Commodore VIC-20 - Page 99

many sections of memory. By changing the character memory
pointer to point to RAM, the character set may be programmed for
any need.
The VIC's standard characters are stored as follows:
HEX DECIMAL DESCRIPTION
8000 32768 Upper case with full graphics
8400 33792 Upper case & graphics—reversed
8800 34816 Upper and lower case with some
graphics
8C0O 35840 Upper & lower with some graphics—re
versed
The register which controls where the chip gets its character
information is at location 36869 decimal (9005 HEX}. Its value is
normally 240 (upper case and graphics) or 242 (upper, lower case).
The programmed character set cannot be put into expansion
RAM, since the VIC chip doesn't have access to that memory.
Therefore, any programmed characters must begin at a memory
location between 4096 and 7168. Since BASIC programs are
normally stored beginning at 4096, and strings start at the top of
available memory and work their way down, precautions must be
taken to protect the character set from being overwritten by BASIC.
If the BASIC program begins at 4096, the normal procedure is to
change the pointers to the top of available RAM at locations 52 and
56 so that they point below the character set. The following chart
shows the possible locations of character sets, and the POKES to
protect them.
Num
ber
240
241
242
243
244
245
246
247
248
249
250
251
252
Location of
Characters
32768
33792
34816
35840
(36864)
(37888)
(38912)
(39936)
(0)
(1024)
(2048)
(3192)
4096
Contents
of Location
Character ROM
Character ROM
Character ROM
Character ROM
VIC Chip, I/O
Color RAM
nothing
nothing
Zero Page RAM
Expansion RAM
Expansion RAM
Expansion RAM
Start of BASIC RAM
POKE
52 & 56
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