[Simh] Limits on MSCP controllers

[Paul Koning]

> On Jun 24, 2019, at 5:27 PM, Timothe Litt <litt at ieee.org> 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