Commodore PET User Manual page 395

The tough part of this exercise is that the screen display code needed to
POKE is different from the PET ASCII code that C$ is assigned. As covered in
Chapter 6, the screen code ois equivalent to a 6-bit ASCII subset with ASCII bit 7
moved over into screen bit 6:
l
-'- r""-----
~_-.,
7 6 5 4 3 2 1 0
~
o
=
eharaeter
1
=
graphie
(shifted)
L
1......-.....,
6-bit sereen
eode
o
=
eharaeter
1
=
graphie
(shifted)
This is what line 150 in the program does with the logic operation:
(ASC(C$)AND128)/2 OR (ASC(C$)AND63)
Well go over this operation piece by piece.
382
ASC(C$)
AND 128
/2
OR
provides the numerical equivalent of the character C$.
As a number, it can be manipulated arithmetically (and
logically).
12810=8016. This AND operation masks off (zeroes) ail
but the high-order bit (bit
7)
of the ASCII code to isolate
it. Regardless of the previous value of C$, after this
operation the term has a value of:
100000002 or 000000002.
Divide by 2. In binary, division by 2 is equivalent to a
logical right shift one bit. It admittedly requires a little
background to come up with this step. After this opera-
tion. the term has a value of:
010000002 or 000000002.
Thus, we have moved bit
7
of the ASCII code into bit 6
and replaced bit
7
with a zero.
This operation will combine the bit configuration
already developed (010000002 or 000000002) with the
bit configuration yet to be discussed. (ASC(C$)AND63).
The OR operation is performed last (see Table 3-4).