Subject: Re: Oooooh... IDE to Q-bus/Unibus adapters.
To: John Wilson <>
From: Thor Lancelot Simon <>
List: port-vax
Date: 07/08/1999 02:42:39
On Thu, Jul 08, 1999 at 02:34:03AM -0400, John Wilson wrote:
> ANYWAY, if we're talking about a MV2/MV3/PDP-11 Q-bus controller, then this
> is a totally pointless flame war.  The throughput of the Q-bus is a tiny
> fraction of what *any* modern hard drive is capable of, so that's where the
> limit is going to be.  Dunno how much rosier the picture is on the desktop
> VAXen.

Well, maybe -- but if you're doing programmed I/O, I expect the bottleneck
will be in how fast your CPU can sling bits.  The performance of the SCSI
on the desktop VAXen, in PIO mode, is so horrific as to make a PIO-only
IDE controller seem well-nigh worthless...

The wire protocol for the "UltraDMA" and "DMA" IDE modes is evidently 
different.  I don't know how, Manuel Bouyer could probably tell you.  The
old-fashioned "DMA IDE controllers" used ISA or EISA motherboard DMA and
just used it to push bytes into the FIFO once you got things started --
that worked with any drive, the new modes do _not_.