Vax 11/780 performance vs Sun 4/280 performance

[John Mashey]

In article <3859 at lynx.UUCP> m5 at lynx.UUCP (Mike McNally) writes:
...
>As a somewhat related side question, what does the Sun 4/SPARC MMU look
>like?  Are lookaside buffer reloads done in software like on the MIPS
>R[23]000?  (Is that really true about the R[23]000 anyhow?)

The Sun-4 MMU, like earlier Suns, doesn't use a TLB, but has SRAMs
for memory maps (16 contexts' worth, compared to 8 in Sun-3/200, for
example).

The R[23]000 indeed do TLB-miss refill handling in software;
this is not unusual in RISC machines: HP Precision and AMD 29K (at least)
do this also.  The overall cost if typically 1% or less of CPU time,
which is fairly competitive with hardware refill, especially since one
of the larger costs on faster machines is the accumulated cache-miss penalty
for fetching PTEs from memory.
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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