On Wed, Nov 21, 2001 at 05:53:30PM -0600, Alfred Perlstein wrote:
> * der Mouse <mouse@Rodents.Montreal.QC.CA> [011121 17:30] wrote:
> > > To 'shovel' 10Mb ethernet across the ISA bus means moving a byte
> > > every 800ns.  I can't remember the exact figure, but a typical ISA
> > > bus cycle is around 600ns (some of the cards I've seen actually add
> > > wait states!).  So for an 8-bit card the data transfer takes 75% of
> > > your cpu time -
> > 
> > Shouldn't that be "75% of your bus bandwidth"?
> 
> No, 100% of your ISA bus bandwidth, probably some thing like
> 10% of your CPU<->memory bus bandwidth, and 75% of your CPU
> time spent stalled in IO.

That's almost certainly only true on an in-order processor with really
dumb I/O hardware attached to it (which, I'll admit, the i486 is).

However, it's not true anyway, because it demonstrably can't be.  I
don't know what was wrong with the way the Unix the person who originally
claimed this was doing its I/O, but in point of fact I've seen slow 486
boxes running NetBSD do well more than 1MB/sec of programmed I/O on the
ISA bus; with 16-bit cards (which you'd use, of course, were you not an
idiot) I've seen them move well more than 3MB/sec.

-- 
Thor Lancelot Simon	                                      tls@rek.tjls.com
    And now he couldn't remember when this passion had flown, leaving him so
  foolish and bewildered and astray: can any man?
						   William Styron