Re: ROM Dump of Amiga Keyboard controller

From: silverdr_at_wfmh.org.pl
Date: Sun, 6 Jul 2014 16:12:38 +0200
Message-ID: <etPan.53b95957.6b8b4567.e391@Szanghaj.local>
On 2014-07-06 at 15:23:15, smf (smf@null.net) wrote:

> >I was also interested how they did it internally. Like if they changed the
> >6502 core (doubtful)
> > or for example wired the two upper address nybbles together or something
> > like that...
>  
> Only 2kb of rom is available because that is all they put inside, don't
> forget they need ram in there as well as IO so they can't can't just
> duplicate 2kb of rom throughout 64kb address space.

RAM and I/O are in the lower half of the 4KiB. It is quite possible that all 4KiB is “duplicated” throughout the whole address space.

> I would imagine that because of certain 6502 features the rom is mapped from
> F800 to FFFF, with ram at least from 0000-0200 (however much they included).

I guess the most important “certain feature” are the H/W vectors.

> The rom and ram might be mirrored at other addresses, you achieve this by
> looking at less address lines not by wiring lines together. They might just
> use A15 for the rom chip select, so the rom is duplicated every 2k from
> 8000-FFFF. If they wanted to make it only appear at F800 then they'd use A15
> AND A14 AND A13 AND A12 AND A11, but it would cost them more and the only
> benefit is the rom isn't mirrored.

I think you misunderstood. Since 2KiB of ROM starts at $800 (NOT $f800) it most probably /has to/ be mirrored if you don’t want to change the CPU core. And I concur with Jim and Gerrit - the cheapest way must have been to just make the last four address bits “don’t care”.



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Received on 2014-07-06 15:00:02

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