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These are the same characters that are directly available on the PET keyboard.
The representation in screen memory is derived from the standard ASCII set by dropping bit
6; giving us a six bit code for the keyboard characters.
The graphic, or shifted characters, set Is represented by a 1 In bit six of the screen memory, giving an
additional 64 displayable characters.
This gives the following table for PET displayable characters. It should be noted that all of the graphics
characters are organized so that they are just a shift from the normal keyboard character.
Ii
01
~
~
~
~
1
1
1
1
I
~I
~
~
1
1
~
~
1
1
I 41
~
1
~
1
~
1
~
1
~,1i:::J
~
......
~
p
..
1
,
I:H:U::H
A
Q
1
•
•
•
...
,
."1"
Ii
k
"
~
I
•
T
1l~11
(;
S
"
~
"
~
• 1....
j)
1
•
4
I
I
111..,
t.
U
"
~
r
I
I
1111..
t
V
..
0
)<.
. .
I
11111
G
W
'(
I
0
I
111....
H
X
<
II
I
4-
-
•
111111
I
V
)
Y
,
1
~
-
1.. , ..
J
"
*
,
•
I
.J
11111
K
L
+
J
+
•
•
lUIl::J
L
,
(
L
1
•
,
11111
M
J
=
\
I
,
"
111..
N
1
)
/
. .
,
•
,
1111
0
+-
/
"
I
"
-
' .
FIgure 2.8.
PET
graphic charaet... Nt
(7
bit
code)
Example: This gives us the following conversions:
Character
In main memory
In screen memory
A
01000001
00000001
+
1100 0001
00000oo1
1
00110001
00110001
~
10110001
01110001
Note the reduction from seven bit ASCII to six bit gives the effect of changing the order
of A and 1. In screen memory, the 8th bit is used to store reverse field. The reverse
field consists of taking the dot pattern from the character generator and reversing It, replacing
a white dot with black and a black dot with a white.
If the operating system is used, it automatically translates the values from ASCII Into the screen
memory representation. Both PRINT and direct Input from the keyboard result in automatic
translation between the screen memory and the main memory.
8
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