Re: A85: FA70-FBFF?
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Re: A85: FA70-FBFF?
Richardlewis wrote:
>
> Okay, interrupt mode 2 uses two numbers, I, and it pulls the other off
> the data bus (I'm still not exactly sure what the data bus is, but if
> someone could explain it). They are both 8 bit numbers, which form a
> word, with I being the high byte. If we placed the table at $FB00,
> then
> the maximum number formed with that would be $FBFF. It could never
> point past $FBFF, ever.
> That number is an address pointer to a number on the interrupt table,
> which it then jumps to, right? It being a 16 bit number, with the
> high
> byte set to $FB, we wouldn't have to worry about intersecting the
> Video
> Memory.
>
> Why does everyone keep saying that the table covers 257 bytes? I'm
> very
> sure that is impossible.
Think about what you just said. The maximum value of the pointer would
be $FBFF. But the pointer points to another pointer. Get it? It is
going to load a 16-bit address from the interrupt table. So if you load
a 16-bit pointer from $FBFF, where does the second byte come from?
That's right... $FC00. Video memory. And if the table starts at $FB00
and ends at $FC00, that sounds like 257 bytes to me...
Hope that helps,
--
Ben Shakal
shakalb98@jhs.net
Quote of the Month:
"Common sense is the collection of prejudices acquired by age 18."
- Albert Einstein
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