Re: 6500/1 ROM

From: Kajtár Zsolt <>
Date: Mon, 17 Sep 2012 20:10:45 +0200
Message-ID: <>
Hash: SHA1

>> 3. If I need to load PRG into RAM, how do I switch execution to it when
>> all accesses go to port anyway? Maybe a precisely timed disconnect of
>> the _RES voltage combined with appropriate data preceding it..
> That's what I figure... Have the CPU fetch a JMP to the beginning of your
> code in RAM, that let go of _RESET and it should start to run your
> program.

I think the "easiest" would be to hook it up to a normal 6502's data bus and
clock both of them simultaneously. After reset the 6502 would feed the right
data at the right time from an external memory to the 6500/1. Now that both
are executing the same code you could fill up the small zero page with the
code required, jump there, execute an instruction which triggers the +10V
release. From now on the 2 CPUs are not coupled anymore, and the programs
diverge after some simple check to find out which CPU it's running on, e.g
by checking if the ram and stack areas are the same. The 6502 could be used
to dump the data in cooperation with the small code in the 6500/1 trough the
I/O ports.

Sounds easy ;)

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