Mask ROM

Yes you can!! the mask ROM as you state is coded at manufacture, you don't need to use all the available space, just as long as you end your code in a way that makes your program realise its reached the end!!

A normal EPROM when 'clean' has all its locations filled with 'FF' or with logic state 'ones' - you only program the part you want to use... i.e. 0 to 12 kb in your case... assuming you have set the code to be at the start of the memory address, if not your code will need to start there.

John.

Thanks for the reply!

One thing I'm still not clear about though. What sort of data would you expect to get when reading from the unused portion of the mask?

When dealing with normal powers of 2, let's say reading a 16Kb ROM as if it were a 32Kb ROM, you'd simply get mirrored data, the same 16Kb, because the upper most address line is not doing anything.

However, when dealing with a non-power of 2, like the 12Kb mask in 16Kb ROM, you would still physically be able to access that portion. Would you get an "open bus" type situation or would you get a mirrored segment?

Thanks,

Tom

Ahhhh that would depend on how you addressed the memory, if it was as a 16kb device then you would expect the micro to 'see' the extra 4 kb as filled with FF or 00 both won't execute anything... But as you say sometimes if you address a 16kb device with a 32kb address the 16kb is mirrored above the first 16kbit.

So yes you would see a mirrored section if you addressed the device as above 16kbit, but your software coding should not allow the micro to see this... or if it does it will execute the same as for the non-mirrored portion.

John.

Thanks again for the quick reply!

I think it makes sense to me know. My understanding of the manufacturing process of mask ROMs is that you start off with a "sheet", so to speak, of metal and then the mask is applied to the sheet and etched by some sort of acid.

So if you had your 12Kb of data made into a 16Kb ROM, the unused area would not be masked, thus getting burned off by the acid. So reading that area would return 00h or ffh depending on how you interpret it.

Is my understanding of this correct?

Thanks,

Tom

Yes that's correct.

John.