| Re: [hatari-devel] Adding cache support for the MegaSTE |
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Le 13/07/2024 à 15:29, Christian Zietz a écrit :
Christian Zietz schrieb:
Results attached.
I struggle to understand the difference between 8 MHz and 16 MHz + 100%
cache hits, though. The come out at $19E = 414 HBLs and $A7 = 167 HBLs,
respectively, which is a 414/167 ≈ 2.5(!) speed increase.
You do a MOVE.W (Ax)+,Dx; DBRA Dy,xxx. The MOVE is 8 cycles, the DBRA is
10 cycles. In 8 MHz mode the DBRA incurs two waitstates (because of
unaligned prefetch from ST RAM). Hence, in 8 MHz mode the loop should
take 8+10+2=20 cycles, equivalent to 40 16-MHz cycles.
Assuming 100% cache hit, in 16-MHz mode the same loop ought to take 8+10
= 18 cycles.
But that only would explain a 2.22 (40/18) speed increase. I wonder if
the rest is due to overhead in interrupt handling in 8-MHz mode.
Do you see a way to modify your test program to run with interrupts
disabled, using, e.g., one of MFP timers for time measurement?
In my case where I want to compare Hatari with MegaSTE, it's not too
much of a problem if counting takes more time, as long as both Hatari
and MegaSTE give the same result
but it's true that an HBL interrupt takes 112 cycles, even with just a
"add" in the interrupt. And when you run in monochrome an HBL is
triggered every 224 cycles at 8 MHz, so half of the time is spent in the
hbl, which is a lot compared to color mode at 50 Hz for example.
and even at 16 MHz you loose a lot of cpu just for the hbl int, which
makes it harder to really measure cache alone with as little overhead as
possible.
I will change the measure mode to use a timer and send a new version.