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

From: Gábor Lénárt <>
Date: Wed, 8 Feb 2012 13:04:34 +0100
Message-ID: <>

On Wed, Feb 08, 2012 at 11:58:17AM +0100, Anders Carlsson wrote:
> Hello,
> >"55-56 Pointer: Limit of memory"?
> Yes, this pointer is used to mark highest available address for
> Basic. As you write, you should subtract it from the pointer Start
> of Basic to get total memory.
> Of course for machine code and data, you can store those in a few
> locations not easily reachable for Basic. As you might have noticed,
> there is a 8K RAM/ROM block at $A000 that is used for autostarting
> cartridges. Quite a lot of us have switchable 32K or larger memory
> expansions, which means this block of memory also might be used for
> data storage. Then you have the 3K block at $0400 which I mentioned
> in the previous post. It is not quite as common to have available at
> the same time one has +8K expansion enabled. For really esotheric
> purposes, some people may even have decoded unused I/O space for
> RAM/ROM, but it is far from standard procedure.

Hmmm, VIC-20 is a bit new topic for me, but it seems to be an
adventurous area :)

I have a VIC-20 board (+ a kbd which was a C16 one originally, without any
case, I got these in this "configuration" as a "gift" when I was something
like 16 years old) somewhere at my parent's home, at least it worked
something like 20 years ago, I haven't even tried since then.  But it seems
it would worth to "play" with the real thing (TM) as well now.

To deal with this: is it possible to switch off (from cartridge port) all
the internal ROMs somehow (hmm and/or I/O)?  Because then it would be really
interesting project to have a memory expansion cartridge (built by me - I
mean) where all of the memory can be used as RAM, if I want that - for sure,
it should be configured from software after disabling IRQ, etc ;) If I use
some kind of SRAM (DRAM has been always "alien" for me ...  it's easier to
do things with SRAM on a hobby level at least), it shouldn't be too hard to
build, as far as I can imagine.  Now then the next is to have more than 64K
RAM, and even pageable :) However I am not so familiar with VIC-20 (as you
can noticed), so my ideas would not work at all maybe.

Btw, one more VIC-20 question: I was reading the description of the VIC-I
chip. It seems, it can supports up to 31 (hmm, 32?) characters within a
line. Then what is the reason to have only 22 by default? Is there a problem
to support more resolution (eg, TV overscan problem, hw limitiation, not
stable, whatever) or "only" some memory problems that it would need more
video RAM which may not available on a stock, unexpanded VIC-20, so CBM
engineers came with the idea to use resolution they have enough available
RAM for? Well, that was only an "as far as I remember" about the VIC-I ...

- Gábor

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Received on 2012-02-08 13:00:08

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