Date   : Sun, 29 Nov 2009 23:42:06 +0000
From   : kevin@... (Kevin Bracey)
Subject: Tube - I/O processor memory questions

John Kortink wrote:
> On Sun, 29 Nov 2009 22:06:25 +0000, Kevin Bracey
> <kevin@...> wrote:
>
>   
>> John Kortink wrote:
>>
>>     
>> Actually, you also get &90-&EE allocated to you
>>     
>
> They're not being used on a standard 6502 2p
> running Tube OS 1.10. But that doesn't mean
> they're not reserved for future use.
>   
Well, they are explicitly documented as being available to languages in 
the original 6502 Second Processor User Guide:

    Page 0 to &EE is available
    Page 1 = stack
    Page 2 = OS indirections and user memory
    Page 3 = error messages
    Page 4 ? 7 are available to the user, although not in BASIC
    Page 8 ? &F7 available to the user


That's sounds like official documentation, not just some bit of 
reverse-engineering. As official as the normal availability of 00-9F.

As ever, of course, the detail level of documentation tends to diminish 
with every successive generation, but I've never seen any hint that this 
was revoked.

Obviously someone could come up with a new 6502 copro that nicked some 
back, just as they could come up with a new MOS that nicked back some of 
00-9F, but that would scupper backwards compatibility for people who had 
believed the documentation and implied promises of the previous generations.

On the other hand, it seems that Acorn may have been over-generous by 
not leaving themselves any slack for future expansion in those docs. 
They should probably have documented some "reserved" bytes...

> In general, it shouldn't matter whether your
> code runs on the I/O side or on the 2p side.
>   
Yeah, which is why those 90-EE bytes aren't terribly useful, even if 
documented. But if you are building separate "HI" versions, you could 
move some variables down for speed in those, I guess.

Kevin