Re: 6500/1 ROM

From: silverdr_at_wfmh.org.pl
Date: Sun, 16 Sep 2012 23:30:58 +0200
Message-Id: <659ADA28-ED84-4364-832B-EC019FB63D9F@wfmh.org.pl>
On 2012-09-16, at 13:05, Gerrit Heitsch wrote:

>>> The test mode is described briefly on page 8.
>> 
>> "... Applying +10V signal to the _RES line places 6500/1 in the test mode. While in this mode all memory fetches are made from Port PC.  [...] A program can be loaded into RAM allowing the contents of the instruction ROM to be dumped to any port for external verification."
>> 
>> Now, how do we understand this?
>> 
>> "Port PC" is probably Port C (Pins PC0-PC7) /me guesses. Eight bits of data (guessing again) but what about addresses? There is not much of a program to be loaded into the whopping 64 bytes of RAM but should be enough for dumping the ROM. Still /how/ can this be loaded into RAM? And executed?
> 
> I wouldn't be surprised it means that no matter what the PC or commands say, every memory access (command and data fetch) will redirected to Port C.
> 
> So you have to expect what it wants to load and then supply the Byte starting from the RESET vector. Sounds like a very painful job to get a program into the thing, but it could be done and 64 Bytes RAM are enough for a simple copy routine that will just dump the ROM onto another port.

Even if it is so, then few more questions come to mind:

1. What about the timing? Since I notice no form of handshaking mentioned, I guess one would need to put proper bits on the port according to cycle-exact fetch/execution times?

2. If it is as you write and [1.] above is solved, then what do I need [to load program into] RAM for? I could supply commands and data until I am done with ROM dump or anything, effectively simulating RAM on the port, couldn't I? I see some contradiction here. Or we understand it wrong.

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..

Yes, in any case sounds like a painful process. Might Bill for example remember something about this stuff and shed some light?

-- 
SD!


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Received on 2012-09-16 22:00:32

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