[Coco] About those EPROMS...

[Mark McDougall]

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,

-- 
|              Mark McDougall                | "Electrical Engineers do it
|  <http://members.iinet.net.au/~msmcdoug>   |   with less resistance!"