Re: News on the remake of the Mill 6809

[mressl@macgui.com (Marc S Ressl)]

Alex Freed wrote:
> OK, a slow boat from China brought a few PCBs and I have assembled one
> card.
> 
> http://alexfreed.com/FPGApple/Mill6809/redesign_mill6809.jpg
> 
> Smaller and simpler than the original due to a CPLD replacing the memory 
> remapping ROM and some glue logic.
> 
> The good news is that it works, but the bad news is that it takes an 
> instrument to see that.
> 
> Here is a snapshot:
> 
> http://alexfreed.com/FPGApple/Mill6809/mill6809_analyzer.PNG
> 
> I'm trying to use the good old demo1.bas from the disk image.
> 
>   480  REM FFFE -> 40FA=JMP $1000
>   490 X =  PEEK (ROM) * 256 +  PEEK (ROM + 1)
>   500  REM
>   510  POKE (X),126: REM  7E
>   520  POKE (X + 1),CODE / 256
>   530  POKE (X + 2),0
>   540  POKE SLOT + 2,128: REM  RUN
> 
> This code wants to write a jump instruction to the address, presumably 
> $40FA, pointed to by the last 2 bytes in standard Apple ][ ROM. 
> Unfortunately my enhanced 2e has $FAC3 at FFFE-FFFF. So as one can see 
> on the analyzer screen the 6809 fetches $FAC3 and jumps to that address. 
> Which of course lives in ROM and can't be patched. Oops...
> 
> Sure I could load a standard monitor into the "language card", switch to 
> it and then try it again, but this is a bit ugly. Also I'm pretty 
> confident the basic counting test will work - after all both CPUs are 
> running. Where should I take a better test? My main concern is that 
> knowing that the LS14 was replaced with LS04 while adding the memory 
> mapping for OS09 I suspect that timing may need to be tuned. Of course 
> my address mapping CPLD is an order of magnitude faster than a ROM used 
> in the original version, but "too fast" can also be an issue. So I need 
> some tests before declaring the card operational. I want something that 
> runs with all 3 versions of the memory map - virgin, flipped A15 and 
> OS09 remapping.
> 
> -ALex.
>

This is really cool! :-) Thanks for all the work.

With the best wishes,

Marc.-