a difficult problem. They can use graphics commands and get colour, or they can use user defined characters and get detail, but what if they want both at the same time? There must be a solution - how else would games like Citadel manage to produce such detailed quality graphics, and at such a speed?
There are, in fact, two possible solutions, both of which involve dea;ing with an area of memory known as screen memory. This is, however, very complicated, and will take an entire article to cover... No prizes for guessing what the subject will be next time!
00000000 50 52 4f 47 52 41 4d 4d 49 4e 47 20 54 45 43 48 |PROGRAMMING TECH| 00000010 4e 49 51 55 45 0d 62 79 20 44 6f 6d 69 6e 69 63 |NIQUE.by Dominic| 00000020 20 46 6f 72 64 0d 50 61 72 74 20 32 3a 20 47 72 | Ford.Part 2: Gr| 00000030 61 70 68 69 63 73 0d 0d 20 20 20 20 20 54 68 65 |aphics.. The| 00000040 20 6d 6f 73 74 20 69 6d 70 6f 72 74 61 6e 74 20 | most important | 00000050 64 65 63 69 73 69 6f 6e 20 77 68 69 63 68 20 79 |decision which y| 00000060 6f 75 20 6d 75 73 74 20 6d 61 6b 65 20 77 68 65 |ou must make whe| 00000070 6e 20 79 6f 75 20 61 72 65 20 0d 77 72 69 74 69 |n you are .writi| 00000080 6e 67 20 74 68 65 20 67 72 61 70 68 69 63 73 20 |ng the graphics | 00000090 72 6f 75 74 69 6e 65 73 20 66 6f 72 20 61 20 70 |routines for a p| 000000a0 72 6f 67 72 61 6d 20 69 73 20 77 68 69 63 68 20 |rogram is which | 000000b0 67 72 61 70 68 69 63 73 20 6d 6f 64 65 20 0d 74 |graphics mode .t| 000000c0 6f 20 75 73 65 2e 20 54 68 65 72 65 20 69 73 20 |o use. There is | 000000d0 73 65 76 65 6e 20 64 69 66 66 65 72 65 6e 74 20 |seven different | 000000e0 64 69 73 70 6c 61 79 20 74 79 70 65 73 20 61 76 |display types av| 000000f0 61 69 6c 61 62 6c 65 20 6f 6e 20 74 68 65 20 0d |ailable on the .| 00000100 65 6c 65 63 74 72 6f 6e 2c 20 61 6e 64 20 61 6e |electron, and an| 00000110 20 65 69 67 68 74 68 20 69 73 20 61 76 61 69 6c | eighth is avail| 00000120 61 62 6c 65 20 6f 6e 20 74 68 65 20 42 42 43 2e |able on the BBC.| 00000130 20 45 61 63 68 20 64 69 73 70 6c 61 79 20 74 79 | Each display ty| 00000140 70 65 20 0d 68 61 73 20 69 74 73 20 6f 77 6e 20 |pe .has its own | 00000150 63 68 61 72 61 63 74 65 72 69 73 74 69 63 73 2c |characteristics,| 00000160 20 61 6e 64 20 79 6f 75 20 6d 75 73 74 20 63 68 | and you must ch| 00000170 6f 6f 73 65 20 77 68 69 63 68 20 69 73 20 6d 6f |oose which is mo| 00000180 73 74 20 73 75 69 74 61 62 6c 65 20 0d 66 6f 72 |st suitable .for| 00000190 20 79 6f 75 72 20 70 61 72 74 69 63 75 6c 61 72 | your particular| 000001a0 20 70 72 6f 67 72 61 6d 2e 20 59 6f 75 20 73 65 | program. You se| 000001b0 6c 65 63 74 20 67 72 61 70 68 69 63 73 20 6d 6f |lect graphics mo| 000001c0 64 65 73 20 62 79 20 75 73 69 6e 67 20 74 68 65 |des by using the| 000001d0 20 0d 4d 4f 44 45 20 78 20 63 6f 6d 6d 61 6e 64 | .MODE x command| 000001e0 2e 20 44 65 74 61 69 6c 73 20 6f 66 20 74 68 65 |. Details of the| 000001f0 20 73 65 76 65 6e 20 64 69 66 66 65 72 65 6e 74 | seven different| 00000200 20 61 76 61 69 6c 61 62 6c 65 20 6d 6f 64 65 73 | available modes| 00000210 20 61 72 65 20 0d 67 69 76 65 6e 20 62 65 6c 6f | are .given belo| 00000220 77 3a 0d 0d 78 20 43 6f 6c 6f 75 72 73 20 47 72 |w:..x Colours Gr| 00000230 61 70 68 69 63 73 3f 20 52 65 73 2f 47 20 20 20 |aphics? Res/G | 00000240 52 65 73 2f 54 20 20 4d 65 6d 6f 72 79 0d 30 20 |Res/T Memory.0 | 00000250 20 20 32 20 20 20 20 20 20 59 65 73 20 20 20 20 | 2 Yes | 00000260 20 20 36 34 30 78 32 35 36 20 38 30 78 33 32 20 | 640x256 80x32 | 00000270 20 20 32 30 4b 0d 31 20 20 20 34 20 20 20 20 20 | 20K.1 4 | 00000280 20 59 65 73 20 20 20 20 20 20 33 32 30 78 32 35 | Yes 320x25| 00000290 36 20 34 30 78 33 32 20 20 20 32 30 4b 0d 32 20 |6 40x32 20K.2 | 000002a0 20 20 31 36 20 20 20 20 20 59 65 73 20 20 20 20 | 16 Yes | 000002b0 20 20 31 36 30 78 32 35 36 20 32 30 78 33 32 20 | 160x256 20x32 | 000002c0 20 20 32 30 4b 0d 33 20 20 20 32 20 20 20 20 20 | 20K.3 2 | 000002d0 20 4e 6f 20 20 20 20 20 20 20 4e 2f 41 20 20 20 | No N/A | 000002e0 20 20 38 30 78 32 35 20 20 20 31 36 4b 0d 34 20 | 80x25 16K.4 | 000002f0 20 20 32 20 20 20 20 20 20 59 65 73 20 20 20 20 | 2 Yes | 00000300 20 20 33 32 30 78 32 35 36 20 34 30 78 33 32 20 | 320x256 40x32 | 00000310 20 20 31 30 4b 0d 35 20 20 20 34 20 20 20 20 20 | 10K.5 4 | 00000320 20 59 65 73 20 20 20 20 20 20 31 36 30 78 32 35 | Yes 160x25| 00000330 36 20 32 30 78 33 32 20 20 20 31 30 4b 0d 36 20 |6 20x32 10K.6 | 00000340 20 20 32 20 20 20 20 20 20 4e 6f 20 20 20 20 20 | 2 No | 00000350 20 20 4e 2f 41 20 20 20 20 20 34 30 78 32 35 20 | N/A 40x25 | 00000360 20 20 38 4b 0d 37 20 20 20 31 36 20 20 20 20 20 | 8K.7 16 | 00000370 4e 6f 20 20 20 20 20 20 20 4e 2f 41 20 20 20 20 |No N/A | 00000380 20 34 30 78 32 35 20 20 20 31 4b 20 3c 2d 20 42 | 40x25 1K <- B| 00000390 42 43 20 4f 4e 4c 59 21 0d 0d 4b 45 59 3a 0d 0d |BC ONLY!..KEY:..| 000003a0 78 3a 20 4d 4f 44 45 20 6e 75 6d 62 65 72 2e 0d |x: MODE number..| 000003b0 0d 43 6f 6c 6f 75 72 73 3a 20 54 68 65 20 6d 61 |.Colours: The ma| 000003c0 78 69 6d 75 6d 20 6e 75 6d 62 65 72 20 6f 66 20 |ximum number of | 000003d0 64 69 66 66 65 72 65 6e 74 20 63 6f 6c 6f 75 72 |different colour| 000003e0 73 20 77 68 69 63 68 20 6d 61 79 20 62 65 20 0d |s which may be .| 000003f0 75 73 65 64 20 6f 6e 20 73 63 72 65 65 6e 20 61 |used on screen a| 00000400 74 20 61 6e 79 20 6f 6e 65 20 74 69 6d 65 2e 0d |t any one time..| 00000410 0d 47 72 61 70 68 69 63 73 3f 3a 20 44 6f 65 73 |.Graphics?: Does| 00000420 20 74 68 69 73 20 67 72 61 70 68 69 63 73 20 6d | this graphics m| 00000430 6f 64 65 20 61 6c 6c 6f 77 20 74 68 65 20 75 73 |ode allow the us| 00000440 65 20 6f 66 20 74 68 65 20 67 72 61 70 68 69 63 |e of the graphic| 00000450 73 20 63 6f 6d 6d 61 6e 64 73 20 28 69 65 2e 20 |s commands (ie. | 00000460 4d 4f 56 45 2c 20 44 52 41 57 2c 20 65 74 63 2e |MOVE, DRAW, etc.| 00000470 29 3f 20 4d 6f 64 65 73 20 33 20 26 20 36 20 64 |)? Modes 3 & 6 d| 00000480 6f 20 6e 6f 74 20 61 6c 6c 6f 77 20 74 68 65 73 |o not allow thes| 00000490 65 20 63 6f 6d 6d 61 6e 64 73 2c 20 61 73 20 74 |e commands, as t| 000004a0 68 65 72 65 20 61 72 65 20 74 68 69 6e 20 68 6f |here are thin ho| 000004b0 72 69 7a 6f 6e 74 61 6c 20 73 74 72 69 70 65 73 |rizontal stripes| 000004c0 20 69 6e 20 62 65 74 77 65 65 6e 20 74 68 65 20 | in between the | 000004d0 72 6f 77 73 20 6f 66 20 63 68 61 72 61 63 74 65 |rows of characte| 000004e0 72 73 2c 20 69 6e 20 77 68 69 63 68 20 6e 6f 74 |rs, in which not| 000004f0 68 69 6e 67 20 63 61 6e 20 62 65 20 64 69 73 70 |hing can be disp| 00000500 6c 61 79 65 64 2e 20 54 68 75 73 2c 20 61 6e 79 |layed. Thus, any| 00000510 20 67 72 61 70 68 69 63 73 20 64 72 61 77 6e 20 | graphics drawn | 00000520 69 6e 20 74 68 65 73 65 20 6d 6f 64 65 73 20 77 |in these modes w| 00000530 6f 75 6c 64 20 68 61 76 65 20 62 6c 61 63 6b 20 |ould have black | 00000540 68 6f 72 69 7a 6f 6e 74 61 6c 20 73 74 72 69 70 |horizontal strip| 00000550 65 73 20 61 63 72 6f 73 73 20 74 68 65 6d 2c 20 |es across them, | 00000560 61 6e 64 20 77 6f 75 6c 64 20 6c 6f 6f 6b 20 72 |and would look r| 00000570 61 74 68 65 72 20 73 69 6c 6c 79 21 20 54 6f 20 |ather silly! To | 00000580 73 65 65 20 77 68 61 74 20 49 20 6d 65 61 6e 2c |see what I mean,| 00000590 20 74 75 72 6e 20 6f 6e 20 79 6f 75 72 20 63 6f | turn on your co| 000005a0 6d 70 75 74 65 72 20 61 6e 64 20 74 79 70 65 3a |mputer and type:| 000005b0 20 4d 4f 44 45 20 36 3a 43 4f 4c 4f 55 52 20 31 | MODE 6:COLOUR 1| 000005c0 32 38 3a 43 4f 4c 4f 55 52 20 30 3a 43 4c 53 0d |28:COLOUR 0:CLS.| 000005d0 0d 52 65 73 2f 47 3a 20 54 68 65 20 72 65 73 6f |.Res/G: The reso| 000005e0 6c 75 74 69 6f 6e 20 6f 66 20 74 68 65 20 73 63 |lution of the sc| 000005f0 72 65 65 6e 20 69 6e 20 74 68 69 73 20 6d 6f 64 |reen in this mod| 00000600 65 20 28 69 65 2e 20 74 68 65 20 6e 75 6d 62 65 |e (ie. the numbe| 00000610 72 20 6f 66 20 70 69 78 65 6c 73 20 61 63 72 6f |r of pixels acro| 00000620 73 73 20 61 6e 64 20 64 6f 77 6e 20 74 68 65 20 |ss and down the | 00000630 73 63 72 65 65 6e 2e 20 47 65 6e 65 72 61 6c 6c |screen. Generall| 00000640 79 2c 20 74 68 65 20 68 69 67 68 65 72 20 74 68 |y, the higher th| 00000650 65 20 76 61 6c 75 65 2c 20 74 68 65 20 73 6d 6f |e value, the smo| 00000660 6f 74 68 65 72 20 79 6f 75 72 20 67 72 61 70 68 |other your graph| 00000670 69 63 73 20 77 69 6c 6c 20 6c 6f 6f 6b 2e 20 4e |ics will look. N| 00000680 6f 74 65 20 74 68 61 74 20 74 68 69 73 20 69 73 |ote that this is| 00000690 20 6e 6f 74 20 74 68 65 20 73 61 6d 65 20 61 73 | not the same as| 000006a0 20 74 68 65 20 70 61 72 61 6d 65 74 65 72 73 20 | the parameters | 000006b0 74 61 6b 65 6e 20 62 79 20 74 68 65 20 4d 4f 56 |taken by the MOV| 000006c0 45 20 61 6e 64 20 44 52 41 57 20 63 6f 6d 6d 61 |E and DRAW comma| 000006d0 6e 64 73 20 2d 20 74 68 65 73 65 20 61 6c 77 61 |nds - these alwa| 000006e0 79 73 20 73 63 61 6c 65 20 74 68 65 20 6e 75 6d |ys scale the num| 000006f0 62 65 72 20 6f 66 20 70 69 78 65 6c 73 20 61 63 |ber of pixels ac| 00000700 72 6f 73 73 20 74 68 65 20 73 63 72 65 65 6e 20 |ross the screen | 00000710 74 6f 20 31 32 38 30 20 61 6e 64 20 64 6f 77 6e |to 1280 and down| 00000720 20 74 68 65 20 73 63 72 65 65 6e 20 74 6f 20 31 | the screen to 1| 00000730 30 32 34 2c 20 73 6f 20 74 68 61 74 20 28 31 32 |024, so that (12| 00000740 38 30 2c 31 30 32 34 29 20 77 69 6c 6c 20 61 6c |80,1024) will al| 00000750 77 61 79 73 20 62 65 20 74 68 65 20 74 6f 70 2d |ways be the top-| 00000760 72 69 67 68 74 20 63 6f 72 6e 65 72 20 6f 66 20 |right corner of | 00000770 74 68 65 20 73 63 72 65 65 6e 2c 20 61 6e 64 20 |the screen, and | 00000780 28 30 2c 30 29 20 74 68 65 20 62 6f 74 74 6f 6d |(0,0) the bottom| 00000790 20 6c 65 66 74 2e 0d 0d 52 65 73 2f 54 3a 20 54 | left...Res/T: T| 000007a0 68 65 20 6e 75 6d 62 65 72 20 6f 66 20 63 68 61 |he number of cha| 000007b0 72 61 63 74 65 72 73 20 77 68 69 63 68 20 66 69 |racters which fi| 000007c0 74 20 61 63 72 6f 73 73 20 74 68 65 20 73 63 72 |t across the scr| 000007d0 65 65 6e 20 69 6e 20 74 68 69 73 20 6d 6f 64 65 |een in this mode| 000007e0 20 77 68 65 6e 20 75 73 69 6e 67 20 74 65 78 74 | when using text| 000007f0 2e 0d 0d 4d 65 6d 6f 72 79 3a 20 54 68 65 20 61 |...Memory: The a| 00000800 6d 6f 75 6e 74 20 6f 66 20 6d 65 6d 6f 72 79 20 |mount of memory | 00000810 75 73 65 64 20 62 79 20 74 68 65 20 67 72 61 70 |used by the grap| 00000820 68 69 63 73 20 6d 6f 64 65 2e 20 49 66 20 79 6f |hics mode. If yo| 00000830 75 20 61 72 65 20 77 72 69 74 69 6e 67 20 61 20 |u are writing a | 00000840 6c 6f 6e 67 20 70 72 6f 67 72 61 6d 2c 20 74 68 |long program, th| 00000850 69 73 20 69 73 20 61 6e 20 69 6d 70 6f 72 74 61 |is is an importa| 00000860 6e 74 20 63 6f 6e 73 69 64 65 72 61 74 69 6f 6e |nt consideration| 00000870 2c 20 61 73 20 79 6f 75 20 6d 61 79 20 6e 6f 74 |, as you may not| 00000880 20 68 61 76 65 20 72 6f 6f 6d 20 66 6f 72 20 74 | have room for t| 00000890 68 65 20 70 72 6f 67 72 61 6d 20 61 66 74 65 72 |he program after| 000008a0 20 73 75 62 74 72 61 63 74 69 6e 67 20 32 30 4b | subtracting 20K| 000008b0 20 66 72 6f 6d 20 74 68 65 20 63 6f 6d 70 75 74 | from the comput| 000008c0 65 72 27 73 20 33 32 4b 20 52 41 4d 20 66 6f 72 |er's 32K RAM for| 000008d0 20 61 20 4d 4f 44 45 20 32 20 73 63 72 65 65 6e | a MODE 2 screen| 000008e0 2e 0d 0d 20 20 20 20 20 48 61 76 69 6e 67 20 64 |... Having d| 000008f0 65 63 69 64 65 64 20 77 68 69 63 68 20 67 72 61 |ecided which gra| 00000900 70 68 69 63 73 20 6d 6f 64 65 20 79 6f 75 20 61 |phics mode you a| 00000910 72 65 20 67 6f 69 6e 67 20 74 6f 20 75 73 65 2c |re going to use,| 00000920 20 79 6f 75 20 0d 6d 75 73 74 20 64 65 63 69 64 | you .must decid| 00000930 65 20 68 6f 77 20 79 6f 75 20 61 72 65 20 67 6f |e how you are go| 00000940 69 6e 67 20 74 6f 20 67 65 74 20 79 6f 75 72 20 |ing to get your | 00000950 67 72 61 70 68 69 63 73 20 6f 6e 74 6f 20 74 68 |graphics onto th| 00000960 65 20 73 63 72 65 65 6e 2e 20 0d 54 68 65 72 65 |e screen. .There| 00000970 20 61 72 65 20 6d 61 6e 79 20 6d 65 74 68 6f 64 | are many method| 00000980 73 20 66 6f 72 20 70 72 6f 64 75 63 69 6e 67 20 |s for producing | 00000990 67 72 61 70 68 69 63 73 2c 20 61 6e 64 20 77 68 |graphics, and wh| 000009a0 69 63 68 20 79 6f 75 20 0d 63 68 6f 6f 73 65 20 |ich you .choose | 000009b0 64 65 70 65 6e 64 73 20 75 70 6f 6e 20 77 68 61 |depends upon wha| 000009c0 74 20 79 6f 75 20 77 61 6e 74 20 74 68 65 20 67 |t you want the g| 000009d0 72 61 70 68 69 63 73 20 66 6f 72 2c 20 61 6e 64 |raphics for, and| 000009e0 20 77 68 61 74 20 79 6f 75 20 0d 77 61 6e 74 20 | what you .want | 000009f0 74 68 65 6d 20 74 6f 20 64 6f 20 28 61 6e 64 2c |them to do (and,| 00000a00 20 6f 66 20 63 6f 75 72 73 65 2c 20 79 6f 75 72 | of course, your| 00000a10 20 70 72 6f 67 72 61 6d 6d 69 6e 67 20 61 62 69 | programming abi| 00000a20 6c 69 74 79 29 2e 20 54 68 65 20 0d 6d 6f 73 74 |lity). The .most| 00000a30 20 6f 62 76 69 6f 75 73 20 69 73 20 74 68 65 20 | obvious is the | 00000a40 50 52 49 4e 54 20 63 6f 6d 6d 61 6e 64 2e 20 54 |PRINT command. T| 00000a50 68 69 73 20 69 73 20 61 20 71 75 69 63 6b 2c 20 |his is a quick, | 00000a60 65 61 73 79 20 77 61 79 20 6f 66 20 0d 70 75 74 |easy way of .put| 00000a70 74 69 6e 67 20 74 65 78 74 20 6f 6e 74 6f 20 74 |ting text onto t| 00000a80 68 65 20 73 63 72 65 65 6e 2e 20 57 68 65 6e 20 |he screen. When | 00000a90 75 73 65 64 20 77 69 74 68 20 74 68 65 20 43 4f |used with the CO| 00000aa0 4c 4f 55 52 20 63 6f 6d 6d 61 6e 64 20 0d 61 6e |LOUR command .an| 00000ab0 64 20 74 68 65 20 54 41 42 28 78 2c 79 29 2c 20 |d the TAB(x,y), | 00000ac0 6d 61 6e 79 20 69 6e 74 65 72 65 73 74 69 6e 67 |many interesting| 00000ad0 20 65 66 66 65 63 74 73 20 63 61 6e 20 62 65 20 | effects can be | 00000ae0 61 63 68 69 65 76 65 64 2e 20 0d 54 68 65 73 65 |achieved. .These| 00000af0 20 61 72 65 2c 20 68 6f 77 65 76 65 72 2c 20 67 | are, however, g| 00000b00 72 65 61 74 6c 79 20 6c 69 6d 69 74 65 64 20 62 |reatly limited b| 00000b10 79 20 74 68 65 20 66 61 63 74 20 74 68 61 74 20 |y the fact that | 00000b20 6f 6e 6c 79 20 6c 65 74 74 65 72 73 20 0d 63 61 |only letters .ca| 00000b30 6e 20 62 65 20 64 69 73 70 6c 61 79 65 64 20 6f |n be displayed o| 00000b40 6e 20 74 68 65 20 73 63 72 65 65 6e 2e 20 54 68 |n the screen. Th| 00000b50 69 73 20 6c 69 6d 69 74 61 74 69 6f 6e 20 6d 61 |is limitation ma| 00000b60 79 20 62 65 20 6f 76 65 72 63 6f 6d 65 20 75 73 |y be overcome us| 00000b70 69 6e 67 20 0d 75 73 65 72 20 64 65 66 69 6e 65 |ing .user define| 00000b80 64 20 63 68 61 72 61 63 74 65 72 73 20 28 73 65 |d characters (se| 00000b90 65 20 74 68 65 20 47 72 61 70 68 69 63 73 20 63 |e the Graphics c| 00000ba0 68 61 70 74 65 72 20 6f 66 20 74 68 65 20 55 73 |hapter of the Us| 00000bb0 65 72 20 4d 61 6e 75 61 6c 20 0d 66 6f 72 20 64 |er Manual .for d| 00000bc0 65 74 61 69 6c 73 29 2e 20 45 76 65 6e 20 75 73 |etails). Even us| 00000bd0 69 6e 67 20 74 68 65 73 65 20 63 68 61 72 61 63 |ing these charac| 00000be0 74 65 72 73 2c 20 68 6f 77 65 76 65 72 2c 20 74 |ters, however, t| 00000bf0 68 65 72 65 20 61 72 65 20 73 65 72 69 6f 75 73 |here are serious| 00000c00 20 0d 6c 69 6d 69 74 61 74 69 6f 6e 73 2e 20 54 | .limitations. T| 00000c10 68 65 79 20 63 61 6e 20 6f 6e 6c 79 20 75 73 65 |hey can only use| 00000c20 20 6f 6e 65 20 63 6f 6c 6f 75 72 20 70 65 72 20 | one colour per | 00000c30 63 68 61 72 61 63 74 65 72 2c 20 61 6e 64 20 73 |character, and s| 00000c40 6f 20 65 76 65 6e 20 0d 69 66 20 79 6f 75 20 6d |o even .if you m| 00000c50 61 6b 65 20 64 69 66 66 65 72 65 6e 74 20 63 68 |ake different ch| 00000c60 61 72 61 63 74 65 72 73 20 64 69 66 66 65 72 65 |aracters differe| 00000c70 6e 74 20 63 6f 6c 6f 75 72 73 2c 20 79 6f 75 72 |nt colours, your| 00000c80 20 73 63 72 65 65 6e 20 77 69 6c 6c 20 0d 73 74 | screen will .st| 00000c90 69 6c 6c 20 61 70 70 65 61 72 20 72 61 74 68 65 |ill appear rathe| 00000ca0 72 20 64 75 6c 6c 2e 20 41 6c 73 6f 2c 20 74 68 |r dull. Also, th| 00000cb0 65 79 20 61 72 65 20 64 69 66 66 69 63 75 6c 74 |ey are difficult| 00000cc0 20 74 6f 20 75 73 65 20 77 68 65 6e 20 0d 64 65 | to use when .de| 00000cd0 61 6c 69 6e 67 20 77 69 74 68 20 6c 61 72 67 65 |aling with large| 00000ce0 20 61 72 65 61 73 20 6f 66 20 74 68 65 20 73 63 | areas of the sc| 00000cf0 72 65 65 6e 2e 20 0d 20 20 20 20 20 54 68 69 73 |reen. . This| 00000d00 20 69 73 20 77 68 65 72 65 20 4d 4f 56 45 2c 20 | is where MOVE, | 00000d10 44 52 41 57 20 61 6e 64 20 50 4c 4f 54 20 61 72 |DRAW and PLOT ar| 00000d20 65 20 75 73 65 66 75 6c 2e 20 55 73 69 6e 67 20 |e useful. Using | 00000d30 74 68 65 73 65 20 0d 63 6f 6d 6d 61 6e 64 73 2c |these .commands,| 00000d40 20 6c 69 6e 65 73 20 63 61 6e 20 62 65 20 64 72 | lines can be dr| 00000d50 61 77 6e 20 61 63 72 6f 73 73 20 74 68 65 20 73 |awn across the s| 00000d60 63 72 65 65 6e 2c 20 61 6e 64 2c 20 75 73 69 6e |creen, and, usin| 00000d70 67 20 50 4c 4f 54 2c 20 0d 74 72 69 61 6e 67 6c |g PLOT, .triangl| 00000d80 65 73 20 63 61 6e 20 62 65 20 64 72 61 77 6e 2e |es can be drawn.| 00000d90 20 54 68 65 73 65 20 63 6f 6d 6d 61 6e 64 73 20 | These commands | 00000da0 61 72 65 20 63 6f 76 65 72 65 64 20 69 6e 20 74 |are covered in t| 00000db0 68 65 20 0d 47 72 61 70 68 69 63 73 20 63 68 61 |he .Graphics cha| 00000dc0 70 74 65 72 20 6f 66 20 74 68 65 20 75 73 65 72 |pter of the user| 00000dd0 20 6d 61 6e 75 61 6c 2e 20 54 68 65 20 6f 74 68 | manual. The oth| 00000de0 65 72 20 6d 61 6a 6f 72 20 61 64 76 61 6e 74 61 |er major advanta| 00000df0 67 65 20 6f 66 20 0d 74 68 65 73 65 20 67 72 61 |ge of .these gra| 00000e00 70 68 69 63 73 20 63 6f 6d 6d 61 6e 64 73 20 69 |phics commands i| 00000e10 73 20 74 68 61 74 20 74 68 65 79 20 63 61 6e 20 |s that they can | 00000e20 62 65 20 75 73 65 64 20 6e 6f 74 20 6a 75 73 74 |be used not just| 00000e30 20 74 6f 20 73 65 74 20 0d 74 68 65 20 63 6f 6c | to set .the col| 00000e40 6f 75 72 20 6f 66 20 61 6e 20 61 72 65 61 2c 20 |our of an area, | 00000e50 62 75 74 20 61 6c 73 6f 20 74 6f 20 69 6e 76 65 |but also to inve| 00000e60 72 74 20 69 74 2e 20 54 68 65 20 47 43 4f 4c 20 |rt it. 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At firs| 000011e0 74 20 73 69 67 68 74 2c 20 69 74 20 6d 61 79 20 |t sight, it may | 000011f0 73 65 65 6d 20 74 68 61 74 20 75 73 65 72 20 64 |seem that user d| 00001200 65 66 69 6e 65 64 20 63 68 61 72 61 63 74 65 72 |efined character| 00001210 73 20 73 65 72 76 65 20 6e 6f 20 70 75 72 70 6f |s serve no purpo| 00001220 73 65 20 6f 6e 63 65 20 61 20 70 72 6f 67 72 61 |se once a progra| 00001230 6d 6d 65 72 20 68 61 73 20 6c 65 61 72 6e 74 20 |mmer has learnt | 00001240 61 62 6f 75 74 20 74 68 65 20 6d 6f 72 65 20 66 |about the more f| 00001250 6c 65 78 69 62 6c 65 20 67 72 61 70 68 69 63 73 |lexible graphics| 00001260 20 63 6f 6d 6d 61 6e 64 73 2c 20 62 75 74 20 74 | commands, but t| 00001270 68 69 73 20 69 73 20 6e 6f 74 20 74 68 65 20 63 |his is not the c| 00001280 61 73 65 2e 20 49 66 20 79 6f 75 20 74 72 79 20 |ase. If you try | 00001290 74 6f 20 70 72 6f 64 75 63 65 20 64 65 74 61 69 |to produce detai| 000012a0 6c 65 64 20 67 72 61 70 68 69 63 73 20 75 73 69 |led graphics usi| 000012b0 6e 67 20 4d 4f 56 45 20 61 6e 64 20 44 52 41 57 |ng MOVE and DRAW| 000012c0 2c 20 79 6f 75 20 77 69 6c 6c 20 66 69 6e 64 20 |, you will find | 000012d0 74 68 61 74 20 79 6f 75 72 20 70 72 6f 67 72 61 |that your progra| 000012e0 6d 20 63 72 61 77 6c 73 20 61 6c 6f 6e 67 20 61 |m crawls along a| 000012f0 74 20 61 6e 20 75 6e 61 63 63 65 70 74 61 62 6c |t an unacceptabl| 00001300 65 20 72 61 74 65 2e 20 55 73 65 72 20 64 65 66 |e rate. User def| 00001310 69 6e 65 64 20 63 68 61 72 61 63 74 65 72 73 2c |ined characters,| 00001320 20 6f 6e 20 74 68 65 20 6f 74 68 65 72 20 68 61 | on the other ha| 00001330 6e 64 2c 20 63 61 6e 20 70 72 6f 64 75 63 65 20 |nd, can produce | 00001340 64 65 74 61 69 6c 20 71 75 69 63 6b 6c 79 2c 20 |detail quickly, | 00001350 65 73 70 65 63 69 61 6c 6c 79 20 69 66 20 74 68 |especially if th| 00001360 65 20 70 61 74 74 65 72 6e 20 72 65 70 65 61 74 |e pattern repeat| 00001370 73 2c 20 73 6f 20 74 68 61 74 20 6f 6e 65 20 63 |s, so that one c| 00001380 68 61 72 61 63 74 65 72 20 64 65 66 69 6e 69 74 |haracter definit| 00001390 69 6f 6e 20 63 61 6e 20 62 65 20 75 73 65 64 20 |ion can be used | 000013a0 72 65 70 65 61 74 65 64 6c 79 20 69 6e 20 73 65 |repeatedly in se| 000013b0 76 65 72 61 6c 20 61 72 65 61 73 20 6f 66 20 74 |veral areas of t| 000013c0 68 65 20 73 63 72 65 65 6e 2e 0d 20 20 20 20 20 |he screen.. | 000013d0 53 6f 2c 20 69 74 20 73 65 65 6d 73 20 74 68 61 |So, it seems tha| 000013e0 74 20 70 72 6f 67 72 61 6d 6d 65 72 73 20 61 72 |t programmers ar| 000013f0 65 20 66 61 63 65 64 20 77 69 74 68 20 61 20 64 |e faced with a d| 00001400 69 66 66 69 63 75 6c 74 20 70 72 6f 62 6c 65 6d |ifficult problem| 00001410 2e 20 54 68 65 79 20 63 61 6e 20 75 73 65 20 67 |. They can use g| 00001420 72 61 70 68 69 63 73 20 63 6f 6d 6d 61 6e 64 73 |raphics commands| 00001430 20 61 6e 64 20 67 65 74 20 63 6f 6c 6f 75 72 2c | and get colour,| 00001440 20 6f 72 20 74 68 65 79 20 63 61 6e 20 75 73 65 | or they can use| 00001450 20 75 73 65 72 20 64 65 66 69 6e 65 64 20 63 68 | user defined ch| 00001460 61 72 61 63 74 65 72 73 20 61 6e 64 20 67 65 74 |aracters and get| 00001470 20 64 65 74 61 69 6c 2c 20 62 75 74 20 77 68 61 | detail, but wha| 00001480 74 20 69 66 20 74 68 65 79 20 77 61 6e 74 20 62 |t if they want b| 00001490 6f 74 68 20 61 74 20 74 68 65 20 73 61 6d 65 20 |oth at the same | 000014a0 74 69 6d 65 3f 20 54 68 65 72 65 20 6d 75 73 74 |time? There must| 000014b0 20 62 65 20 61 20 73 6f 6c 75 74 69 6f 6e 20 2d | be a solution -| 000014c0 20 68 6f 77 20 65 6c 73 65 20 77 6f 75 6c 64 20 | how else would | 000014d0 67 61 6d 65 73 20 6c 69 6b 65 20 43 69 74 61 64 |games like Citad| 000014e0 65 6c 20 6d 61 6e 61 67 65 20 74 6f 20 70 72 6f |el manage to pro| 000014f0 64 75 63 65 20 73 75 63 68 20 64 65 74 61 69 6c |duce such detail| 00001500 65 64 20 71 75 61 6c 69 74 79 20 67 72 61 70 68 |ed quality graph| 00001510 69 63 73 2c 20 61 6e 64 20 61 74 20 73 75 63 68 |ics, and at such| 00001520 20 61 20 73 70 65 65 64 3f 0d 20 20 20 20 20 54 | a speed?. T| 00001530 68 65 72 65 20 61 72 65 2c 20 69 6e 20 66 61 63 |here are, in fac| 00001540 74 2c 20 74 77 6f 20 70 6f 73 73 69 62 6c 65 20 |t, two possible | 00001550 73 6f 6c 75 74 69 6f 6e 73 2c 20 62 6f 74 68 20 |solutions, both | 00001560 6f 66 20 77 68 69 63 68 20 69 6e 76 6f 6c 76 65 |of which involve| 00001570 20 64 65 61 3b 69 6e 67 20 77 69 74 68 20 61 6e | dea;ing with an| 00001580 20 61 72 65 61 20 6f 66 20 6d 65 6d 6f 72 79 20 | area of memory | 00001590 6b 6e 6f 77 6e 20 61 73 20 73 63 72 65 65 6e 20 |known as screen | 000015a0 6d 65 6d 6f 72 79 2e 20 54 68 69 73 20 69 73 2c |memory. This is,| 000015b0 20 68 6f 77 65 76 65 72 2c 20 76 65 72 79 20 63 | however, very c| 000015c0 6f 6d 70 6c 69 63 61 74 65 64 2c 20 61 6e 64 20 |omplicated, and | 000015d0 77 69 6c 6c 20 74 61 6b 65 20 61 6e 20 65 6e 74 |will take an ent| 000015e0 69 72 65 20 61 72 74 69 63 6c 65 20 74 6f 20 63 |ire article to c| 000015f0 6f 76 65 72 2e 2e 2e 20 4e 6f 20 70 72 69 7a 65 |over... No prize| 00001600 73 20 66 6f 72 20 67 75 65 73 73 69 6e 67 20 77 |s for guessing w| 00001610 68 61 74 20 74 68 65 20 73 75 62 6a 65 63 74 20 |hat the subject | 00001620 77 69 6c 6c 20 62 65 20 6e 65 78 74 20 74 69 6d |will be next tim| 00001630 65 21 0d |e!.| 00001633
.