Appendix
B:
Memory Usage
305
Jfyou
are using the map to
PEEK
at
or
POKE
into a memory location,
use the decimal value. Machine code
or
assembly language programs
should access the hex value.
In some cases two
or
more memory locations are indicated
on
the table.
When two locations are indicated, they are to be taken as a single two-byte
address. The VIC
20
stores such numbers in low-byte, high-byte order.
To
better understand this, consider the first two-byte address.
Hex
Address
0001-0002
Decimal
Address Description
1-2
Vector for
USR
If
the
USR
program began
at
memory location 1234 (hex), the
numbers would be stored as follows:
Address
0001
0002
Stored Value
34
(low byte)
12
(high byte)
NOTE: All values stored in pointers like the one above are hexadec-
imal.
For
more information
on
hexadecimal-decimal conversion, see
Appendix D.
6560
VIC
Display Chip Functions
The VIC's 6560 video display chip can be accessed by manipulating the
values
of
memory locations 36864-36879 (decimal). The actual function
of
each memory location (control register)
is
indicated in Table B-3.
In
some
cases the control registers are broken up into smaller control units (bits). In
these instances the functions are shown by a separation between numbered
blocks.
Each control register
is
a single byte in memory.
To
change single bits
within the bytes you will need to change the decimal value in such a way
that
the single binary bit you want to access changes.
Try the following example. Memory register 36879 changes the border
color
and
the field color.
It
also controls whether the characters are dis-
played as reversed
or
non-reversed images.
In
the
table you can see
that
the
byte
is
broken up into three sections. These sections contain the numbers 20,
21, and
22.
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