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shipwren/PT3

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.

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Filename: shipwren/PT3
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File size: 6ACF bytes
Load address: 0000
Exec address: 0000
Duplicates

There is 1 duplicate copy of this file in the archive:

File contents
Programming Technique
Part 3: Graphics - Screen Memory

Written by Dominic Ford

At the end of the last article, you could program graphics in colour using
the built in MOVE and DRAW commands in BASIC, and could program fast,
detailed graphics in black and white using user defined characters. But
there was a problem if we wanted graphics in both colour and detail, as
by both methods the production of such graphics would be incredibly slow.

There is a solution to this problem, which involves taking advantage of the
way in which the computer's video system operates. The colour of each pixel
on your monitor is determined the data sent to the monitor by your computer -
thats fairly obvious. But if you have drawn a very complex set of shapes on
the computer screen, perhaps using a BASIC program which took several hours
to calculate and draw the result you see on the screen, how does the 
computer remember the colour of every pixel on the screen, and know what
data to send to your monitor? The answer is simple: within your computer's
memory there is an area set aside to store the colour of every pixel on the
screen, and when the monitor requests the colour of a particular pixel, the
computer simply reads the colour of the pixel from this area of memory.
This area of memory is called "Screen Memory". It can be considered as a
sort of giant table, holding data for the colours of all of the pixels in
a huge matrix of memory locations.

But of what use is this to the programmer you might be wondering. The
answer is that when you use user defined characters, MOVE and DRAW commands,
or any other method of putting anything on the screen, the computer simply
alters a few values in this giant table. But the computer takes time to
process your MOVE and DRAW commands to work out which values in the table
to alter. If you could bypass this stage, and set the values in this giant
table yourself without using the computer's general purpose graphics
commands, you can get exactly the result that you want, and much faster
than you would achieve if you were doing the operation through the
computer's commands. These have to process the parameters which you give,
and then calculate where abouts in the giant table you want to alter,
before they can actually do any useful work. Thus a lot of computer time
is wasted in performing calculations, which can actually be skipped if the
programmer is careful.

There are two possible ways of achieving this idea, both of which are useful
in different situations:

1. Save screen memory to disc.

This idea is fairly simple to understand. You, as the programmer, write a
program to plot the artwork for your program onto the screen, taking
however many hours it may take to complete all of the calculations
required to plot the graphics. You run this program once on your computer,
so that your masterpiece is stored in the screen memory of your computer
(ie. is on the monitor of your computer). You than transfer the entire
contents of the screen memory on your computer to floppy disc.

Then, in the final program which you want the user to operate, you include
a command to load this data back from floppy disc into the screen memory
of their computer. In a matter of a couple of seconds, graphics which took
possible hours to calculate will appear on their screen. This is a
technique which I have used in several of my programs. The main menu of
Shipwrecked II: Jupiter 3, where you select Play, Instructions, Game
Complete or Cheat with the starfield backdrop was done in this way - the
screen took nearly an hour for my computer to calculate, but I was able
to make it appear in seconds on your screen. I should also mention the title
screen to Shipwreck with the animated sea, not to mention the endless EUG
title screens I have done in this way - the Christmas EUG 35 screen with
the swirly blue background (credited erroneously to Robert Sprowson) took
two whole days to plot!

In order to achieve this, you simply need to understand two BASIC commands:

*SAVE Filename nnnn mmmm
*LOAD Filename nnnn

These load and save areas of the computer's memory to floppy disc or cassette
with the filename givan in place of the work Filename, starting at memory
location nnnn, and continuing until memory location mmmm (where nnnn and mmmm
are both in hexidecimal - base 16). Don't worry if you don't entirely
understand this - it is not necessary to entirely understand the workings
of these commands, or of hexidecimal to use them to store and retrieve
screen memory. All you need to do is substitute nnnn and mmmm for the
addresses of the start and finish of screen memory. The location of these
within your computer varies depeding upon which graphics mode you are using
(which you selected using the MODE command - see previous tutorial if you
are unsure of what this is), but is always constant for any particular
mode. the values for the different modes are given below:

Mode            nnnn    mmmm   NOTES:
0               3000    7FFF
1               3000    7FFF   - nnnn and mmmm values given to the left are
2               3000    7FFF   all in hexidecimal. Therefore they can be
3               4000    7FFF   substituted directly into the *LOAD and *SAVE
4               5800    7FFF   commands with no conversion required.
5               5800    7FFF
6               6000    7FFF   - Values are not valid for the BBC model A in
7               7C00    7FFF   modes 4,5,6 or 7.

Examples:

Suppose you are in mode 2, and want to store the screen to a floppy. From
the table above, nnnn is 3000 and mmmm is 7FFF for mode 2. Therefore, to
save the screen under the filename "Screen", you would type:

*SAVE Screen 3000 7FFF

Now, if you want to restore the screen again, and put your graphics back
on the screen again, you would use *LOAD. Notice that this time only a
value for nnnn is required, and not for mmmm:

*LOAD Screen 3000

You can try this out now, if you switch on your Electron, and insert a work
disc into your floppy drive. If you use ADFS, you may need to use *MOUNT
to select the floppy disc, and be able to write to it. Now enter mode 2
by typing:

MODE 2

Now type some text on the screen which will represent the graphics which
you want to save. When you have finished, save the contents of screen memory
to floppy disk as shown above, by typing:

*SAVE Screen 3000 7FFF

When the disk drive has finished, clear the text off the screen by typing:

CLS

The text which you typed onto the screen has now been wiped from the
computer's memory, but should still be stored on the floppy disc. We can
show that this is so by typing:

*LOAD Screen 3000

Your text should now reappear on the screen. You can repeat this with any
kind of complex graphics on screen, and it will still work. The same can
be done in any other graphics mode, simply by changing the values of nnnn
and mmmm to those given in the table above. For example in mode 5 you
would use:

*SAVE Screen 5800 7FFF

followed by:

*LOAD Screen 5800 7FFF

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.

If you are not familiar with machine code, then you can probably skip the next
section on how to directly access screen memory, as it is impossible
to achieve any speed by accessing screen memory directly in BASIC - your
program will crawl along incredibly slowly. You could still use such a
technique in preparing a screen which you intend to save to floppy using
the technique described before with *SAVE and *LOAD.

Firstly, it is important to know how screen memory is laid out in memory.
The exact position of each character within the memory of your computer
varies from one graphics mode to another, but the basic principals are the
same:

- The screen area is divided up into characters. These are blocks of eight 
  pixels by eight pixels, and are the same size as the rectangular block
  of space taken up by one letter when you print text to the screen. Screen
  memory is divided up into blocks, with each the characters being defined
  by one of these blocks.
- The screen is stored row by row, with the top row first and the bottom
  row last.
- Within each of these rows, characters are stored one by one, from left to
  right.
- In all modes, screen memory ends at &7FFF (ie. at the end of RAM)

Footnote: By convention, memory addresses are specified in hexidecimal. I
          use the standard Acorn '&' prefix to indicate hexidecimal values.
          Those used to dealing with PCs are any other computers may be used
          to the American convention of using a dollar prefix in its place.
          It should be noted that the Elk expects & and will not accept $.

Example:

Suppose I want to draw a blob in character square (6,7), in mode 1. What is 
the address of this square? (given that each character takes 16 bytes of
screen memory to define in mode 1 - we'll come to how to calculate this in
a minute).

Well - looking at the table earlier, screen memory starts at &3000 in mode 1.
We want to find the start of row 7 in screen memory. This row has 7 rows of
characters above it, and which will be stored before it. Each of these rows
consists of 40 characters, which take up 16 bytes each. Therefore the total
memory taken up by these rows is 7 * 40 * 16 = 4480 bytes. [Each character
is 16 bytes, each row consists of 40 of these characters, therefore we
multiply by 40, and as there are 7 of these complete rows, we multiply by
7 to get the value for seven of these rows]

Now we have that the starting address of row 7 is at:

&3000 + 4480 = &4180

Now that we have found where abouts in memory this row starts, we need to
locate character 6. This should be fairly easy - we know that characters are
stored from left to right, so just add on the memory required to store six
characters to the left of the one we want, i.e. 16 * 6 = 96 bytes [Six
characters, each taking 16 bytes to store].

Therefore the address of character (6,7) will be:

&4180 + 96 = &41E0

Answer: Character (6,7) is stored from &41E0 to &41EF

Just to check the answer is correct, type this simple loop:

10 MODE 1
20 FOR ad=&41E0 TO &41EF
30 ?ad=255
40 NEXT

And yes, a white splodge does appear in square (6,7).

This whole procedure can actually be refined down, and expressed as a simple
formula:

Address = Base + ( y * No. characters per line + x ) * Memory per character

Where: Address= The memory location of the start of the character you want.
       Base   = Start of screen memory in current graphics mode.
       (x,y)  = Co-ordinate of square you want to access (same as PRINT TAB)

Since the number of characters per line, the number of bytes per character,
and the base address of screen memory are all constant for any particular
graphics mode, the values for the graphics mode you are using can be
substituted into the equation in your program. For mode 1, for example, you
would use:

Address = &3000 + (40 * y + x) * 16

Before continuing, one note should be borne in mind. You may remember that
a while back I mentioned that when you type text which overflows off the
bottom of the screen, the computer makes a note that each line is stored
in memory one line lower than it should be, but does not actually move
anything around in screen memory. The same effect can cause problems with
direct access to screen memory as well. The simple rule for beginners is,
NEVER attempt to use direct screen memory access when there is even the
slightest possibility that the screen might have been allowed to scroll up
at all (e.g. after using INPUT to get input from the user - how do you
know that they didn't type so much that it went of the bottom of the screen?)
If you know what you are doing, this rule can be ignored, and high-speed
scrolling can be achieved as a result, but that is not for the beginner! I
can tell you from first hand experience, that creating scrolling in Jupiter
3 by this technique was a nightmare - especially when the operating system
calls you need to make to achieve this are completely different on the BBC
to the Elk, so you need to find out which platform you are operating on,
and then make the relavant calls for that platform!

Now that you can locate a the memory occupied by a character in screen
memory, you need to know how the bytes within this block are laid out, so that
you can have pixel by pixel control over your monitor screen. This is also
required if you are to be able to calculate the amount of memory taken up
by each character's definition in a particular mode.

The graphics modes can be divided into three categories: monochrome modes
(ie. two colours), four colour modes, and sixteen colour modes. All of the
Elk's modes fall into these three groupings. Which of these colours each
pixel of the screen is depends upon the status of a number of binary bits.
These have two states, 0 or 1. Now, in a monochrome mode, one bit is
sufficient to define the colour of one pixel (bits have two states, 0 or 1,
and the pixels have two states, colour 0 or colour 1). In a four colour mode,
on the other hand, one bit will not be sufficient, as four different states
are required. Two bits will be enough to store the colour of one pixel,
however:

Bit 1 state     Bit 2 state     Colour
  0               0               0
  0               1               1
  1               0               2
  1               1               3

Those familiar with binary will spot the pattern straight away - simply
convert the binary value of the two bits into decimal, and you have the
logical colour number. A very similar pattern applies to MODE 2 - the Elk's
16 colour mode. Each pixel requires 4 bits to store its colour (2^4=16).
Once again, the colour of the pixel is the logical colour whose number is
given when the four bits are put together to form a binary value:

Bit 1 state  Bit 2 state  Bit 3 state  Bit 4 state  Colour  Default
  0            0            0            0            0       BLK
  0            0            0            1            1       RED
  0            0            1            0            2       GRN
  0            0            1            1            3       YEL
  0            1            0            0            4       BLE
  0            1            0            1            5       MGT
  0            1            1            0            6       CYN
  0            1            1            1            7       WHT
  1            0            0            0            8     BLK/WHT
  1            0            0            1            9     RED/CYN
  1            0            1            0            10    GRN/MGT
  1            0            1            1            11    YEL/BLE
  1            1            0            0            12    BLE/YEL
  1            1            0            1            13    MGT/GRN
  1            1            1            0            14    CYN/RED
  1            1            1            1            15    WHT/BLK

[Default: Default setting of this logical colour when MODE 2 is first
entered]

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
VDU23. 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. 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. This time, though, not
all of the pixels in the character are being defined, and only the left
most 4 pixels. The right-most four pixels are defined in a second downward
vertical sweep. Thus the total memory allocation for each character is
sixteen bytes. The layout is shown more clearly using a diagram. 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 hexidecimal:

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 the 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
left-most, D is right-most), the lay out 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.

The same 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) bit clears the two bits defining the
pixel that we want to change. The OR &88 then sets the two bits that we
want set. This way we ensure that when we are going way 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 the pattern in these bits - each pixel requires
a two bit definition, but these two bits are not next to each other in
the byte - 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 similar to that found in four
colour modes, except that each byte can only store two pixels (each
occupies four bits). As with previous cases, the pixels are stored
in a number of vertical sweeps starting at the left of the character,
and moving to the right. This time, though, four vertical sweeps are
required to cover the entire character, not two as with four colour
modes:

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 hexadecimal. Values in right half of left diagram
should have &10 added to them (only least significant character given)

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 that once again bits defining the pixels A and B are not
stored together, but those defining the least/most significant
bits are stored together.

00000000  50 72 6f 67 72 61 6d 6d  69 6e 67 20 54 65 63 68  |Programming Tech|
00000010  6e 69 71 75 65 0d 50 61  72 74 20 33 3a 20 47 72  |nique.Part 3: Gr|
00000020  61 70 68 69 63 73 20 2d  20 53 63 72 65 65 6e 20  |aphics - Screen |
00000030  4d 65 6d 6f 72 79 0d 0d  57 72 69 74 74 65 6e 20  |Memory..Written |
00000040  62 79 20 44 6f 6d 69 6e  69 63 20 46 6f 72 64 0d  |by Dominic Ford.|
00000050  0d 41 74 20 74 68 65 20  65 6e 64 20 6f 66 20 74  |.At the end of t|
00000060  68 65 20 6c 61 73 74 20  61 72 74 69 63 6c 65 2c  |he last article,|
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000000b0  64 20 44 52 41 57 20 63  6f 6d 6d 61 6e 64 73 20  |d DRAW commands |
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000000d0  75 6c 64 20 70 72 6f 67  72 61 6d 20 66 61 73 74  |uld program fast|
000000e0  2c 0d 64 65 74 61 69 6c  65 64 20 67 72 61 70 68  |,.detailed graph|
000000f0  69 63 73 20 69 6e 20 62  6c 61 63 6b 20 61 6e 64  |ics in black and|
00000100  20 77 68 69 74 65 20 75  73 69 6e 67 20 75 73 65  | white using use|
00000110  72 20 64 65 66 69 6e 65  64 20 63 68 61 72 61 63  |r defined charac|
00000120  74 65 72 73 2e 20 42 75  74 0d 74 68 65 72 65 20  |ters. But.there |
00000130  77 61 73 20 61 20 70 72  6f 62 6c 65 6d 20 69 66  |was a problem if|
00000140  20 77 65 20 77 61 6e 74  65 64 20 67 72 61 70 68  | we wanted graph|
00000150  69 63 73 20 69 6e 20 62  6f 74 68 20 63 6f 6c 6f  |ics in both colo|
00000160  75 72 20 61 6e 64 20 64  65 74 61 69 6c 2c 20 61  |ur and detail, a|
00000170  73 0d 62 79 20 62 6f 74  68 20 6d 65 74 68 6f 64  |s.by both method|
00000180  73 20 74 68 65 20 70 72  6f 64 75 63 74 69 6f 6e  |s the production|
00000190  20 6f 66 20 73 75 63 68  20 67 72 61 70 68 69 63  | of such graphic|
000001a0  73 20 77 6f 75 6c 64 20  62 65 20 69 6e 63 72 65  |s would be incre|
000001b0  64 69 62 6c 79 20 73 6c  6f 77 2e 0d 0d 54 68 65  |dibly slow...The|
000001c0  72 65 20 69 73 20 61 20  73 6f 6c 75 74 69 6f 6e  |re is a solution|
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000069f0  5b 4d 4f 53 54 20 53 49  47 4e 49 46 49 43 41 4e  |[MOST SIGNIFICAN|
00006a00  54 5d 20 20 20 20 41 33  20 42 33 20 41 32 20 42  |T]    A3 B3 A2 B|
00006a10  32 20 41 31 20 42 31 20  41 30 20 42 30 20 20 20  |2 A1 B1 A0 B0   |
00006a20  20 5b 4c 45 41 53 54 20  53 49 47 4e 49 46 49 43  | [LEAST SIGNIFIC|
00006a30  41 4e 54 5d 0d 0d 4e 6f  74 69 63 65 20 74 68 61  |ANT]..Notice tha|
00006a40  74 20 6f 6e 63 65 20 61  67 61 69 6e 20 62 69 74  |t once again bit|
00006a50  73 20 64 65 66 69 6e 69  6e 67 20 74 68 65 20 70  |s defining the p|
00006a60  69 78 65 6c 73 20 41 20  61 6e 64 20 42 20 61 72  |ixels A and B ar|
00006a70  65 20 6e 6f 74 0d 73 74  6f 72 65 64 20 74 6f 67  |e not.stored tog|
00006a80  65 74 68 65 72 2c 20 62  75 74 20 74 68 6f 73 65  |ether, but those|
00006a90  20 64 65 66 69 6e 69 6e  67 20 74 68 65 20 6c 65  | defining the le|
00006aa0  61 73 74 2f 6d 6f 73 74  20 73 69 67 6e 69 66 69  |ast/most signifi|
00006ab0  63 61 6e 74 0d 62 69 74  73 20 61 72 65 20 73 74  |cant.bits are st|
00006ac0  6f 72 65 64 20 74 6f 67  65 74 68 65 72 2e 0d     |ored together..|
00006acf

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