From: adrie@ica.philips.nl (Adrie Koolen)
Newsgroups: comp.sys.sun.hardware
Subject: Re: New viking sparc processor?
Date: 14 Apr 92 15:13:14 GMT
Organization: Philips Consumer Electronics, Eindhoven, The Netherlands

In article <TUTUI.92Apr14033920@silk.imel.kyoto-u.ac.jp> tutui@silk.imel.kyoto-u.ac.jp (Takashi &) writes:
>Adaptec SCSI controller for ATs have 10MB sync scsi data transfer
>rate!! That makes high use of the DMA bus band width. While,
>setting aside the sparcserver, Sparcstations have 2-5M/s SCSI
>controller in it. In case of SS2, it have two controllers, to make
>high use of data BUS , disks should be put both in internal and in
>external. The disk to attach should be changed to SCSI-II drives.
>And cheaper sparcstations have only one SCSI.

I'm not a very experienced PC hardware guy, heck, I'm a software programmer
who's experience is on SparcStations, so excuse me if I didn't get it
right.

I think that there are a few misconceptions here. First, you're only talking
about SCSI disk controllers. If you would get 10MB/s out of a disk over the
AT-bus into your PC (and I doubt that you could, see below), then your PC
would have come to a halt. You could not do anything else on the bus anymore
(on my PC, when doing DMA, the processor is stopped, regardless whether the
external cache is used or not). So you can't play ADPCM audio from CDROM to
the soundblaster or perform video scrolls, moves, etc. Of course, this is
less of a problem on an Sbus equipped Sparc.

Second, the Adaptec controllers have a configurable bus speed which has a
maximum of 10MB/s bus speed. Problem is that an ISA bus can't handle such a
speed and my brother, who has an AH1540, had to reduce the speed to 5MB/s.
At that speed, my Maxtor LXT340S transferred 1.5MB/s so it wouldn't even
reach the 5MB/s. On my SparcStation IPC, I got 1.75MB/s, so the maximum
speed of the drive is definitely higher than the 1.5MB/s. My brother
recently bought a Quantum LP240S, which gave 2MB/s on my SparcStation. I'm
curious to know what he gets out of it on his PC :-) . Don't get me wrong,
I really like the Adaptec SCSI controllers and 1.5MB/s on a PC is quite
fast, but an ISA bus can't handle 10MB/s (that's 200ns per 16 bits cycle)!

Finally, the Adaptec 154x controllers are bus masters, so they don't use DMA.
This morning, I (and a few collegues of mine) studied a DMA cycle on my PC
via a logic analyser. After receiving a DMA request (DRQ), it takes
approximately 2us for the DMA controller (and the 386) to produce an
acknowledge (DACK). After the DACK, it takes another microsecond to perform
a parallel IO read/memory write. 10MB/s via DMA over an ISA bus? I'm sorry,
but that's physically impossible (according to the ISA-bus spec)!

>While smart PC users use ADAPTEC controllers. EISA bus is faster.
>Putting several SCSI (cheap) controllers, it seems to have >= performance
>compared with ordinal sparcs.

Again, you're comparing the SCSI controllers, not the bus systems! You're
right in that the SparcStations use a SCSI-1 controller which maxes out at
5MB/s. Hardly any disk drive reaches this speed, but technology goes fast
so Sun should update it's hardware and allow at least fast SCSI (10MB/s).
You won't get 10MB/s on an ISA bus. Look at memory mapped SVGA adapters and
compare BSDI/386 or MS windows with X11R5 on a SparcStation and you should
get a remarkable difference!

EISA is a lot faster (I read) and is definitely the way to go. I'm anxious
to know how fast a memory mapped EISA framebuffer is. Of course, as I can't
get 100MB/s sustained on my SparcStation, I suppose that you don't get
33MB/s on an EISA.

Adrie Koolen (adrie@ica.philips.nl)
Philips Consumer Electronics, Eindhoven, the Netherlands
Newsgroups: comp.sys.sun.hardware