Re: Hardware emulation of 6509 using 6502?

From: Mia Magnusson <mia_at_plea.se>
Date: Sun, 25 Feb 2018 00:15:48 +0100
Message-ID: <20180225001548.00001e17@plea.se>
Den Sat, 24 Feb 2018 12:01:47 -0700 (MST) skrev Dr Jefyll
<laughton@cyg.net>:
> Luckily that's pretty easy.  But if page crossings are allowed then
> things get harder, because when cycle 6 arrives it might be the data
> access or it might be the opcode fetch of the next instruction.  We
> need to know whether or not to flip to P3-P0 flip back to their
> previous state --  and there can't be any mistake about this!  So,
> what I'm saying is: allowing page crossings on cross-bank accesses
> requires extra resources.

Isn't the sync signal good enough?

The possible risk/problem I see is if the flipping of P0-P3 is too slow
so they bounce when they are supposed to be stable. Especially the
DRAMs are sensitive to this. I'm not sure if they are used in the RAS
and/or CAS part of a DRAM access cycle. Let's hope they only control
CAS, then it's less of a problem and there is also far more room for
slow timing.

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Received on 2018-02-25 01:01:24

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