Re: Switchless ROMs

Date: Thu, 29 Dec 2016 10:57:42 +0100
Message-Id: <>
> On 2016-12-28, at 23:19, Jim Brain <> wrote:
> On 12/28/2016 12:20 PM, smf wrote:
>> On 28/12/2016 17:34, Jim Brain wrote:
>>> access $fffd (jmp)
>>> <lots of time elapses for VIC badline>
>>> access $fffd (low byte of vector)
>> How do you tell between <lots of time elapses for VIC badline> and
>> access $fffd
>> <lots of time elapses when CPU only accesses RAM>
>> access $fffc
> Not sure you do.  Would ther be an issue in not worrying about it?

If we want to keep compatibility - I am afraid the answer is "yes". A simple example: a program uses the RAM area under KERNAL as a temporary storage and reads from the consecutive addresses there. I know for a fact that such programs exist. So you would need to monitor the configuration bits or the _CS or ... The next example is copying KERNAL from ROM to RAM - lots of programs to this in order to modify a few things in the KERNAL. Here monitoring the _CS won't help as the program reads from ROM locations and you know what happens when you don't differentiate between the _RST induced reads and the same done by the program.


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Received on 2016-12-29 10:00:02

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