Just had an insight here: if the game is running on MSX 2 or above, I can use unrolled OUTI's, even after VBlank. V9938 works with 15 cycles apart from each OUT operation, instead of the 29 cycles of TMS9918.
So I simply have to check MSXID3 ($002d) for a value different than 0.
It's working perfectly (tested on openMSX) on MSX 2, 2+ and TR.
Yes, you can use OTIR or unrolled OUTI if it's MSX2 or above.
The simplest way is indeed to check the BIOS ID value at $2D, although by doing so, you will be missing MSX1 computers equipped with the V9938 (they are rare, but they're out there). Alternatively, you could check the VDP identification number in S#1.
My routine to get when a frame takes more than one Vblank finally works.
How do you do it?
Can you share the reference? I'll search for it in the GitHub.
TIA
Actually is very simple, I save the current Jiffy at the frame start and compare it to jiffy at the end of main loop:
MainLoop:
ld hl, BIOS_JIFFY ; (v-blank sync)
ld a, (hl)
.waitVBlank:
cp (hl)
jr z, .waitVBlank
; ----------------------------------------------------------------
; Save Jiffy to check if previous frame ended
ld a, (hl)
ld (CurrentJiffy), a
; Main loop code here
; ......
; ......
; ......
; ......
; ......
ld a, (BIOS_JIFFY)
ld b, a
ld a, (CurrentJiffy)
cp b
call nz, .frameSkip
jp MainLoop
Now I see that I should put these two pieces of code inside IFDEF DEBUG directives.
I use the same procedure (waiting for VBLANK to start the main loop) in my game engine.
This trick to see if the loop took longer than a frame is simple yet efficient.
Nice.
