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shipwren/+PTB1
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/+PTB1 |
| Read OK: | ✔ |
| File size: | 2881 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 » +PTB1
- Personal collection » Acorn hard disk » misc » misc3 » shipwren/+PTB1
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. Some graphics modes, for example, allow
you to have many different colours on screen, but poor detail, while
others allow you to have very great detail but few colours. 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 129:COLOUR 0:CLS
The background should be entirely white, but there are horizontal
stripes where the screen can be no colour except black.
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 the computer's 32K of memory which is 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. If this
is the case, you must use a mode which uses less memory, such as mode 5.
My advice in choosing which mode to use would be to try, if at
all possible, to use one of the 20K modes. The detail of MODE 1, and
the colours of MODE 2, are both far superior to the quality of MODE 5.
If you are an efficient programmer, you should find that it is perfectly
possible to fit a reasonably lengthed program into memory at the same
time as 20K of screen. Shipwrecked, for example, used MODE 2 for its
graphics, as does Shipwrecked II on this issue.
Fitting a program into memory as well as a large screen memory is
fairly easy when you are using a tape or DFS filing system, but this
can prove difficult with ADFS, because 4K of the memory normally used
to store programs is taken away for the ADFS to use, leaving you only a
mere 4.5K free for programs. Assuming that once you have loaded the
program you do not intend to use the disk again, there is a way around
this. If you switch over to a tape filing system, the ADFS memory becomes
redundant and can be reclaimed for BASIC use. Once you have done this, of
course, you cannot access disks without resetting the computer, so load
and save options in your program are restricted to tape only. This
technique will be explained in greater depth in a later article on
memory management.
After you have decided which mode to use, and so you know how
many colours are available, you must decide what colours these are
going to be. In MODE 2, where you have 16 colours, you have access
to all of the colours available, and so this choice does not affect
you. In 4 colour modes, on the other hand, you only initially have
access to black, red, yellow and white. These are a warm selection
of colours, but you might want a colder selection in your program,
or just simply a set of colours which go well with your screen design.
To do this you use the command:
VDU 19,logical colour number,actual colour number,0,0,0
The logical colour number is the number that you use with the
colour command to select that particular colour. In a four colour
mode this must be between 0 and 3, and in a two colour mode this
must be either 0 or 1. The actual colour number is the number of
the colour which you want to assign to that logical colour. The
actual colour numbers are given below:
0-Black 8-Flashing black/white
1-Red 9-Flashing red/cyan
2-Green 10-Flashing grenn/magenta
3-Yellow 11-Flashing yellow/blue
4-Blue 12-Flashing blue/yellow
5-Magenta (purple) 13-Flashing magenta/green
6-Cyan (light blue) 14-Flashing cyan/red
7-White 15-Flashing white/black
Looking at the table above, you might wonder what the difference
is between flashing black/white and flashing white/black. If you look
at them both seperately, there is little difference, but if you look
at them together, you see that when one is black the other is white,
and vice versa. So, for example, if you put a logo on the screen
with the logo in colour 8 and the background in colour 15, the
screen flashes between white on black and black on white. To
demonstrate this, turn on your Electron and type:
MODE 6:VDU 19,1,8,0,0,0:VDU 19,0,15,0,0,0
You can take this further. Four is the maximum number of colours
allowed on screen at any one time. Therefore for different screens
you can have different selections of colours. The main menu screen
might be in warm colours, and then subsequent menu screens in colder
colours. The changing of these logical colours is called "Palette
Switching". You can go even further and create simply animation
using palette switching; an effect which is called "Colour Cycling".
If, for example, you put a row of colour 3 above a row of colour 2,
with a row of colour 1 below that, and colour 0 at the base.
Initially, you set all logical colours to black. Then, if you
set logical colour 0 to white, the base appears. Now set logical
colour 1 to white as well, the bar moves up the screen. Repeat
this with colours 2 and 3, and you have a simple animation. This
is even more powerful in MODE 2, where you can go through 16 rows
up. This is demonstrated at the end of the tutorial.
Having decided which graphics mode you are going to use, and
which colours to have on the screen, 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. This is demonstrated at
the end of the tutorial.
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
colourful graphics, and at such a speed?
There are, in fact, two possible solutions, both of which
involve dealing with an area of memory known as screen memory.
This is, however, very complicated, and will be discussed further
in the next article. 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 0a 62 79 20 44 6f 6d 69 6e 69 63 |NIQUE.by Dominic| 00000020 20 46 6f 72 64 0a 50 61 72 74 20 32 3a 20 47 72 | Ford.Part 2: Gr| 00000030 61 70 68 69 63 73 0a 0a 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 0a 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 0a 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 0a |ailable on the .| 00000100 45 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 0a 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 0a 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 53 6f 6d 65 20 67 | program. Some g| 000001b0 72 61 70 68 69 63 73 20 6d 6f 64 65 73 2c 20 66 |raphics modes, f| 000001c0 6f 72 20 65 78 61 6d 70 6c 65 2c 20 61 6c 6c 6f |or example, allo| 000001d0 77 20 0a 79 6f 75 20 74 6f 20 68 61 76 65 20 6d |w .you to have m| 000001e0 61 6e 79 20 64 69 66 66 65 72 65 6e 74 20 63 6f |any different co| 000001f0 6c 6f 75 72 73 20 6f 6e 20 73 63 72 65 65 6e 2c |lours on screen,| 00000200 20 62 75 74 20 70 6f 6f 72 20 64 65 74 61 69 6c | but poor detail| 00000210 2c 20 77 68 69 6c 65 20 0a 6f 74 68 65 72 73 20 |, while .others | 00000220 61 6c 6c 6f 77 20 79 6f 75 20 74 6f 20 68 61 76 |allow you to hav| 00000230 65 20 76 65 72 79 20 67 72 65 61 74 20 64 65 74 |e very great det| 00000240 61 69 6c 20 62 75 74 20 66 65 77 20 63 6f 6c 6f |ail but few colo| 00000250 75 72 73 2e 20 59 6f 75 20 73 65 6c 65 63 74 20 |urs. You select | 00000260 0a 67 72 61 70 68 69 63 73 20 6d 6f 64 65 73 20 |.graphics modes | 00000270 62 79 20 75 73 69 6e 67 20 74 68 65 20 4d 4f 44 |by using the MOD| 00000280 45 20 78 20 63 6f 6d 6d 61 6e 64 2e 20 44 65 74 |E x command. Det| 00000290 61 69 6c 73 20 6f 66 20 74 68 65 20 73 65 76 65 |ails of the seve| 000002a0 6e 20 0a 64 69 66 66 65 72 65 6e 74 20 61 76 61 |n .different ava| 000002b0 69 6c 61 62 6c 65 20 6d 6f 64 65 73 20 61 72 65 |ilable modes are| 000002c0 20 67 69 76 65 6e 20 62 65 6c 6f 77 3a 0a 0a 78 | given below:..x| 000002d0 20 43 6f 6c 6f 75 72 73 20 47 72 61 70 68 69 63 | Colours Graphic| 000002e0 73 3f 20 52 65 73 2f 47 20 20 20 52 65 73 2f 54 |s? Res/G Res/T| 000002f0 20 20 4d 65 6d 6f 72 79 0a 30 20 20 20 32 20 20 | Memory.0 2 | 00000300 20 20 20 20 59 65 73 20 20 20 20 20 20 36 34 30 | Yes 640| 00000310 78 32 35 36 20 38 30 78 33 32 20 20 20 32 30 4b |x256 80x32 20K| 00000320 0a 31 20 20 20 34 20 20 20 20 20 20 59 65 73 20 |.1 4 Yes | 00000330 20 20 20 20 20 33 32 30 78 32 35 36 20 34 30 78 | 320x256 40x| 00000340 33 32 20 20 20 32 30 4b 0a 32 20 20 20 31 36 20 |32 20K.2 16 | 00000350 20 20 20 20 59 65 73 20 20 20 20 20 20 31 36 30 | Yes 160| 00000360 78 32 35 36 20 32 30 78 33 32 20 20 20 32 30 4b |x256 20x32 20K| 00000370 0a 33 20 20 20 32 20 20 20 20 20 20 4e 6f 20 20 |.3 2 No | 00000380 20 20 20 20 20 4e 2f 41 20 20 20 20 20 38 30 78 | N/A 80x| 00000390 32 35 20 20 20 31 36 4b 0a 34 20 20 20 32 20 20 |25 16K.4 2 | 000003a0 20 20 20 20 59 65 73 20 20 20 20 20 20 33 32 30 | Yes 320| 000003b0 78 32 35 36 20 34 30 78 33 32 20 20 20 31 30 4b |x256 40x32 10K| 000003c0 0a 35 20 20 20 34 20 20 20 20 20 20 59 65 73 20 |.5 4 Yes | 000003d0 20 20 20 20 20 31 36 30 78 32 35 36 20 32 30 78 | 160x256 20x| 000003e0 33 32 20 20 20 31 30 4b 0a 36 20 20 20 32 20 20 |32 10K.6 2 | 000003f0 20 20 20 20 4e 6f 20 20 20 20 20 20 20 4e 2f 41 | No N/A| 00000400 20 20 20 20 20 34 30 78 32 35 20 20 20 38 4b 0a | 40x25 8K.| 00000410 37 20 20 20 31 36 20 20 20 20 20 4e 6f 20 20 20 |7 16 No | 00000420 20 20 20 20 4e 2f 41 20 20 20 20 20 34 30 78 32 | N/A 40x2| 00000430 35 20 20 20 31 4b 20 3c 2d 20 42 42 43 20 4f 4e |5 1K <- BBC ON| 00000440 4c 59 21 0a 0a 4b 45 59 3a 0a 0a 78 3a 20 4d 4f |LY!..KEY:..x: MO| 00000450 44 45 20 6e 75 6d 62 65 72 2e 0a 0a 43 6f 6c 6f |DE number...Colo| 00000460 75 72 73 3a 20 54 68 65 20 6d 61 78 69 6d 75 6d |urs: The maximum| 00000470 20 6e 75 6d 62 65 72 20 6f 66 20 64 69 66 66 65 | number of diffe| 00000480 72 65 6e 74 20 63 6f 6c 6f 75 72 73 20 77 68 69 |rent colours whi| 00000490 63 68 20 6d 61 79 20 62 65 20 0a 75 73 65 64 20 |ch may be .used | 000004a0 6f 6e 20 73 63 72 65 65 6e 20 61 74 20 61 6e 79 |on screen at any| 000004b0 20 6f 6e 65 20 74 69 6d 65 2e 0a 0a 47 72 61 70 | one time...Grap| 000004c0 68 69 63 73 3f 3a 20 44 6f 65 73 20 74 68 69 73 |hics?: Does this| 000004d0 20 67 72 61 70 68 69 63 73 20 6d 6f 64 65 20 61 | graphics mode a| 000004e0 6c 6c 6f 77 20 74 68 65 20 75 73 65 20 6f 66 20 |llow the use of | 000004f0 74 68 65 20 67 72 61 70 68 69 63 73 20 0a 63 6f |the graphics .co| 00000500 6d 6d 61 6e 64 73 20 28 69 65 2e 20 4d 4f 56 45 |mmands (ie. MOVE| 00000510 2c 20 44 52 41 57 2c 20 65 74 63 2e 29 3f 20 4d |, DRAW, etc.)? M| 00000520 6f 64 65 73 20 33 20 26 20 36 20 64 6f 20 6e 6f |odes 3 & 6 do no| 00000530 74 20 61 6c 6c 6f 77 20 74 68 65 73 65 20 0a 63 |t allow these .c| 00000540 6f 6d 6d 61 6e 64 73 2c 20 61 73 20 74 68 65 72 |ommands, as ther| 00000550 65 20 61 72 65 20 74 68 69 6e 20 68 6f 72 69 7a |e are thin horiz| 00000560 6f 6e 74 61 6c 20 73 74 72 69 70 65 73 20 69 6e |ontal stripes in| 00000570 20 62 65 74 77 65 65 6e 20 74 68 65 20 72 6f 77 | between the row| 00000580 73 20 0a 6f 66 20 63 68 61 72 61 63 74 65 72 73 |s .of characters| 00000590 2c 20 69 6e 20 77 68 69 63 68 20 6e 6f 74 68 69 |, in which nothi| 000005a0 6e 67 20 63 61 6e 20 62 65 20 64 69 73 70 6c 61 |ng can be displa| 000005b0 79 65 64 2e 20 54 68 75 73 2c 20 61 6e 79 20 67 |yed. Thus, any g| 000005c0 72 61 70 68 69 63 73 20 0a 64 72 61 77 6e 20 69 |raphics .drawn i| 000005d0 6e 20 74 68 65 73 65 20 6d 6f 64 65 73 20 77 6f |n these modes wo| 000005e0 75 6c 64 20 68 61 76 65 20 62 6c 61 63 6b 20 68 |uld have black h| 000005f0 6f 72 69 7a 6f 6e 74 61 6c 20 73 74 72 69 70 65 |orizontal stripe| 00000600 73 20 61 63 72 6f 73 73 20 74 68 65 6d 2c 20 0a |s across them, .| 00000610 61 6e 64 20 77 6f 75 6c 64 20 6c 6f 6f 6b 20 72 |and would look r| 00000620 61 74 68 65 72 20 73 69 6c 6c 79 21 20 54 6f 20 |ather silly! To | 00000630 73 65 65 20 77 68 61 74 20 49 20 6d 65 61 6e 2c |see what I mean,| 00000640 20 74 75 72 6e 20 6f 6e 20 79 6f 75 72 20 63 6f | turn on your co| 00000650 6d 70 75 74 65 72 20 0a 61 6e 64 20 74 79 70 65 |mputer .and type| 00000660 3a 20 4d 4f 44 45 20 36 3a 43 4f 4c 4f 55 52 20 |: MODE 6:COLOUR | 00000670 31 32 39 3a 43 4f 4c 4f 55 52 20 30 3a 43 4c 53 |129:COLOUR 0:CLS| 00000680 0a 54 68 65 20 62 61 63 6b 67 72 6f 75 6e 64 20 |.The background | 00000690 73 68 6f 75 6c 64 20 62 65 20 65 6e 74 69 72 65 |should be entire| 000006a0 6c 79 20 77 68 69 74 65 2c 20 62 75 74 20 74 68 |ly white, but th| 000006b0 65 72 65 20 61 72 65 20 68 6f 72 69 7a 6f 6e 74 |ere are horizont| 000006c0 61 6c 20 0a 73 74 72 69 70 65 73 20 77 68 65 72 |al .stripes wher| 000006d0 65 20 74 68 65 20 73 63 72 65 65 6e 20 63 61 6e |e the screen can| 000006e0 20 62 65 20 6e 6f 20 63 6f 6c 6f 75 72 20 65 78 | be no colour ex| 000006f0 63 65 70 74 20 62 6c 61 63 6b 2e 0a 0a 52 65 73 |cept black...Res| 00000700 2f 47 3a 20 54 68 65 20 72 65 73 6f 6c 75 74 69 |/G: The resoluti| 00000710 6f 6e 20 6f 66 20 74 68 65 20 73 63 72 65 65 6e |on of the screen| 00000720 20 69 6e 20 74 68 69 73 20 6d 6f 64 65 20 28 69 | in this mode (i| 00000730 65 2e 20 74 68 65 20 6e 75 6d 62 65 72 20 6f 66 |e. the number of| 00000740 20 0a 70 69 78 65 6c 73 20 61 63 72 6f 73 73 20 | .pixels across | 00000750 61 6e 64 20 64 6f 77 6e 20 74 68 65 20 73 63 72 |and down the scr| 00000760 65 65 6e 2e 20 47 65 6e 65 72 61 6c 6c 79 2c 20 |een. Generally, | 00000770 74 68 65 20 68 69 67 68 65 72 20 74 68 65 20 76 |the higher the v| 00000780 61 6c 75 65 2c 20 0a 74 68 65 20 73 6d 6f 6f 74 |alue, .the smoot| 00000790 68 65 72 20 79 6f 75 72 20 67 72 61 70 68 69 63 |her your graphic| 000007a0 73 20 77 69 6c 6c 20 6c 6f 6f 6b 2e 20 4e 6f 74 |s will look. Not| 000007b0 65 20 74 68 61 74 20 74 68 69 73 20 69 73 20 6e |e that this is n| 000007c0 6f 74 20 74 68 65 20 73 61 6d 65 20 0a 61 73 20 |ot the same .as | 000007d0 74 68 65 20 70 61 72 61 6d 65 74 65 72 73 20 74 |the parameters t| 000007e0 61 6b 65 6e 20 62 79 20 74 68 65 20 4d 4f 56 45 |aken by the MOVE| 000007f0 20 61 6e 64 20 44 52 41 57 20 63 6f 6d 6d 61 6e | and DRAW comman| 00000800 64 73 20 2d 20 74 68 65 73 65 20 61 6c 77 61 79 |ds - these alway| 00000810 73 20 0a 73 63 61 6c 65 20 74 68 65 20 6e 75 6d |s .scale the num| 00000820 62 65 72 20 6f 66 20 70 69 78 65 6c 73 20 61 63 |ber of pixels ac| 00000830 72 6f 73 73 20 74 68 65 20 73 63 72 65 65 6e 20 |ross the screen | 00000840 74 6f 20 31 32 38 30 20 61 6e 64 20 64 6f 77 6e |to 1280 and down| 00000850 20 74 68 65 20 73 63 72 65 65 6e 20 0a 74 6f 20 | the screen .to | 00000860 31 30 32 34 2c 20 73 6f 20 74 68 61 74 20 28 31 |1024, so that (1| 00000870 32 38 30 2c 31 30 32 34 29 20 77 69 6c 6c 20 61 |280,1024) will a| 00000880 6c 77 61 79 73 20 62 65 20 74 68 65 20 74 6f 70 |lways be the top| 00000890 2d 72 69 67 68 74 20 63 6f 72 6e 65 72 20 6f 66 |-right corner of| 000008a0 20 74 68 65 20 0a 73 63 72 65 65 6e 2c 20 61 6e | the .screen, an| 000008b0 64 20 28 30 2c 30 29 20 74 68 65 20 62 6f 74 74 |d (0,0) the bott| 000008c0 6f 6d 20 6c 65 66 74 2e 0a 0a 52 65 73 2f 54 3a |om left...Res/T:| 000008d0 20 54 68 65 20 6e 75 6d 62 65 72 20 6f 66 20 63 | The number of c| 000008e0 68 61 72 61 63 74 65 72 73 20 77 68 69 63 68 20 |haracters which | 000008f0 66 69 74 20 61 63 72 6f 73 73 20 74 68 65 20 73 |fit across the s| 00000900 63 72 65 65 6e 20 69 6e 20 74 68 69 73 20 0a 6d |creen in this .m| 00000910 6f 64 65 20 77 68 65 6e 20 75 73 69 6e 67 20 74 |ode when using t| 00000920 65 78 74 2e 0a 0a 4d 65 6d 6f 72 79 3a 20 54 68 |ext...Memory: Th| 00000930 65 20 61 6d 6f 75 6e 74 20 6f 66 20 74 68 65 20 |e amount of the | 00000940 63 6f 6d 70 75 74 65 72 27 73 20 33 32 4b 20 6f |computer's 32K o| 00000950 66 20 6d 65 6d 6f 72 79 20 77 68 69 63 68 20 69 |f memory which i| 00000960 73 20 75 73 65 64 20 62 79 20 0a 74 68 65 20 67 |s used by .the g| 00000970 72 61 70 68 69 63 73 20 6d 6f 64 65 2e 20 49 66 |raphics mode. If| 00000980 20 79 6f 75 20 61 72 65 20 77 72 69 74 69 6e 67 | you are writing| 00000990 20 61 20 6c 6f 6e 67 20 70 72 6f 67 72 61 6d 2c | a long program,| 000009a0 20 74 68 69 73 20 69 73 20 61 6e 20 69 6d 70 6f | this is an impo| 000009b0 72 74 61 6e 74 20 0a 63 6f 6e 73 69 64 65 72 61 |rtant .considera| 000009c0 74 69 6f 6e 2c 20 61 73 20 79 6f 75 20 6d 61 79 |tion, as you may| 000009d0 20 6e 6f 74 20 68 61 76 65 20 72 6f 6f 6d 20 66 | not have room f| 000009e0 6f 72 20 74 68 65 20 70 72 6f 67 72 61 6d 20 61 |or the program a| 000009f0 66 74 65 72 20 0a 73 75 62 74 72 61 63 74 69 6e |fter .subtractin| 00000a00 67 20 32 30 4b 20 66 72 6f 6d 20 74 68 65 20 63 |g 20K from the c| 00000a10 6f 6d 70 75 74 65 72 27 73 20 33 32 4b 20 52 41 |omputer's 32K RA| 00000a20 4d 20 66 6f 72 20 61 20 4d 4f 44 45 20 32 20 73 |M for a MODE 2 s| 00000a30 63 72 65 65 6e 2e 20 49 66 20 74 68 69 73 20 0a |creen. If this .| 00000a40 69 73 20 74 68 65 20 63 61 73 65 2c 20 79 6f 75 |is the case, you| 00000a50 20 6d 75 73 74 20 75 73 65 20 61 20 6d 6f 64 65 | must use a mode| 00000a60 20 77 68 69 63 68 20 75 73 65 73 20 6c 65 73 73 | which uses less| 00000a70 20 6d 65 6d 6f 72 79 2c 20 73 75 63 68 20 61 73 | memory, such as| 00000a80 20 6d 6f 64 65 20 35 2e 0a 0a 20 20 20 20 20 4d | mode 5... M| 00000a90 79 20 61 64 76 69 63 65 20 69 6e 20 63 68 6f 6f |y advice in choo| 00000aa0 73 69 6e 67 20 77 68 69 63 68 20 6d 6f 64 65 20 |sing which mode | 00000ab0 74 6f 20 75 73 65 20 77 6f 75 6c 64 20 62 65 20 |to use would be | 00000ac0 74 6f 20 74 72 79 2c 20 69 66 20 61 74 20 0a 61 |to try, if at .a| 00000ad0 6c 6c 20 70 6f 73 73 69 62 6c 65 2c 20 74 6f 20 |ll possible, to | 00000ae0 75 73 65 20 6f 6e 65 20 6f 66 20 74 68 65 20 32 |use one of the 2| 00000af0 30 4b 20 6d 6f 64 65 73 2e 20 54 68 65 20 64 65 |0K modes. 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