[Coco] About those EPROMS...

[Dave Philipsen]

Yes, I used to work on arcade video games back in the 80s and I don't 
remember which game it was but I remmeber looking at the schematic and 
wondering why all that extra circuitry existed between the processor and 
the EPROMs.  In reality, though, all you would have had to do was to 
plug in some sort of emulator into the processor socket and you could 
read the code from the EPROMs if you wanted to disassemble it.  I've 
also seen circuits where the address lines and data lines were all 
scrambled around....not too hard to figure out, but....

Dave Philipsen


On 4/9/2015 6:34 PM, Mark McDougall wrote:
> On 9/04/2015 11:02 PM, Dave Philipsen wrote:
>
>> Ok, but the ROM itself, even if encrypted, must be read in order for the
>> program to be executed.  The processor places an address on the address
>> lines, strobes whatever output enables are necessary, and the data at 
>> that
>> address appears on the data lines.  Right?
>
> Of course. Stephen's original statement is a little ambiguous and not 
> technically correct. A ROM has no way of knowing whether you are 
> executing code or reading data. If an external device can read ROM 
> contents - encrypted or not - then so can a 'dumping circuit'.
>
> Encryption schemes used in the past ranged from simple mangled address 
> and/or data lines to sophisticated block cipher schemes with keys 
> embedded in custom processors, and everything in between. Good 
> examples can be found in retro arcade video games. None of this 
> however changes the fact that the ROM contents can be read directly by 
> simply driving the address bus.
>
> Regards,
>