Date   : Fri, 08 May 2009 10:35:54 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: ReCo6502 now available

John Kortink wrote:
> ReCo6502 now available
 
While perusing the ReCo6502 documentation I started thinking about
the possibility of being able to do data transfer to the flat 65816
memory model, ie to be able to *LOAD or *SAVE anywhere in the 512K
using "normal" addresses, not having to page banks in and do a
chunk at a a time.
 
I realised I was rethinking the same thoughts I'd had about flat
memory access for my PDP-11 system.
 
If no memory paging has occured, then *LOADing to &0000xxxx would
load data to memory at &0000xxxx, just as with the 6502 Tube client
MOS. However, if a bank has been paged (eg bank 4 paged in at
&4000) and 64K of data is LOADed to &00000000, then that data will
be loaded to &000000-&003FFF, &010000-&013FFFF, &008000-&00FFFF.
 
That's ok, until one starts thinking about being able to load/save
to the flat address space. If, for instance, *LOAD file 14000 loads
data to &014000 in the flat address space, then that implies that
*LOAD file 4000 would load to &004000 in the flat address space.
Which as we've just seen above, if running as a 6502 and a
different bank is paged in would not be the case.
 
These are exactly the circular thoughts I've had with the PDP-11
system some months ago.
 
Now obviously, loading to &0000xxxx has to load to the bottom 64K
whatever the memory mapping as that's all a 6502 program will be
able to see. Which means that using the data address as a flat
memory address would be unworkable.
 
With the PDP-11 I started thinking of using &00xxxxxx for the
bottom 64K in whatever mapping it happened to be, and, for
instance, &01xxxxxx for flat memory addressing. That would give
access to 16M of memory in a flat memory model.
 
(or &02xxxxxx or &04xxxxxx, etc. all the way up to &8xxxxxxx).
 
The 65816 has resurrected uncompleted thoughts on how to address
memory for data transfers for the PDP-11. Any thoughts?
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
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