Re: Quiescing Nubus DMA?

[Hauke Fath <hauke%Espresso.Rhein-Neckar.DE@localhost>]

At 18:08 Uhr -0600 02.12.2010, Donald Lee wrote:
>It's hard to imagine any OS running reliably if it does not "take charge"
>of such a peripheral card at boot time, to reset and initialize it.

Well, NetBSD/mac68k does that, to the extent that it knows the hardware.

>How does MacOS do this?  I would think it would *have* to be done
>in the ROM, since nothing else would be safe from the DMA engine.

The Nubus is sent a power-on reset signal, and the ROM startup routine
sends another two reset pulses, to be sure. Since the Mac expects to be
able to boot from the Jackhammer, it sets up the card pretty early on.

>I would expect that NuBus would have some sort of reset sequence, and
>that would *have* to "park" the card.

It does, and that would. According to DC&D, the Nubus reset line can be
toggled by software, so that would be a quick and dirty way to discipline
the Jackhammer.

The main problem with applying this sledgehammer is that we lose access to
any Nubus graphics cards, which only MacOS (rather: the board's ROM driver,
on behalf of the Macintosh ROMs) knows how to set up. AFAICS, that is the
main reason why native boots (i.e. bypassing the Macintosh operating
system) never got any traction on !OF Macintoshes.

        hauke

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Hauke Fath                        <hauke%Espresso.Rhein-Neckar.DE@localhost>
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