Re: ROM Dump of Amiga Keyboard controller

From: smf <smf_at_null.net>
Date: Sun, 6 Jul 2014 14:22:40 +0100
Message-ID: <9882054601E64382BBCC52512CB3E624@smf>
>> again the datasheet if they really meant "$(0)FFF" and not just shortened 
>> the full "$FFFF"
>> ;-) But it also clearly states that only 2KiB of ROM is available. 
>> Impressive.

>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.

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).

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.


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Received on 2014-07-06 14:00:03

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