Re: Some questions

["John R Pierce" <pierce@hogranch.com>]

Duane Clark <Duane.Clark@jpl.nasa.gov> writes
> 
> John R Pierce wrote:
> > 
> > Most all PC (ie. pentium systems) won't generate burst reads, only
> > burst writes.  There's no guarantee that you won't get a target retry abort
> > somewhere in the middle of that burst.
> 
> Yow!! This is a truly scary statement. I am new to PCI and was in the
> process of a board design (for in-house use). The board is currently
> planned to be used in a 166MHz pentium Compaq running Solaris X86. I
> would be hard pressed to meet system needs without burst read
> capability. So what is preventing this? Is it built into the bridge or
> other hardware or a feature of the operating system or just what??? Do
> other systems allow burst reads? Do I need to implement bus mastering?
> If I do so, would the PC bridge allow burst writes to system memory?

Well, basically, the pentium processor will only do burst reads on locations
that are cacheable.  This pretty much eliminates all forms of IO devices as
caching them would cause more problems than good.

The best way to transfer data from a peripheral to main memory is with
busmastering.  You can in theory achieve 100Mbyte/sec, but 60-80Mbyte/sec are
probably more typical.

-jrp
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