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

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Hi

as a summer job, I worked on supporting the MegaSTE external 16 KB cache 
in Hatari :)

Thanks to Christian's tests on his MegaSTE by running some custom cache 
test programs it was possible to precisely measure the cache speed in 
various scenario, as well as confirming some cache logic (such as 
"reading a byte from RAM will in fact cache the whole 16 bit word")

Attached is a small program that will toggle between "8 MHz - no cache" 
and "16 MHz - cache" when Hatari runs in MegaSTE mode (will work on 
reald MegaSTE too)

You can run it from TOS 2 or EmuTos in MegaSTE mode and see for yourself 
the speed improvement when cache is enabled (for example gembench 
reports between 139% and 190% speed compared to normal 8 MHz)

Note that the cache needs to be flushed on every transfer when using 
Gemdos HD emulation, or normal FDC / HDC. Else this would transfer data 
to RAM without invalidating the cache (which has no means to know RAM 
has been modified), thus creating crashes in the emulated program sooner 
or later.


For programs making use of the MegaSTE's cache + 16 MHz speed, you can 
try the "Little -Me- Demo" by Overlanders
https://www.pouet.net/prod.php?which=90542

The demo will switch to 16 MHz with cache during the 3d parts.


Nicolas

Attachment: cac16.prg
Description: Binary data

; Toggle the MegaSTE between "8 MHz - no cache" and "16 MHz - cache"
;
; 2024/09/03	Nicolas Pomarede


        opt     x+

	; supervisor mode
	pea     0
	move.w  #$20,-(sp)
	trap    #1
	addq.l  #6,sp
	
	move.b	$ffff8e21.w,d0
	btst	#1,d0
	beq.s	set_16			; freq is 8 MHz, set to 16 MHz

;-- set cpu to 8 MHz, no cache
set_8
	move.b	#0,$ffff8e21.w
	bra.s	end

;-- set cpu to 16 MHz with cache
set_16
	move.b	#3,$ffff8e21.w

end
	clr.w   -(sp)			; exit program
	trap    #1