Okay. Deep breath - and preparing to be embarrassed

Garth Wilson has a LOT of extremely useful stuff on here. if you are reading this Garth - THANK YOU!

However. I am somewhat lost with one small part of Garth's address decoding primer - http://wilsonminesco.com/6502primer/addr_decoding.html



I am fine with the RAM and ROM and with VIA1, 2 and 3
When it comes to the ACIAs, I am fine with ACIA1 but I don't see the need to use the addresses 4204, 4208 and 420C. They will work, but what's wrong with 4200, 4201, 4202 and 4203?
Similarly for ACIA3 - the addresses show will work but what's wrong with 4100, 1, 2 & 3?

A little further on Garth has this table, where he talks about unique addresses:



again, I understand the RAM and ROM but the VIAs and ACIAs don't match up with the previous table. I would have expected them to be address subsets of entries in the previous table.

for example, for
VIA1, which I calculate to be selected anywhere from $6000 - $7FFF, its unique range (i.e. when it is selected but nothing else, is $6000 - $60FF - so, I would use $6000-$600F in my programs.
VIA2, which I calculate to be selected anywhere from $5000 - $5FFF AND $7000-$7FFF, its unique range (i.e. when it is selected but nothing else, is $5000 - $50FF - so, I would use $5000-$500F in my programs.
(I similarly don't understand VAI3 or the ACIAS)

I am loath to think Garth has made a mistake, its highly likely it's me. Can someone put me right please!

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Thanks for the reply Garth. I didn't spot A2 and A3 going to RS0 and RS1 of ACIAs 2 & 3.

Could you help me out with my second query - I don't understand the second table where (for example) it says

but those addresses are not in the range $6000-$7FFF which is where VIA1 is selected.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Thanks Garth, I understand it all now. Looks great.

me <== happy

For anyone else, like me, who likes to see all this stuff visually, here is a diagram (of the whole memory map) I made so I could visualise what was going on. The coloured blocks are the addresses of the devices that are selected. The red entries are the ones that only have one device selected.

Thanks for this visual representation! This is what my brain needed.

I'm a visual person, too. Here's a diagram I made for myself long ago;

Cheerful regards.