Hard Disk Drive
Joergen: (Again)
RE: DMA and the C Ltd. SCSI Controller.
I will try to clear the air once and for all on DMA for
Hard Disk Access, why we chose not to use DMA and the net effect
on the end user.
First, we built our own DMA drive before we built the non-
DMA unit we are marketing. True DMA (ala Atari ST) is not possible
on the Amiga, even though a HD is "DMA" doesn't mean that is fast.
Our production non-DMA drive ranged from 'about as fast' (+/- 15%)
on opreations like loading Dpaint, Modula compilations from disk,
Database operations and opening windows full of icons; to 'half as
fast' on operations like loading some long files. In many cases,
like loading V.I.P. and Analyze! files, where the loading is done
from the program while computation was going on, the non-DMA drive
was actually faster. (Has something to do with DMA Interrupts,
which I don't understand, even after our techies tried to explain
it to me.) – So, overal speed of a DMA HD is faster, but not all
that much.
Second, the DMA version cost 50% more that the non-DMA unit.
Which would have meant the retail would have been well over our
'under a thousand dollars' target price.
Last, the DMA unit would not have been as easy to adapt to
our upcoming SCSI Local Area Network as our non-DMA unit. The DMA
activity, it seems, does at times preclude (over-ride?) the multi-
tasking ability of the Amiga, a situation which would be very very
difficult to deal with when your network is designed to run as a
high-priority task in one window of the machine.
Your analysis was mostly correct, we are using a pseudo-
DMA 'cashe' scheme. We are grabbing 5K hunks (optimal by our
tests) and dropping them into memory using the DMA channel. This
allows us to set our task priortiy down far enough to allow higher
priority (multi-) tasks to over-ride us.
E.J. Lippert II
C Ltd.
Wichita, KS
(316) 267-6321