🚀 go-pugleaf

In <15152@suns5.crosfield.co.uk> pdg@crosfield.co.uk writes:
> 
> Well I got a pointer to an answer about these new
> Cyrix chips, the Cx486SLC and the Cx486DLC
> (thanks Henrik Vallgren).
> 
> It seems that the chief performance gain (over a 80386)
> comes from the cache.  So, unless your motherboard has
> been designed for these chips, there's not much of a gain
> at all.  There is a software fix (for DOS) which causes
> the cache to be flushed every time some external
> device DMAs to the motherboard, but that reduces the
> performance gain.

The cache is definitely the main source of the 486DLC's performance
gain over the 80386. This can be easily deduced from the performance
of Intel's RapidCAD chip which is basically a 486 without the cache
and with a 386 pin out. While the RapidCAD offers roughly twice the
floating point performance of the Intel 387DX, my benchmarks showed
it to be at most 25% faster than the 80386. Typical performance gain
for integer applications is more like 15% though. This definitely
shows that the 386 bus interface (2 clock cycles per bus cycle, no
burst mode) is a bottleneck for the RapidCAD that can only be overcome
with an internal cache (the above mentioned benchmarks were run on a
machine with a fast 128 kB direct-mapped write-thru cache with one
write buffer). When executing register-to-register instructions, the
RapidCAD will execute them faster than they can be delivered thru the
slow 386 bus interface. When executing instructions that access memory
the most time is spent in the memory access so one doesn't see a big
improvement over the 386 either. 

The German computer magazine c't has shown that for heavy DMA traffic
(e.g. playing a WAV file thru the Soundblaster), the 486DLC with the
cache switched on is slower than with the cache switched off due to
the frequent cache flushes. 


> Paul
> -- 

Norbert
-------------------------------------------------------------------------------
Norbert Juffa       email: S_JUFFA@IRAVCL.IRA.UKA.DE         Live and let live!