Commodore VIC-20 User Manual page 322

308
The VIC 20 User Guide
will increment the value of the four high-order bits by 1. Try the following
program:
10 REM ,SCREEN COLOR*
20 9=36879
30 A~=PEEK(36879)/16+1
40
PRINT"::rJ'HERE ARE 16 POSSIBLE"
50
PRINT"SCREEN COLORS"
60 PRINT"THE SCREEN
IS
NOW"
7' 2 1
PRINT"COLOR
"iA::::
80 PRINT"TO CHANOE TO A HIGHER"
90
PR I NT" NUMBER (COLOR),
HiT
+
II
100 PRINT"TO CHANGE TO A LOWER"
11' 2 1
PRINT"NUMBER (COLOR), Hit _"
120 GET
Vt: IF VI=""
THEN 120
i30 IF V$
=
U+" THEN 150
135 iF V$ (> "-"
THEN 120
140 IF PEEK (36879)
>
11 THEN POKE 36879,(PEEK(36879»-16
145 GOTO 10
150
IF
PEEK (36879)
<
239 THEN POKE 36879, (PEEK(36879»+16
160 OOTO 10
To toggle a single bit such as the
REVERSE VIDEO
switch, use the AND
and OR functions. For example,
To turn on bit 8 of 36879 enter POKE 36879, PEEK(36879) OR 8.
To turn it off enter POKE 36879, PEEK(36879) AND NOT 8.
Kemal Memory Locations
Kernals are machine-callable routines that are available either from
BASIC using the USR or SYS instructions or from your own machine
language programs. Table B-4lists the names and memory locations of the
KERNAL programs available in the VIC 20.