Appendix B Memory Usage
307
TABU
1-3: VIC Chip Functions by Memory Location (continued)
6 This bit controls the size of the character matrix.
If
it
is
0, the matrix will
be
normal
(8
X
8).
If
it
is
I,
the characters will
be
8 wide
by
16
tall.
7 Raster value. The light pen
is
adjusted using this number.
8 Screen memory pointer. This location determines the section of memory the VIC chip
will
look at for its display information.
9 Character set location. Changing the value in this location
will
point the
VI
C to another
section of memory for its characters. This may
be
used to develop a custom character set.
10
Horizontal position of light pen
is
stored
in
this location.
11
Vertical position oflight pen
is
stored in this location.
12
X-Pot. Digitized value
of
variable resistance game controller
is
stored here.
13
V-Pot. Digitized value of variable resistance game controller
is
stored here.
14
Lowest sound frequency register. The larger the number, the higher the frequency.
15
Middle sound frequency register. The larger the number, the higher the frequency.
16
Highest sound frequency register. The larger the number, the higher the frequency.
17
White noise generator. The higher the number, the higher the frequency of the sound.
18
Volume control. Ranges from 0 (off) to
15
(maximum volume).
19
Auxiliary color register.
20
Screen color register.
21
Reverse video switch
(I
= normal; 0 = reverse).
22
Border color.
There are four bits in the "nibble"
that
make up the first (high-order)
bits
of
memory location 36879.
That
means that the screen can be set to any
of
15
different colors.
To
change the four screen color bits without affecting
the rest
of
the byte you will need
to
determine what "decimal values"
(because the BASIC interpreter only understands decimal) will affect the
four high-order bits and not the low-order bits.
You can think
of
a byte as being made up
of
single bits
that
each have a
particular decimal value. From right to left these are
128
64
32
16
8 4 2
Therefore, incrementing the number in location 36879 by
15
(8+4+2+
1)
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