Path: utzoo!attcan!uunet!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!apple!oliveb!amiga!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Bus Speeds doc & question
Message-ID: <11908@cbmvax.commodore.com>
Date: 29 May 90 17:35:16 GMT
References: <1890@mindlink.UUCP>
Reply-To: daveh@cbmvax (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 74

In article <1890@mindlink.UUCP> a186@mindlink.UUCP (Harvey Taylor) writes:
>In <11765@cbmvax.commodore.com>, daveh@cbmvax.commodore.com (Dave Haynie)

>|In article <1990May23.021804.12469@uncecs.edu> urjlew@uncecs.edu
>|(Rostyk Lewyckyj) writes:

>|Yeah, I remember that.  And what that table doesn't show you is that
>|68030 to NuBus interface on the Mac IIs runs about 5 MB/s.

>    How do you know this? I'm not doubting you in particular, I'm just
> looking for an EE101 answer to how to measure &/calculate bus speeds.

Well, I know this mainly because I've seen the figure quoted over and
over again in discussions of the Mac NuBus.  But the basic information
on the bus interface is that NuBus is a synchronous bus clocked at 10MHz.
The minimum bus cycle is two 10MHz clocks (20 MB/s), though most devices 
run in three clocks (13MB/s).  A basic Mac II or IIx runs a 15.8MHz bus, 
and must sync up to the NuBus to achieve a transfer.  It must take at 
least 100ns, worst case, to sync up simply based on the 10MHz clock of 
NuBus, though I recall hearing it's more like a worst-case of 150ns. 
Once data transfer is acknowledged on the NuBus, this must be translated
into 68030 DSACK.  I don't have the NuBus specification handy to be
sure how soon the 68030 can react to data transfer being acknowledged.

In any case, it's the 68030 to NuBus translation that results in the 
maximum effective bus speed of NuBus.  You rarely have a perfectly
efficient bus translation in any bus converter.  Some work quite very
efficiently, others are trouble.  In general, the more similar the
two buses are, the better the conversion efficiency.  So you might
expect Microchannel to 80386 conversion to work better than 68030
to NuBus conversion, since NuBus looks far less like 68030 bus than
Microchannel resembles 80386.

The other stumbling point is clock conversion.  The Mac II, IIx, and
IIci systems each have a main CPU clock that have no relation to the
NuBus clock, so synchronizers much be employed at various points to
avoid any metastabilities in the 68030-to-NuBus cycle conversion state
machine.  On the NeXT machine (which apparently runs a CMOS level NuBus 
with a 12.5MHz NuBus clock and 25MHz system clock) or a Mac IIfx (which 
has a 20MHz local system bus), the bus converter doesn't have to work 
out any major asynchronicities, so the CPU to bus interface on either 
of these systems has a chance, depending on its implementation, of being
much more efficient.

>|That says nothing about how fast other bus masters might go, nor how fast a
>|future Amiga may be able to drive Zorro III.

>   Same questions...

Well, for timing the maximum theoretical speeds for Zorro III full and 
multiple cycles, I simply pulled out "The Zorro III Bus Specification by Dave 
Haynie", flip to "Chapter 5: Timing", and add up the various lengths of
different cycle events for each type of cycle.  That figure sets the minimum 
possible cycle time on the bus, in nanoseconds in this case.  A cycle
transfers 4 bytes, so if you know ns/byte, it's easy to find bytes/ns, 
and ultimately, megabytes/second.  Again, actual Zorro III cycle time is
based on the efficiency of the Zorro III bus master and Zorro III bus slave
acting together.  

A 68030 isn't a perfect Zorro III bus master, so you'll find that, for a 
given memory chip speed, a 68030 will talk to a 68030 memory design faster
than a Zorro III memory design.  It'll be harder or more expensive to build
a Zorro III memory board that'll go 20MB/s with a 68030 (I built one, but 
it uses expensive SRAM) than it would be to build a native 68030 memory
board that'll go 20MB/s with a 68030.  That's rather common on I/O buses,
and a well designed I/O bus master will be able to drive the Zorro III bus
significantly faster than the 68030, or most any CPU, can.

>      Harvey Taylor      Meta Media Productions

-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	"I have been given the freedom to do as I see fit" -REM