gauging interest in VAX 6000-530

From: Mike Cheponis <mac_at_Wireless.Com>
Date: Mon Oct 25 19:02:35 1999

On Mon, 25 Oct 1999, Carlos Murillo-Sanchez wrote:

>Mike: you wanted numbers.
>
>The 11/780 is one machine on which I do have a few recently acquired
>docs. In page 2-2 of the 1977-78 VAX 11/780 architecture handbook there
>is a nice figure of the cpu and different interconnects. Basically, the
>cpu, main memory and I/O subsystems were tied to the Synchronous
>Backplane Interconnect (SBI), 13.3 MB/s, cycling at 200ns.

Great! Numbers! Thanks, Carlos. OK, SBI = 106 Mbits/sec.

For comparison, the 486's Main Bus runs at 848 Mbits/sec [1]. That's 8 times
as fast as the 11/780 SBI bus.

>The console
>was attached to the CPU directly thorugh a separate channel. Any device
>could request control of the SBI and control was granted using a priority
>scheme. The I/O subsystem could hold one unibus adaptor (1.5 MB/s) and
>up to four massbus adaptors (2MB/s each).

Unibus = 12 Mb/s (Coincidentally, USB is -also- 12 Mb/s ...)

Massbuss = 16 Mb/s each.

(For a rough comparison, the 64-bit 66 MHz PCI bus is over 4 Gbits/sec; so
 that particular modern PC bus has 57 times more bandwidth than a Unibus
 and four Massbus adaptors, combined!)


>The cpu had 8KB of two-way
>set-associative write-through cache (and the instruction set allowed for
>generation of very compact code). The cache monitored I/O <=main memory
>transfers and updated accordingly.

The 486 also has 8K of cache, and is approximately 30 times as fast as the
11/780 running the Dhrystone 2.1 benchmark.

>Given this architecture, I think that there is reason to assume that an
>11/780 might service 100 users over serial lines faster than any 486-66
>PC . Just think of the interrupt service overhead of 100 users typing
>simultaneously, even with 16 byte FIFO buffers.


Well, let's see. The ISA16 bus runs at 8.33 MHz, 4 clocks, 16 bits per
cycle. That's only 33.3 Mb/s, comparable to a Unibus plus a Massbus.

(And, of course, most 486dx2/66 machines used at least an EISA bus [2],
 running 32 bits at 8.33 MHz x two clock edges = 533 Mbits/sec.)


So if we are able to plug multiport intelligent serial cards into our EISA
bus, and to use a standard EISA SCSI controller, the numbers say that the
dx2/66 would crush the 11/780 by a factor of at least 5 (which is
533 Mb/s / 106 Mb/s) or perhaps as much as 30, depending on the task.


>By the way, this little book is great reading... It really puts things
>in perspective to see the great 11/780 implementation 22 years ago...

I remember the book well; I wish I still had my copy!

>Carlos.


Thanks again, -Mike


[1] Intel i486 Microprocessor Manual, Page 1. Doc #240440-002 November 1989

[2] Indispensable PC Hardware Book, 3rd edition, p. 552
Received on Mon Oct 25 1999 - 19:02:35 BST