Re: [hatari-devel] Adding cache support for the MegaSTE

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

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.