Hardware freaks Unite (on this one)

[Adrian Godwin]

In article <100 at fbits.ttank.com> Mariusz at fbits.ttank.com (Mariusz Stanczak) writes:
>In article <6386 at acorn.co.uk>, agodwin at acorn.co.uk (Adrian Godwin) writes:
>> A number of companies sell daughter boards - some of them based on the
>> Motorola app. note - as an upgrade for the Commodore Amiga. It seems to 
>
>	Yes, so do others for the Mac... the question for me would be
>    are those boards system specific, i.e. do they assume (and "solve")
>    anything about the hardware/software quirks of the given system
>    for which they are marketed?

They may assume things about the bus timing - the critical timings on one
system may differ from those on another, and these add-ons aren't going to be as
well specified as say, an emulator. One card I know of adds some memory at a
fixed address (though it might be possible to change it with a different PAL)
- however, it does it at an address that can't exist on a 24-bit-address
68000 so it can't possibly conflict. Control registers may - or may not -
be similar.

>> inside an A500, they may be useable on the 3b1. This would at least reduce 
>> the problem to the software part [...]
>
>	So, as above... what would be the pitfals(sp).  If such a "card"
>    wouldn't be hardware specific, and (correct me anybody) since most

As I understand it, mostly to the kernel. The higher 680x0 processors
have a different interrupt/exception stack frame to the 68000, and the 
system needs also to recognise memory that isn't in it's normal addressing 
range. A different mechanism is used to load the PSR.

Perhaps there are other differences too - it's only a drop-in replacement 
for the Amiga because its OS knows about alternative processors.

Even so, some applications do things they shouldn't, and fall over. This
should be less of a problem on the 3b1, because Unix applications are much
better behaved than those for OSs without process protection - but don't
forget to fix the diagnostics as well as the real OS ! Or use an upgrade
card that makes it possible to switch between a 68020 and a real 68000.

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Adrian Godwin                                        (agodwin at acorn.co.uk)