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94
dots, the printhead
would have to slow down and back up to
print both dots-not
very efficient! To avoid this inefficiency,
this printer will not allow you to define a character
like Figure
8-5. (Actually, you can define it, but when it prints, your printer
will leave out the overlapping
dots, so that it would print like
Figure 8-4.)
n Rule 2: Each row is divided into three bytes
Now it's time to give our creative side a break and get down to
some basic arithmetic.
Each vertical column (which has a max-
imum of 24 dots) is first divided into three groups of eight dots.
Each group of eight dots is represented
by one byte, which con-
sists of eight bits. That's where the numbers down the left side
of the grid come in. Notice that there is a number for each row
of dots and that each number is twice the number below it. By
making these numbers powers of two we can take any combina-
tion of dots in a vertical column and assign them a unique value.
Some examples will make this clearer. As shown in Figure 8-6,
if we add the numbers
for the dots that print in a column, the
sum will be a number in the range of 0 to 255. Each number
from 0 - 255 represents
a unique combination
of dots.
128
O-128
64
0 -
64
0
-
64
32
0 -
32
0
-
32
0
-
32
16
0 -
16
0 -
16
8
O-8
.-a
4
o-4
o-4
2
o-2
O-2
0
-2
1
O-l
O-1
-
_____
Sum
103
58
255
Figure 8-6.
By adding the values of each dot in a column, you'll
get
a unique description
for any combination
of dots.
So, add up the values of the dots in each cloumn using this
system. In Figure 8-7 we've shown our grid with the sums of the
columns filled in across the bottom (see if these agree with your
answers!). Across the top of the grid you've probably noticed
the cryptic labeling of each column: dl, d2, ~3, etc. These labels
correspond
to the labels in the command
syntax statement,
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