Commodore VIC-20 - Page 101

Type:
PRINT FRE(O)
The number displayed is the amount of memory space ieft
unused. Now type the following:
POKE 52, 28: POKE56, 28: CLR
Now type:
PRINT FRE(O)
See the change? BASIC now thinks it has 512 bytes [ess memory
to work with, The memory you just reclaimed from BASIC is where
you are going to put your character set, safe from actions of BASIC.
The next step is to put your characters into RAM. When you
begin, there is random data beginning at 7168. You must put
character patterns in RAM (in the same style as the ones in ROM)
for the VIC to use.
The following one line program moves 64 characters from ROM
to your character set RAM:
FOR I- 7166 TO 7679: POKE I, PEEK(I+ 25600): NEXT
Now POKE 36869 with 255, Nothing happens, right? Well,
almost nothing. The VIC is now getting its character Information
from your RAM, instead of from ROM. But since we copied the
characters from ROM exactfy, no difference can be seen . . . yet.
You can easily change the characters now. Clear the screen and
type an @ sign. Move the cursor down a couple of lines, then type:
FOR I = 7168 TO 7163 + 7:POKE I, 255 - PEEK(i) : NEXT
You just created a reversed @ sign!
VtC TIP: Reversed characters are just characters with their bit
patterns in character memory reversed!
Now move the cursor up to the program again and hit return
again to re-reverse the character (bring it back to normal). By
looking at the table of screen display codes, you can figure out
where in RAM each character is. Just remember that each
character takes eight memory locations to store.
Here are a few examples just to get you started:
CHARAC
TER
8
A
!
>
DISPLAY
CODE
0
1
33
62
CURRENT STARTING LOCATION
IN RAM
7168
7176
7432
7664
35