Re: C64 MMU POC

Den Wed, 16 Oct 2019 14:50:07 +0200 skrev Zoran Davidovac
<zoran.davidovac_at_gmail.com>:
> Hi Jim,
> 
> How about a different idea for MMU?
> 
> My idea is to page both RAM and ROM and I would preferred that if you
> are creating MMU, you create a MMU board that will replace not just
> ROM and RAM but also to replace failing PLAs, it will lower power
> consumption on every c64, repair it and give it longer life.
> 
> Tech Details:
> 
> In stock c64 kernal is on $E000 $FFFF (last 8K) while c64 can access
> to Kernel ROM and execute it, VIC can access only to RAM under kernel
> even with KERNAL ROM turned on. Writing to KERNAL ROM space actually
> write to RAM under.
> 
> So you could page not only RAM under $E000-$FFFF ie graphic for
> VIC,but also replace KERNAL ROM in same $E000-$FFFF code for CPU
> 
> That would give us two types of pagging:
> 
> -8K in another words you can swap 8k at the time $E000-$FFFF RAM, so
> you can also change every 8K block to fill all the data RAM and
> potential ROM (every second 8K RAM)
> 
> -16K as 8k at the time $E000-$FFFF RAM and 8k at the time $E000-$FFFF
> ROM read only.
> 
> this give us to data as 8K graphic and 8K code or 2x8K code if you
> are turning of ROM on/off
> 
> That would actually give a boost to C64 hardware and development life
> with new RAM and PLA replacement, and with its 16K gain "in single 8K
> place"  we can have up to 512MB ram (with 2x8bit register for
> paging), but I would suggest to go max to 16MB as then this can be re
> implemented in code U64, cameleon etc, so eventually would have some
> compatibility.
> 
> Problems:
> -there would be several PCBs for such solution
> -ram replacement for c64 mode in c128 would be also possible with a
> few wires  or second PCB under c128 rom.

Side tracK: isn't this kind of expansion something that could be done
specifically for the C128 without supporting the C64 (but supporting
C64 mode on a C128)? My reasoning is that if someone wants something
C64 compatible but with more features they would likely anyway want a
C128.

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