Re: [stella] some questions about the a2600

Subject: Re: [stella] some questions about the a2600
From: Erik Mooney <emooney@xxxxxxxxxxxxxxxx>
Date: Tue, 25 May 1999 16:40:21 -0400 (EDT)
> 1. how are interrupts (BRK, IRQ, NMI) handled by the 2600?

There are no interrupts on the 2600.

> 2. what exactly is the point of having missile and ball objects and how
> are they used
>     in already existing roms?

To have more than two objects on a scanline, you can use the missile
and/or ball objects.  Combat uses the missiles for the players' missiles,
Super Breakout uses the ball for the ball, etc.

> 3. should the 3 VSYNC and 37 VBLANK lines be applied as soon as
> execution
>     begins, or is it possible to have a blank 1st frame in which
> initialization and other
>     housekeeping goes on?

You can do whatever you want at the beginning.. whenever you start
generating VSYNCs, the TV will pick up on it and start displaying.

> 4. what exactly are overscan lines and how are they done?

Overscan lines are scanlines that come after (below) the visible TV area,
before you do the VSYNC to start the next frame.  There should be 30
overscan lines.  You can either leave them blank, or display something
during them, though not all TVs will show objects you put there.

> 5. is it true that the 6507 can only address 13 bits of information (i
> read somewhere
>     that it can, but then i read somewhere else that it could only read
> 12, maybe it was
>     a typo) and can this be used to any effect at all?

The 6507 has only 13 address lines, for a total of 8K addressable space.
However, the topmost one is used as a selector between the TIA's register
address space and the cartridge ROM space.  In other words, the TIA
occupies the lower half of that 8K and the cartridge ROM occupies the
upper half.

> 6. has anyone ever written a home brewed game that had bank switching
> (is larger
>     than 4K)?

Yes.  I believe there was an 8K bankswitched version of the text adventure
Dark Mage, by.... <blanking>... Greg Troutman?  There's also been at least
a couple 6k bankswitched Supercharger games.


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