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From: smith@iwblsys.UUCP (Mickey Smith)
Newsgroups: comp.dcom.lans,comp.sys.intel,comp.protocols.ibm
Subject: Re: MBII, VME performance
Summary: MBII bus speed
Keywords: transactions per second
Message-ID: <286@iwblsys.UUCP>
Date: 6 Jul 89 16:23:36 GMT
References: <1796@cbnewsh.ATT.COM> <1284@motmpl.UUCP>
Followup-To: poster
Organization: Intel Corp.
Lines: 39

In article <1284@motmpl.UUCP>, ron@motmpl.UUCP (Ron Widell) writes:
>
> Well it's been quite a while since I've looked at either of the specs (and
> I'm at home right now) (WARNING-possible memory parity error) but it seems
> to me that the theoretical max for MBII was about 40 MBytes/sec (one 4-byte
> transfer every 100nS) while the theoretical max for VME was about 48 Mbytes-
> per-second (one 4-byte transfer every 60 nS).
> Neither of them ever (to my knowledge) got that. But both of them, given the
> proper memory system, could comfortably run in the 20-30 Mbyte/sec range.

MultiBus II backplane speed is based on a 10 MHz synchronous clock and
a 32 bit data path. MultiBus II will only make transfers across the bus
at this rate(40 Megabytes/sec), no faster and no slower. 

As far as bus performance, consider this. The transfer rate from one
memory subsystem to another is dictated by the slowest memory subsystem
speed. This is true of VME as well as Multibus II when we compare such
memory to memory data movements. But since there is a bus interface
part on Multibus II (the Message Passing Coprocessor) that decouples
the local bus from the backplane bus, the time spent holding the
backplane for the transfer is held to a percent of total bus bandwidth.
To illistrate this:

Time to transfer from memory A to memory B:

     |----------------------------------------------------------|

Packetized transfers on the backplane bus:

     |--|        |--|       |--|       |--|       |--|       |--|

The time between the packetized transfers is usable for other agents
on the bus to carry out other functions.

-- 
Mickey Smith, Regional Software Specialist  | Standard disclaimer, I speak
Intel Corporation                           | only for myself
7071 Orchard Lake Road                      |
W. Bloomfield, MI 48332  (313) 851-8096     |