Thanks for that insight, Bob.
I wasn't anywhere near DEC when this happened, but it do match my
perceptions of the whole thing very well.
MSCP was definitely an improvement on Massbus in pretty much every way.
Possibly with the exception of if you wanted to throw together some
small piece of code to just control the drive. MSCP always requires a
bit more plumbing.
But it is nice that I can still throw new drives onto my "old" RSX
system, and with MSCP it just works, and I can use all the capacity of
that new drive... Noone probably even dreamed of 8G drives on a PDP-11
back then... But it works fine. No changes to anything needed.
Johnny
On 2019-06-25 17:43, Bob Supnik wrote:
True. My first assignment at DEC was managing the "New Disk Subsystem"
(NDS) advanced development project, which led eventually to both the
HSC50 and the UDA50. Among the goals of the project were
1. To move ECC correction off the host and into the disk subsystem, so
that much more powerful and complex ECC codes could be used.
2. To move bad block replacement off the host and into the disk subsystem.
3. To provide a uniform software interface for radically different disk
drives.
4. To abstract away all device geometry information.
5. To implement command queuing and to perform all performance
optimization, particularly seek optimization, in the disk subsystem.
From my recollection, the Massbus was actually regarded as the
counterexample. First, the Massbus cables were massive and unwieldy; the
radial drive interconnect for the RA-class drives were a direct
reaction. Second, the Massbus exposed drive geometries, which made it
impossible to implement a new disk type without a driver update; with
MSCP, the subsystem reported capacities, and the operating system could
set up a file structure accordingly... sometimes.
What the two protocols had in common was standardizing the format of the
software [register (Massbus) / packet (MSCP)] interface. All Massbus
disks used the same register sets... except when they didn't (the RP/RM
divergence). All MSCP disks used the same packet formats... except for
errors. But it was a lot better than the "every disk is unique"
nonsense. DG had a standardized disk interface from the get-go.
/Bob
On 6/25/2019 8:10 AM, [email protected] wrote:
Message: 8
Date: Tue, 25 Jun 2019 08:10:08 -0400
From: Paul Koning<[email protected]>
To: Timothe Litt<[email protected]>
Cc: SIMH<[email protected]>
Subject: Re: [Simh] Limits on MSCP controllers
Message-ID:<[email protected]>
Content-Type: text/plain; charset=us-ascii
On Jun 24, 2019, at 5:27 PM, Timothe Litt<[email protected]> wrote:
As is often the case, things turn out to be complicated. Here's a
more detailed version. In an off-list note, Bob pointed out that
MSCP originated in a project he managed that was to develop the "next
generation" disk controller - a forerunner of the UDA. ...
However the similarities came to pass, I found viewing DSA as an
evolved Massbus to be a useful model, with a lot of support for that
perspective in the specifications. MSCP contains the high-level
protocol of Massbus drivers (and much more) through the drive control
logic/formatter. SI replaces the DCL/formatter to drive "bus" of
Massbus -- SI is serial, ruggedized and capable of quite long runs.
But it carries much the same low level drive commands. (Note that
there's a long history of serializing parallel buses as technology
evolves, e.g. PCI -> PCIe -> CSI, a.k.a. quickPath). The host ports
(UQSSP,KLIPA,etc) replace the registers and DMA channels. Command
and function names from Massbus spec & drivers often appear in the
MSCP spec versions that I used...
Very interesting. I never thought of MSCP as a descendant of earlier
DEC storage architectures. Perhaps because all I really saw was what
the UDA50 exposes, which from the programmer's point of view is
radically different from, say, the RP04 or RK05.
On the host ports and message based I/O, that same approach appears
earlier in the KMC11 and its derivatives such as the DMC11 network
controller. Were those an influence on the message based host port
design?
paul
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