Re: L68K: Re: Afterburner and Linux

From: Bernd Harries <>
Date: Sat, 6 Jun 1998 13:43:17 +0200 (MEST)

Hi Guys!

I don't get it. Either the espressions below are ambiguous or something
else is wrong.

> The only problem the AB040 poses on top of that is not doing bus
> snooping (plus maybe other weirdness we didn't discover yet). But
> the ball is back in your court now - I hope I could prove that
> Linux works on the AB040 when you don't cache the RAM.
This is my / the problem! What is THE RAM? |||

It should be clear that if caching is disabled completely no
inconsistencies can ever occur. And weather snoopiing is disabled/broken
or not is simply irrelevant then.

But that should not be the point. The point is that noone should wonder
that 16 Bit wide RAM is NOT connected directly to the 68040 and cannot be

Snooping is a PASSIVE monitoring of the 32 Address and 32 Data pins of the
68040 chip while it itself doesn't drive the Address lines.

And I wouldn't believe that the Afterburner's 32 Bit wide FastRAM could
not be snooped if nescessary. I wrote 'could' here because that is a case
that should be irrelevant on the Falcon AB because no other Busmaster
but the 68040 could access Addresses above 16 MB (0x00FFFFFF). Neither
the DMA unit nor the 68030 could access there because they have only 24
address lines connected ;-( and if they drive the bus the AB considers
A24..A31 to be zero and thus FastRAM is NEVER accessed.

I don't know if you all know what that means:

It solves the major problem itself. :-) The fastram should be cache-able
because 2 reasons speak for it:

1. I see no reason why the 32 bit wide FastRAM cannot be snooped.
2. Noone can generate inconsistencies there and snooping is irrelevant.

That the 16 bit wide RAM cannot be snooped should not surprise. Now it
simply becomes a game if someone feels the need to cache parts of that
RAM anyway. I would not do so for now beacuse it's complicated that leaves
the cache entrys for other areas.

Further the 16 bit wide RAM should only be used for special purposes
(Graphics, DMA-Buffers, Swap-space)

If soneone defines an area in the 16 bit wide RAM which is only accessed
by the 68040 there is no reason not to map it cacheable although it cannot
be snooped.

Bernd Harries Tel.: +49 421 809 7351 priv. | Linux-m68k +49 421 457 3966 offi. | Medusa T40
Received on lø. juni 06 1998 - 17:16:00 CEST

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