[Coco] Learning CPU Architecture and Digital Design

[jdaggett at gate.net]

On 17 Feb 2013 at 4:30, Kip Koon wrote:

> Hello Everyone,
> 
> I know at least two people on this list have studied CPU Architecture and
> Digital Design at some point, because there are two versions of the 6x09
> processor core in existence ready to burn into an FPGA chip.  My interests
> have begun to be peeked in that direction as well.  I'd like to expand the
> 6x09 core (not sure which one to pick yet) into a core with a 24-bit address
> bus and keep the same 8-bit data bus while at the same time preserving the
> complete instruction set, if possible.  Has anyone else ever had this idea?
> Paging memory into and out of the 64K address space would be a thing of the
> past and we would not have to learn a new assembly language to utilize it.
> I'd like to eventually burn this into an FPGA chip and use it in an expanded
> version of my 6809 computer project.  I'd also like to study this subject in
> detail.  In order to facilitate my studies, which book(s) would you all
> suggest I look into getting?  Thank you all for your help.
> 
> Kip
> 
>  


Kip

Expanding the address bus to 24 bits is a bit more difficult than just saying one would like to. 

Simple way would be to do what some expanded 8 bit procesors have done in the past. That 
would be to have an internal register that acts as the highest order byte of the Program 
counter that can be transferred into from say one of the accumalator. Adding that in is doable 
but a bit clunky. The use of a page memory map unit like the GIME chip has is by far a bit 
cleaner way in my opinion. That can be expanded to 8 bits. Expansion of the task registers in 
the GIME chip would be of great help for OS9. That would allow more than two tasks to be 
multiplexed easier. That way you can allocate blocks of memory for multiple tasks and not be 
bothered saving contexts of either one of the two present tasks. That would also require OS9 
to be modified to handle the extra task registers. 

Adding instructions to the core of the 6809 is not all that difficult. The mechanics of a two byte 
instruction are already there with the multiple pages. The HC11 has four pages as compared 
to three for the 6809/6309. There is room to add a fourth page to the 6809/6309 if needed. 
Even with the 6309 opcodes there is still a lot of space in page 2 and page three of the 
opcode map. Once you understand how the second opcode byte, often called the pre byte, 
works, it is rather a simple task of adding in extra instructions. 

just my thoughts 
james