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eug_titles/PT3/+P4
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/+P4 |
| Read OK: | ✔ |
| File size: | 156A 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/+P4
- Personal collection » Acorn hard disk » misc » misc3 » eug_titles/PT3/+P4
File contents
So... the Electron has modes where the definition of the colour of a
certain pixel can take 1, 2 or 4 bits. As each byte of the electron's
memory consists of 8 seperate bits, this means that in monochrome modes
we can fit 8 pixels into each byte (8/1=8). In four colour modes (where
each pixel requires two bits) we can fit 4 pixels into each byte (8/2=4).
Finally in MODE 2 (the sixteen colour mode where each pixel requires 4
bits) we can fit two pixels into each byte. Let's now look at how each is
laid out in more depth:
MONOCHROME MODES:
These are laid out in the simplest way, as eight pixels fit into each
byte. The character, remember, consists of an 8-by-8 square of pixels. The
data is stored in the same way as user defined characters are defined
using VDU 23. The first byte of the definition defines the top row of
eight pixels; the second defines the second row down, etc. Thus the entire
character can be defined in eight bytes, and this is the number of bytes
of the Elk's memory which is used by each character's definition. Note -
the most significant bit of each byte defines the left-most pixel of the
row, and the least significant bit defines the right-most pixel (just as
with VDU 23 definitions).
FOUR COLOUR MODES:
This time each byte can only hold four pixels. Once again, each byte holds
a number of pixels which are in a horizontal line. This time, as before,
the first eight bytes run down in a vertical line. The difference this
time, though, is that not all of the pixels in the character are being
defined by the one sweep, and instead only the left-most four pixels can
be defined. The right-most four pixels are defined in a second downward
vertical sweep. Thus the total memory allocation for the definition of
each character is sixteen bytes. The layout of these is shown more clearly
by the diagram below. This shows the eight-by-eight block of pixels in a
character, and each has the offset of the memory location where this pixel
is stored from the start of the character definition. Offsets are given in
hexadecimal:
PIXEL-BY-PIXEL LAYOUT MEMORY FLOW
00008888 0 8
11119999 | |
2222AAAA | |
3333BBBB | |
4444CCCC | |
5555DDDD | |
6666EEEE \ / \ /
7777FFFF 7 F
Each of these sixteen bytes, as already discussed, contains four pixel
definitions. But within each byte's eight bits, they are not laid out as
you might at first imagine. If we call the four pixels A, B, C and D (A is
the left-most, D is the right-most), the layout is:
[MOST SIGNIFICANT] A1 B1 C1 D1 A0 B0 C0 D0 [LEAST SIGNIFICANT]
Where: A1: Most significant bit of A's definition } Two bit definition
A0: Least significant bit of A's definition } of pixel A
A simialar pattern holds for B, C and D.
EXAMPLE:
Let's suppose we want to set the colour of the left-most pixel in a byte
of a four-colour mode to be colour 3. We would use:
?address=(?address AND &77) OR &88
Explanation - The (?address AND &77) part clears the two bits defining the
pixel that we want to change. The OR &88 then sets the two bits that we
want to set. This way we ensure that when we are going say from colour 1
to colour 2, the least significant bit of the definition is cleared as
well as the most significant bit being set.
It is important to notice tha pattern in these bits - each pixel requires
a two bit definition, but these two bits are not next to each other in the
byte as one might expect - instead all of the high-bytes are stored
together, and all of the low-bytes are stored together.
SIXTEEN COLOUR MODES
The pattern for sixteen colour modes is very simialr to that found in four
colour modes, except that each byte can only store two pixels (each
occupies four bites). As with previous cases, the pixels are stored in a
number of vertical sweeps starting at the left of the character, and
moving right. This time four vertical sweeps are required to cover all of
the character (there are eight pixels across, and each sweep covers two of
them). The layout is shown below:
PIXEL-BY-PIXEL LAYOUT MEMORY FLOW
00880088 0 8 10 18
11991199 | | | |
22AA22AA | | | |
33BB33BB | | | |
44CC44CC | | | |
55DD55DD | | | |
66EE66EE \ / \ / \ / \ /
77FF77FF 7 F 17 1F
****
NB: Offsets given in hexadecinal. Values in the right half of the above
diagram (ie. the right four pixels, which have * underneath) should have
&10 added to them - only the least significant character of each
hexadecimal value is given. I am sure you can see my problem - the
hexadecimal values are two digits long, and I've only got one character in
which to put them!
The pattern of the layout of the pixel data in each of the bytes is also
similar to that for the four colour modes (the two pixels are A and B, and
each comprises of four bits 0-3, where 0 is least significant and 3 is
most significant).
[MOST SIGNIFICANT] A3 B3 A2 B2 A1 B1 A0 B0 [LEAST SIGNIFICANT]
Notice once again that bits defining the pixels A and B are not stored
together, but those defining the least/most significant bits are stored
together.
MORE ON THE SUBJECT ON SCREEN MEMORY IN NEXT EUG...
00000000 53 6f 2e 2e 2e 20 74 68 65 20 45 6c 65 63 74 72 |So... the Electr| 00000010 6f 6e 20 68 61 73 20 6d 6f 64 65 73 20 77 68 65 |on has modes whe| 00000020 72 65 20 74 68 65 20 64 65 66 69 6e 69 74 69 6f |re the definitio| 00000030 6e 20 6f 66 20 74 68 65 20 63 6f 6c 6f 75 72 20 |n of the colour | 00000040 6f 66 20 61 0d 63 65 72 74 61 69 6e 20 70 69 78 |of a.certain pix| 00000050 65 6c 20 63 61 6e 20 74 61 6b 65 20 31 2c 20 32 |el can take 1, 2| 00000060 20 6f 72 20 34 20 62 69 74 73 2e 20 41 73 20 65 | or 4 bits. As e| 00000070 61 63 68 20 62 79 74 65 20 6f 66 20 74 68 65 20 |ach byte of the | 00000080 65 6c 65 63 74 72 6f 6e 27 73 0d 6d 65 6d 6f 72 |electron's.memor| 00000090 79 20 63 6f 6e 73 69 73 74 73 20 6f 66 20 38 20 |y consists of 8 | 000000a0 73 65 70 65 72 61 74 65 20 62 69 74 73 2c 20 74 |seperate bits, t| 000000b0 68 69 73 20 6d 65 61 6e 73 20 74 68 61 74 20 69 |his means that i| 000000c0 6e 20 6d 6f 6e 6f 63 68 72 6f 6d 65 20 6d 6f 64 |n monochrome mod| 000000d0 65 73 0d 77 65 20 63 61 6e 20 66 69 74 20 38 20 |es.we can fit 8 | 000000e0 70 69 78 65 6c 73 20 69 6e 74 6f 20 65 61 63 68 |pixels into each| 000000f0 20 62 79 74 65 20 28 38 2f 31 3d 38 29 2e 20 49 | byte (8/1=8). I| 00000100 6e 20 66 6f 75 72 20 63 6f 6c 6f 75 72 20 6d 6f |n four colour mo| 00000110 64 65 73 20 28 77 68 65 72 65 0d 65 61 63 68 20 |des (where.each | 00000120 70 69 78 65 6c 20 72 65 71 75 69 72 65 73 20 74 |pixel requires t| 00000130 77 6f 20 62 69 74 73 29 20 77 65 20 63 61 6e 20 |wo bits) we can | 00000140 66 69 74 20 34 20 70 69 78 65 6c 73 20 69 6e 74 |fit 4 pixels int| 00000150 6f 20 65 61 63 68 20 62 79 74 65 20 28 38 2f 32 |o each byte (8/2| 00000160 3d 34 29 2e 0d 46 69 6e 61 6c 6c 79 20 69 6e 20 |=4)..Finally in | 00000170 4d 4f 44 45 20 32 20 28 74 68 65 20 73 69 78 74 |MODE 2 (the sixt| 00000180 65 65 6e 20 63 6f 6c 6f 75 72 20 6d 6f 64 65 20 |een colour mode | 00000190 77 68 65 72 65 20 65 61 63 68 20 70 69 78 65 6c |where each pixel| 000001a0 20 72 65 71 75 69 72 65 73 20 34 0d 62 69 74 73 | requires 4.bits| 000001b0 29 20 77 65 20 63 61 6e 20 66 69 74 20 74 77 6f |) we can fit two| 000001c0 20 70 69 78 65 6c 73 20 69 6e 74 6f 20 65 61 63 | pixels into eac| 000001d0 68 20 62 79 74 65 2e 20 4c 65 74 27 73 20 6e 6f |h byte. Let's no| 000001e0 77 20 6c 6f 6f 6b 20 61 74 20 68 6f 77 20 65 61 |w look at how ea| 000001f0 63 68 20 69 73 0d 6c 61 69 64 20 6f 75 74 20 69 |ch is.laid out i| 00000200 6e 20 6d 6f 72 65 20 64 65 70 74 68 3a 0d 0d 4d |n more depth:..M| 00000210 4f 4e 4f 43 48 52 4f 4d 45 20 4d 4f 44 45 53 3a |ONOCHROME MODES:| 00000220 0d 0d 54 68 65 73 65 20 61 72 65 20 6c 61 69 64 |..These are laid| 00000230 20 6f 75 74 20 69 6e 20 74 68 65 20 73 69 6d 70 | out in the simp| 00000240 6c 65 73 74 20 77 61 79 2c 20 61 73 20 65 69 67 |lest way, as eig| 00000250 68 74 20 70 69 78 65 6c 73 20 66 69 74 20 69 6e |ht pixels fit in| 00000260 74 6f 20 65 61 63 68 0d 62 79 74 65 2e 20 54 68 |to each.byte. Th| 00000270 65 20 63 68 61 72 61 63 74 65 72 2c 20 72 65 6d |e character, rem| 00000280 65 6d 62 65 72 2c 20 63 6f 6e 73 69 73 74 73 20 |ember, consists | 00000290 6f 66 20 61 6e 20 38 2d 62 79 2d 38 20 73 71 75 |of an 8-by-8 squ| 000002a0 61 72 65 20 6f 66 20 70 69 78 65 6c 73 2e 20 54 |are of pixels. T| 000002b0 68 65 0d 64 61 74 61 20 69 73 20 73 74 6f 72 65 |he.data is store| 000002c0 64 20 69 6e 20 74 68 65 20 73 61 6d 65 20 77 61 |d in the same wa| 000002d0 79 20 61 73 20 75 73 65 72 20 64 65 66 69 6e 65 |y as user define| 000002e0 64 20 63 68 61 72 61 63 74 65 72 73 20 61 72 65 |d characters are| 000002f0 20 64 65 66 69 6e 65 64 0d 75 73 69 6e 67 20 56 | defined.using V| 00000300 44 55 20 32 33 2e 20 54 68 65 20 66 69 72 73 74 |DU 23. The first| 00000310 20 62 79 74 65 20 6f 66 20 74 68 65 20 64 65 66 | byte of the def| 00000320 69 6e 69 74 69 6f 6e 20 64 65 66 69 6e 65 73 20 |inition defines | 00000330 74 68 65 20 74 6f 70 20 72 6f 77 20 6f 66 0d 65 |the top row of.e| 00000340 69 67 68 74 20 70 69 78 65 6c 73 3b 20 74 68 65 |ight pixels; the| 00000350 20 73 65 63 6f 6e 64 20 64 65 66 69 6e 65 73 20 | second defines | 00000360 74 68 65 20 73 65 63 6f 6e 64 20 72 6f 77 20 64 |the second row d| 00000370 6f 77 6e 2c 20 65 74 63 2e 20 54 68 75 73 20 74 |own, etc. Thus t| 00000380 68 65 20 65 6e 74 69 72 65 0d 63 68 61 72 61 63 |he entire.charac| 00000390 74 65 72 20 63 61 6e 20 62 65 20 64 65 66 69 6e |ter can be defin| 000003a0 65 64 20 69 6e 20 65 69 67 68 74 20 62 79 74 65 |ed in eight byte| 000003b0 73 2c 20 61 6e 64 20 74 68 69 73 20 69 73 20 74 |s, and this is t| 000003c0 68 65 20 6e 75 6d 62 65 72 20 6f 66 20 62 79 74 |he number of byt| 000003d0 65 73 0d 6f 66 20 74 68 65 20 45 6c 6b 27 73 20 |es.of the Elk's | 000003e0 6d 65 6d 6f 72 79 20 77 68 69 63 68 20 69 73 20 |memory which is | 000003f0 75 73 65 64 20 62 79 20 65 61 63 68 20 63 68 61 |used by each cha| 00000400 72 61 63 74 65 72 27 73 20 64 65 66 69 6e 69 74 |racter's definit| 00000410 69 6f 6e 2e 20 4e 6f 74 65 20 2d 0d 74 68 65 20 |ion. Note -.the | 00000420 6d 6f 73 74 20 73 69 67 6e 69 66 69 63 61 6e 74 |most significant| 00000430 20 62 69 74 20 6f 66 20 65 61 63 68 20 62 79 74 | bit of each byt| 00000440 65 20 64 65 66 69 6e 65 73 20 74 68 65 20 6c 65 |e defines the le| 00000450 66 74 2d 6d 6f 73 74 20 70 69 78 65 6c 20 6f 66 |ft-most pixel of| 00000460 20 74 68 65 0d 72 6f 77 2c 20 61 6e 64 20 74 68 | the.row, and th| 00000470 65 20 6c 65 61 73 74 20 73 69 67 6e 69 66 69 63 |e least signific| 00000480 61 6e 74 20 62 69 74 20 64 65 66 69 6e 65 73 20 |ant bit defines | 00000490 74 68 65 20 72 69 67 68 74 2d 6d 6f 73 74 20 70 |the right-most p| 000004a0 69 78 65 6c 20 28 6a 75 73 74 20 61 73 0d 77 69 |ixel (just as.wi| 000004b0 74 68 20 56 44 55 20 32 33 20 64 65 66 69 6e 69 |th VDU 23 defini| 000004c0 74 69 6f 6e 73 29 2e 0d 0d 46 4f 55 52 20 43 4f |tions)...FOUR CO| 000004d0 4c 4f 55 52 20 4d 4f 44 45 53 3a 0d 0d 54 68 69 |LOUR MODES:..Thi| 000004e0 73 20 74 69 6d 65 20 65 61 63 68 20 62 79 74 65 |s time each byte| 000004f0 20 63 61 6e 20 6f 6e 6c 79 20 68 6f 6c 64 20 66 | can only hold f| 00000500 6f 75 72 20 70 69 78 65 6c 73 2e 20 4f 6e 63 65 |our pixels. Once| 00000510 20 61 67 61 69 6e 2c 20 65 61 63 68 20 62 79 74 | again, each byt| 00000520 65 20 68 6f 6c 64 73 0d 61 20 6e 75 6d 62 65 72 |e holds.a number| 00000530 20 6f 66 20 70 69 78 65 6c 73 20 77 68 69 63 68 | of pixels which| 00000540 20 61 72 65 20 69 6e 20 61 20 68 6f 72 69 7a 6f | are in a horizo| 00000550 6e 74 61 6c 20 6c 69 6e 65 2e 20 54 68 69 73 20 |ntal line. This | 00000560 74 69 6d 65 2c 20 61 73 20 62 65 66 6f 72 65 2c |time, as before,| 00000570 0d 74 68 65 20 66 69 72 73 74 20 65 69 67 68 74 |.the first eight| 00000580 20 62 79 74 65 73 20 72 75 6e 20 64 6f 77 6e 20 | bytes run down | 00000590 69 6e 20 61 20 76 65 72 74 69 63 61 6c 20 6c 69 |in a vertical li| 000005a0 6e 65 2e 20 54 68 65 20 64 69 66 66 65 72 65 6e |ne. The differen| 000005b0 63 65 20 74 68 69 73 0d 74 69 6d 65 2c 20 74 68 |ce this.time, th| 000005c0 6f 75 67 68 2c 20 69 73 20 74 68 61 74 20 6e 6f |ough, is that no| 000005d0 74 20 61 6c 6c 20 6f 66 20 74 68 65 20 70 69 78 |t all of the pix| 000005e0 65 6c 73 20 69 6e 20 74 68 65 20 63 68 61 72 61 |els in the chara| 000005f0 63 74 65 72 20 61 72 65 20 62 65 69 6e 67 0d 64 |cter are being.d| 00000600 65 66 69 6e 65 64 20 62 79 20 74 68 65 20 6f 6e |efined by the on| 00000610 65 20 73 77 65 65 70 2c 20 61 6e 64 20 69 6e 73 |e sweep, and ins| 00000620 74 65 61 64 20 6f 6e 6c 79 20 74 68 65 20 6c 65 |tead only the le| 00000630 66 74 2d 6d 6f 73 74 20 66 6f 75 72 20 70 69 78 |ft-most four pix| 00000640 65 6c 73 20 63 61 6e 0d 62 65 20 64 65 66 69 6e |els can.be defin| 00000650 65 64 2e 20 54 68 65 20 72 69 67 68 74 2d 6d 6f |ed. The right-mo| 00000660 73 74 20 66 6f 75 72 20 70 69 78 65 6c 73 20 61 |st four pixels a| 00000670 72 65 20 64 65 66 69 6e 65 64 20 69 6e 20 61 20 |re defined in a | 00000680 73 65 63 6f 6e 64 20 64 6f 77 6e 77 61 72 64 0d |second downward.| 00000690 76 65 72 74 69 63 61 6c 20 73 77 65 65 70 2e 20 |vertical sweep. | 000006a0 54 68 75 73 20 74 68 65 20 74 6f 74 61 6c 20 6d |Thus the total m| 000006b0 65 6d 6f 72 79 20 61 6c 6c 6f 63 61 74 69 6f 6e |emory allocation| 000006c0 20 66 6f 72 20 74 68 65 20 64 65 66 69 6e 69 74 | for the definit| 000006d0 69 6f 6e 20 6f 66 0d 65 61 63 68 20 63 68 61 72 |ion of.each char| 000006e0 61 63 74 65 72 20 69 73 20 73 69 78 74 65 65 6e |acter is sixteen| 000006f0 20 62 79 74 65 73 2e 20 54 68 65 20 6c 61 79 6f | bytes. The layo| 00000700 75 74 20 6f 66 20 74 68 65 73 65 20 69 73 20 73 |ut of these is s| 00000710 68 6f 77 6e 20 6d 6f 72 65 20 63 6c 65 61 72 6c |hown more clearl| 00000720 79 0d 62 79 20 74 68 65 20 64 69 61 67 72 61 6d |y.by the diagram| 00000730 20 62 65 6c 6f 77 2e 20 54 68 69 73 20 73 68 6f | below. This sho| 00000740 77 73 20 74 68 65 20 65 69 67 68 74 2d 62 79 2d |ws the eight-by-| 00000750 65 69 67 68 74 20 62 6c 6f 63 6b 20 6f 66 20 70 |eight block of p| 00000760 69 78 65 6c 73 20 69 6e 20 61 0d 63 68 61 72 61 |ixels in a.chara| 00000770 63 74 65 72 2c 20 61 6e 64 20 65 61 63 68 20 68 |cter, and each h| 00000780 61 73 20 74 68 65 20 6f 66 66 73 65 74 20 6f 66 |as the offset of| 00000790 20 74 68 65 20 6d 65 6d 6f 72 79 20 6c 6f 63 61 | the memory loca| 000007a0 74 69 6f 6e 20 77 68 65 72 65 20 74 68 69 73 20 |tion where this | 000007b0 70 69 78 65 6c 0d 69 73 20 73 74 6f 72 65 64 20 |pixel.is stored | 000007c0 66 72 6f 6d 20 74 68 65 20 73 74 61 72 74 20 6f |from the start o| 000007d0 66 20 74 68 65 20 63 68 61 72 61 63 74 65 72 20 |f the character | 000007e0 64 65 66 69 6e 69 74 69 6f 6e 2e 20 4f 66 66 73 |definition. Offs| 000007f0 65 74 73 20 61 72 65 20 67 69 76 65 6e 20 69 6e |ets are given in| 00000800 0d 68 65 78 61 64 65 63 69 6d 61 6c 3a 0d 0d 50 |.hexadecimal:..P| 00000810 49 58 45 4c 2d 42 59 2d 50 49 58 45 4c 20 4c 41 |IXEL-BY-PIXEL LA| 00000820 59 4f 55 54 20 20 20 20 20 4d 45 4d 4f 52 59 20 |YOUT MEMORY | 00000830 46 4c 4f 57 0d 0d 30 30 30 30 38 38 38 38 20 20 |FLOW..00008888 | 00000840 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00000850 20 20 30 20 20 20 20 20 20 38 0d 31 31 31 31 39 | 0 8.11119| 00000860 39 39 39 20 20 20 20 20 20 20 20 20 20 20 20 20 |999 | 00000870 20 20 20 20 20 20 20 7c 20 20 20 20 20 20 7c 0d | | |.| 00000880 32 32 32 32 41 41 41 41 20 20 20 20 20 20 20 20 |2222AAAA | 00000890 20 20 20 20 20 20 20 20 20 20 20 20 7c 20 20 20 | | | 000008a0 20 20 20 7c 0d 33 33 33 33 42 42 42 42 20 20 20 | |.3333BBBB | 000008b0 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 000008c0 20 7c 20 20 20 20 20 20 7c 0d 34 34 34 34 43 43 | | |.4444CC| 000008d0 43 43 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |CC | 000008e0 20 20 20 20 20 20 7c 20 20 20 20 20 20 7c 0d 35 | | |.5| 000008f0 35 35 35 44 44 44 44 20 20 20 20 20 20 20 20 20 |555DDDD | 00000900 20 20 20 20 20 20 20 20 20 20 20 7c 20 20 20 20 | | | 00000910 20 20 7c 0d 36 36 36 36 45 45 45 45 20 20 20 20 | |.6666EEEE | 00000920 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 5c | \| 00000930 20 2f 20 20 20 20 5c 20 2f 0d 37 37 37 37 46 46 | / \ /.7777FF| 00000940 46 46 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |FF | 00000950 20 20 20 20 20 20 37 20 20 20 20 20 20 46 0d 0d | 7 F..| 00000960 45 61 63 68 20 6f 66 20 74 68 65 73 65 20 73 69 |Each of these si| 00000970 78 74 65 65 6e 20 62 79 74 65 73 2c 20 61 73 20 |xteen bytes, as | 00000980 61 6c 72 65 61 64 79 20 64 69 73 63 75 73 73 65 |already discusse| 00000990 64 2c 20 63 6f 6e 74 61 69 6e 73 20 66 6f 75 72 |d, contains four| 000009a0 20 70 69 78 65 6c 0d 64 65 66 69 6e 69 74 69 6f | pixel.definitio| 000009b0 6e 73 2e 20 42 75 74 20 77 69 74 68 69 6e 20 65 |ns. But within e| 000009c0 61 63 68 20 62 79 74 65 27 73 20 65 69 67 68 74 |ach byte's eight| 000009d0 20 62 69 74 73 2c 20 74 68 65 79 20 61 72 65 20 | bits, they are | 000009e0 6e 6f 74 20 6c 61 69 64 20 6f 75 74 20 61 73 0d |not laid out as.| 000009f0 79 6f 75 20 6d 69 67 68 74 20 61 74 20 66 69 72 |you might at fir| 00000a00 73 74 20 69 6d 61 67 69 6e 65 2e 20 49 66 20 77 |st imagine. If w| 00000a10 65 20 63 61 6c 6c 20 74 68 65 20 66 6f 75 72 20 |e call the four | 00000a20 70 69 78 65 6c 73 20 41 2c 20 42 2c 20 43 20 61 |pixels A, B, C a| 00000a30 6e 64 20 44 20 28 41 20 69 73 0d 74 68 65 20 6c |nd D (A is.the l| 00000a40 65 66 74 2d 6d 6f 73 74 2c 20 44 20 69 73 20 74 |eft-most, D is t| 00000a50 68 65 20 72 69 67 68 74 2d 6d 6f 73 74 29 2c 20 |he right-most), | 00000a60 74 68 65 20 6c 61 79 6f 75 74 20 69 73 3a 0d 0d |the layout is:..| 00000a70 5b 4d 4f 53 54 20 53 49 47 4e 49 46 49 43 41 4e |[MOST SIGNIFICAN| 00000a80 54 5d 20 20 41 31 20 42 31 20 43 31 20 44 31 20 |T] A1 B1 C1 D1 | 00000a90 41 30 20 42 30 20 43 30 20 44 30 20 20 5b 4c 45 |A0 B0 C0 D0 [LE| 00000aa0 41 53 54 20 53 49 47 4e 49 46 49 43 41 4e 54 5d |AST SIGNIFICANT]| 00000ab0 0d 0d 57 68 65 72 65 3a 20 41 31 3a 20 4d 6f 73 |..Where: A1: Mos| 00000ac0 74 20 73 69 67 6e 69 66 69 63 61 6e 74 20 62 69 |t significant bi| 00000ad0 74 20 6f 66 20 41 27 73 20 64 65 66 69 6e 69 74 |t of A's definit| 00000ae0 69 6f 6e 20 20 7d 20 54 77 6f 20 62 69 74 20 64 |ion } Two bit d| 00000af0 65 66 69 6e 69 74 69 6f 6e 0d 20 20 20 20 20 20 |efinition. | 00000b00 20 41 30 3a 20 4c 65 61 73 74 20 73 69 67 6e 69 | A0: Least signi| 00000b10 66 69 63 61 6e 74 20 62 69 74 20 6f 66 20 41 27 |ficant bit of A'| 00000b20 73 20 64 65 66 69 6e 69 74 69 6f 6e 20 7d 20 20 |s definition } | 00000b30 6f 66 20 70 69 78 65 6c 20 41 0d 0d 41 20 73 69 |of pixel A..A si| 00000b40 6d 69 61 6c 61 72 20 70 61 74 74 65 72 6e 20 68 |mialar pattern h| 00000b50 6f 6c 64 73 20 66 6f 72 20 42 2c 20 43 20 61 6e |olds for B, C an| 00000b60 64 20 44 2e 0d 0d 45 58 41 4d 50 4c 45 3a 0d 0d |d D...EXAMPLE:..| 00000b70 4c 65 74 27 73 20 73 75 70 70 6f 73 65 20 77 65 |Let's suppose we| 00000b80 20 77 61 6e 74 20 74 6f 20 73 65 74 20 74 68 65 | want to set the| 00000b90 20 63 6f 6c 6f 75 72 20 6f 66 20 74 68 65 20 6c | colour of the l| 00000ba0 65 66 74 2d 6d 6f 73 74 20 70 69 78 65 6c 20 69 |eft-most pixel i| 00000bb0 6e 20 61 20 62 79 74 65 0d 6f 66 20 61 20 66 6f |n a byte.of a fo| 00000bc0 75 72 2d 63 6f 6c 6f 75 72 20 6d 6f 64 65 20 74 |ur-colour mode t| 00000bd0 6f 20 62 65 20 63 6f 6c 6f 75 72 20 33 2e 20 57 |o be colour 3. W| 00000be0 65 20 77 6f 75 6c 64 20 75 73 65 3a 0d 0d 3f 61 |e would use:..?a| 00000bf0 64 64 72 65 73 73 3d 28 3f 61 64 64 72 65 73 73 |ddress=(?address| 00000c00 20 41 4e 44 20 26 37 37 29 20 4f 52 20 26 38 38 | AND &77) OR &88| 00000c10 0d 0d 45 78 70 6c 61 6e 61 74 69 6f 6e 20 2d 20 |..Explanation - | 00000c20 54 68 65 20 28 3f 61 64 64 72 65 73 73 20 41 4e |The (?address AN| 00000c30 44 20 26 37 37 29 20 70 61 72 74 20 63 6c 65 61 |D &77) part clea| 00000c40 72 73 20 74 68 65 20 74 77 6f 20 62 69 74 73 20 |rs the two bits | 00000c50 64 65 66 69 6e 69 6e 67 20 74 68 65 0d 70 69 78 |defining the.pix| 00000c60 65 6c 20 74 68 61 74 20 77 65 20 77 61 6e 74 20 |el that we want | 00000c70 74 6f 20 63 68 61 6e 67 65 2e 20 54 68 65 20 4f |to change. The O| 00000c80 52 20 26 38 38 20 74 68 65 6e 20 73 65 74 73 20 |R &88 then sets | 00000c90 74 68 65 20 74 77 6f 20 62 69 74 73 20 74 68 61 |the two bits tha| 00000ca0 74 20 77 65 0d 77 61 6e 74 20 74 6f 20 73 65 74 |t we.want to set| 00000cb0 2e 20 54 68 69 73 20 77 61 79 20 77 65 20 65 6e |. This way we en| 00000cc0 73 75 72 65 20 74 68 61 74 20 77 68 65 6e 20 77 |sure that when w| 00000cd0 65 20 61 72 65 20 67 6f 69 6e 67 20 73 61 79 20 |e are going say | 00000ce0 66 72 6f 6d 20 63 6f 6c 6f 75 72 20 31 0d 74 6f |from colour 1.to| 00000cf0 20 63 6f 6c 6f 75 72 20 32 2c 20 74 68 65 20 6c | colour 2, the l| 00000d00 65 61 73 74 20 73 69 67 6e 69 66 69 63 61 6e 74 |east significant| 00000d10 20 62 69 74 20 6f 66 20 74 68 65 20 64 65 66 69 | bit of the defi| 00000d20 6e 69 74 69 6f 6e 20 69 73 20 63 6c 65 61 72 65 |nition is cleare| 00000d30 64 20 61 73 0d 77 65 6c 6c 20 61 73 20 74 68 65 |d as.well as the| 00000d40 20 6d 6f 73 74 20 73 69 67 6e 69 66 69 63 61 6e | most significan| 00000d50 74 20 62 69 74 20 62 65 69 6e 67 20 73 65 74 2e |t bit being set.| 00000d60 0d 0d 49 74 20 69 73 20 69 6d 70 6f 72 74 61 6e |..It is importan| 00000d70 74 20 74 6f 20 6e 6f 74 69 63 65 20 74 68 61 20 |t to notice tha | 00000d80 70 61 74 74 65 72 6e 20 69 6e 20 74 68 65 73 65 |pattern in these| 00000d90 20 62 69 74 73 20 2d 20 65 61 63 68 20 70 69 78 | bits - each pix| 00000da0 65 6c 20 72 65 71 75 69 72 65 73 0d 61 20 74 77 |el requires.a tw| 00000db0 6f 20 62 69 74 20 64 65 66 69 6e 69 74 69 6f 6e |o bit definition| 00000dc0 2c 20 62 75 74 20 74 68 65 73 65 20 74 77 6f 20 |, but these two | 00000dd0 62 69 74 73 20 61 72 65 20 6e 6f 74 20 6e 65 78 |bits are not nex| 00000de0 74 20 74 6f 20 65 61 63 68 20 6f 74 68 65 72 20 |t to each other | 00000df0 69 6e 20 74 68 65 0d 62 79 74 65 20 61 73 20 6f |in the.byte as o| 00000e00 6e 65 20 6d 69 67 68 74 20 65 78 70 65 63 74 20 |ne might expect | 00000e10 2d 20 69 6e 73 74 65 61 64 20 61 6c 6c 20 6f 66 |- instead all of| 00000e20 20 74 68 65 20 68 69 67 68 2d 62 79 74 65 73 20 | the high-bytes | 00000e30 61 72 65 20 73 74 6f 72 65 64 0d 74 6f 67 65 74 |are stored.toget| 00000e40 68 65 72 2c 20 61 6e 64 20 61 6c 6c 20 6f 66 20 |her, and all of | 00000e50 74 68 65 20 6c 6f 77 2d 62 79 74 65 73 20 61 72 |the low-bytes ar| 00000e60 65 20 73 74 6f 72 65 64 20 74 6f 67 65 74 68 65 |e stored togethe| 00000e70 72 2e 0d 0d 53 49 58 54 45 45 4e 20 43 4f 4c 4f |r...SIXTEEN COLO| 00000e80 55 52 20 4d 4f 44 45 53 0d 0d 54 68 65 20 70 61 |UR MODES..The pa| 00000e90 74 74 65 72 6e 20 66 6f 72 20 73 69 78 74 65 65 |ttern for sixtee| 00000ea0 6e 20 63 6f 6c 6f 75 72 20 6d 6f 64 65 73 20 69 |n colour modes i| 00000eb0 73 20 76 65 72 79 20 73 69 6d 69 61 6c 72 20 74 |s very simialr t| 00000ec0 6f 20 74 68 61 74 20 66 6f 75 6e 64 20 69 6e 20 |o that found in | 00000ed0 66 6f 75 72 0d 63 6f 6c 6f 75 72 20 6d 6f 64 65 |four.colour mode| 00000ee0 73 2c 20 65 78 63 65 70 74 20 74 68 61 74 20 65 |s, except that e| 00000ef0 61 63 68 20 62 79 74 65 20 63 61 6e 20 6f 6e 6c |ach byte can onl| 00000f00 79 20 73 74 6f 72 65 20 74 77 6f 20 70 69 78 65 |y store two pixe| 00000f10 6c 73 20 28 65 61 63 68 0d 6f 63 63 75 70 69 65 |ls (each.occupie| 00000f20 73 20 66 6f 75 72 20 62 69 74 65 73 29 2e 20 41 |s four bites). 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