Re: VIC-20 basic memory start + basic stub + etc

Hello,

VIC-20 has three locations for start of Basic:

Unexpanded: $1001
+3K expanded: $0401
+8K expanded or more: $1201

For both unexpanded and +3K machine, the video matrix is default at $1E00 
and colour memory at $9600.

For machine expanded with +8K or more, the video matrix is default at $1000 
and colour memory at $9400.

These changes are mostly due to Basic needs a continuous memory block to 
operate, while for practical matters it is easier to lower top of RAM to 
make room for custom graphics or machine code routines on an unexpanded 
machine rather than moving start of Basic upwards.

Easiest is if you target +8K or more expansion, as the memory setup will be 
the same for 8K, 16K, 24K expansion. Maximum continuous memory then is 
27.5K. You can also note the 3K memory block at $0400 can be filled, but due 
to the video matrix has to be somewhere between $1000 and $1FFF (internal 
RAM), you would have to load any code or data going into that block 
separately. Some people made internal modifications to their VIC-20 by 
piggy-backing 3K, which allows the video matrix and custom graphics to be at 
$0400 at the cost of making their computer incompatible with the others.

Best regards

-- 
Anders Carlsson


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