In my uni lecture, I am covering address decoding. I understand that memory addresses are decoded to find if they point to the RAM, ROM, or I/O, by way of the address (in the example of BBC Micro) being in the RAM range, user ROM range or OS ROM range (e.g. 0x0000 to 0x7FFF is the range of RAM in the address space).

Addresses can be decoded as little as possible, that is, the minimum number of bits in the address are checked to determine the chip select signal, the signal which determines which chip to access. This approach allows for 'address shadows'. Now I'm not sure what this means, I'm repeating my lecturer.

What I understand address shadows to be are addresses which will point to the same place in memory as another, but only because we are decoding the address as little as possible.

The example my professor gave is of the original Apple Macintosh. The SCC READ chip had addresses from 90000 to A0000, but it only had 4 or 5 registers. This meant the same registers were repeated in memory, which is what an address shadow or mirror is.

Could someone expand on or correct this definition, because web searches have not turned up much fruitful information.



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