Re: Hardware emulation of 6509 using 6502?

I'm guessing that since the B and P series use virtually identical memory maps that it was easier just to put a small amount of ram in the same place on both machines. You'll also note that in the P500 the character ROM is mapped in, but on the B-series that space is empty (since the 6545 circuitry reads it directly).
I actually designed a board to re-map memory from bank (something) into Bank 15 way back in the day. I ended up killing my B128 since I didn't really fully understand how the 6509/B128 really worked back then (and not sure even now)...
For it's time I don't think the 6509 was a bad idea. It was a CPU that's 6502 compatible so there was a large software base, yet extended the memory to 1MB without changing a lot of things. Sure, they probably should have designed a fully 16-bit CPU but the 6509 would have been a fairly simple and cheap upgrade to do.
Steve

      From: Mia Magnusson <mia@plea.se>
 To: cbm-hackers@musoftware.de 
 Sent: Tuesday, March 6, 2018 2:52 PM
 Subject: Re: Hardware emulation of 6509 using 6502?
   
Den Mon, 5 Mar 2018 21:37:23 +0000 (UTC) skrev Steve Gray
<sjgray@rogers.com>:
> There was a 24K ram cart for the CBM-II series. It adds 24K into bank
> 15. I wonder if the P500 could use that ram for graphics/sprites?

Well, apart from the color ram with it's special access, it seems like
the 1k sram is the only thing VIC-II can see that resides in bank 15.

I really don't understand why this sram is there at all instead of just
map a part of the 64k dram in bank 0 as a 1k window in bank 15.

> IMHO, Commodore should have fully populated BANK 15 with 64K ram and
> made it possible to bank out the ROMs as the C64 does. This would
> have made the machines 100 times more useful and would have allowed
> large programs to run without all the overhead required to access
> kernal routines from other banks. Steve

Someone who were any good with software should had pulled some
emergency break before they actually sent the 6509 into production :)

What they could had done is fill all non-rom non-i/o parts of bank 15
with a mirror of bank 0.

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