Re: ROM Dump of Amiga Keyboard controller

From: Segher Boessenkool <>
Date: Sat, 5 Jul 2014 13:33:55 -0500
Message-ID: <>
> Hm, according to the datasheet, the 6500/1 has the ROM located at 
> $0800-$0FFF. So unless it's wrong, it doesn't make sense to dump any 
> other pages than 8 to F. Unassigned space will read whatever is on the 
> bus at that moment (like reading unassigned space in the C64) and the 
> RAM and I/O contents don't matter.
> Page 0 and 1 are most likely the 64 Bytes RAM mirrored multiple times.

It looks like ithe chip decodes addresses like this:

-- The top four address bits are disregarded;
-- If the next two address bits are not both zero, ROM is selected;
-- Otherwise, if bit 7 is 0 RAM is selected; otherwise I/O.

ROM uses the low eleven bits for addressing (so you get a mirror
0400..07ff -> 0c00..0fff).  RAM uses the low seven bits for addressing
(but the top half reads as zeroes; writes presumably are swallowed).
I/O uses the low four bits for addressing (so lots of mirrors).

In summary:

----00--0xxxxxxx  RAM
----00--1---xxxx  I/O
----01xxxxxxxxxx  ROM
----1-xxxxxxxxxx  ROM

or perhaps

----00--00xxxxxx  RAM
----00--01------  open
----00--1---xxxx  I/O
----01xxxxxxxxxx  ROM
----1-xxxxxxxxxx  ROM


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Received on 2014-07-05 19:02:14

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