Re: [Amforth-devel] Atmega1280

[Matthias T. <mt...@we...>]

Hi Andy,

> Looking at the quote below.
>
> Are we actually saying here that the top 64k is not accessible then on a 
> 128k device ??
>   

no no, the 128Kb are fully usable. No (further) restrictions are in place

> That the memory is arranged in Amforth in words (ie 16 bit) ??
>   

It depends. The key to understand it is the concept of an address unit.
One address unit provides atomic access to data. For RAM one
address unit contains 1 byte. One flash address unit has 2bytes.

Since amforth is a 16bit forth, there are 64K address units. That
translates to 64KB RAM and 128KB Flash (separate address
ranges, @ and i@ (and e@) have nothing in common). At that
level there is no difference between any controller, that amforth
supports.

At the assembly level (where you put your hands into the dirty
details) the flash has surprisingly both a 16bit and a 8bit
access schema, that needs to be taken into account. And the bigger
controllers need a different instruction  to read the (whole) flash. The
details are well hidden in the readflashcell and writeflashcell assembly
macros. For the full details you'll definitly need to read the data sheets
from Atmel.

But I hope, that the at90can128 file is a good starting point for the
atmega1281.

> Or that it is paged and therefore takes a bit more time to access ??
>   

The flash pages are not exposed, they are used internally only.
I may re-arrange the code to provide access to the pages in future
releases, e.g. to implement a block device for flash that is not
accessible otherwise (eg on the atmega256).

The increased access time comes from the additional code needed
to access the flash, but thats not that much. Only a few CPU cycles.

still more confused? sorry, but be assured: the atmega128 has nothing
that cannot be used with amforth and is not really slower than the
smaller device types.

Matthias