Is a MSX2 machine slower than a MSX1 one?

Is this an existing program/game or own code ?
There are minor differences in the bios between msx1 and msx2.
Some routines involve slotswitching to the msx2 subrom which add some extra cycles.
Interrupt routines are nearly identical but machines with a diskdrive use some extra cycles. But this can be ruled out by starting the machine with the SHIFT key pressed.

Hi, jltursan!

If you don't need the BIOS reading the keyboard into OLDKEY and NEWKEY system variables (quite likely, as you are already reading the cursors/joystick elsewhere), you can skip a long part of the ISR by adding this near the end of your H.TIMI hook (or maybe you are already doing it):

; Tricks BIOS' KEYINT to skip keyboard scan, TRGFLG, OLDKEY/NEWKEY, ON STRIG...
	xor	a
	ld	[SCNCNT], a
	ld	[INTCNT], a

I don't think this solves the MSX-MSX2 differences, but may free some cycles per frame for you. As the keyboard scan is every third frame, you will also get more stable color border bars :-P

Good job with that AGD engine!!

P.S.: A friend of mine wants to port one of its games (made in AGD) to MSX. Is there any possibility of getting an alpha or beta version of your AGD engine?
Thanks!

Hi all! Where Can I find these patches that mostly disable the BIOS ISR? I'd like to know more, thanks!

I measured in openMSX while idling in Basic without diskdrive (boot with SHIFT key).

• The MSX1 BIOS ISR takes 871 cycles. Every 3rd frame it takes 5278 cycles.
• The NTSC MSX2 BIOS ISR takes 933 cycles. Every 2nd frame it takes 5310 cycles.
• The PAL MSX2 BIOS ISR takes 4109 cycles. Every 2nd frame it takes 5310 cycles.

The 3rd / 2nd frame thing is when SCNCNT reaches 0, then it does keyboard scanning etc. On MSX1 SCNCNT is always reset to 3 regardless of region so it runs the expensive path every 3 frames. On NTSC MSX2 this resets to 2 (so slower on average). And on PAL MSX2 this is reset to 1, in other words it always runs! (Although there still seems to be some part that’s skipped further in.)

The latter is quite shocking actually, I did know PAL scanned more frequently but I thought NTSC did it every 3 and PAL every 2 frames. Not so! Every frame’s pretty bad, even though of course there’s plenty of frame time comparatively, until you switch the VDP to 60 Hz that is!

So in the basis the MSX2 ISR is only marginally slower (due to the addition of light pen support). But it became much slower because they messed with the SCNCNT reset value, especially on PAL. Though the worst case didn’t change, even on MSX1 you will get frames where the ISR is about as slow as on MSX2.

Either way, the simplest code solution is to manually write SCNCNT yourself (e.g. in a H.TIMI hook). I think this is better than making a custom ISR. Set it to 0 continuously to make it always take the fast path (and scan manually). Set it to JIFFY & 1 to make PAL MSX2 behave as NTSC MSX2 does. Set it to JIFFY % 3 to make MSX2 behave as MSX1, or more easily JIFFY & 3 which then scans every 4 frames.

The machines I tested this in were Yamaha CX5M (PAL), Sony HB-75P (PAL), Philips NMS 8245 (PAL) and Panasonic FS-A1 (NTSC).

p.s. The increased scan frequency does probably make MSX2 machines actually feel a bit more responsive to the user (less latency).

Not sure about that. The BIOS code in MSX1 is :

LD      HL,SCNCNT
DEC     (HL)            ;Need to scan?
JR      NZ,INTRET       ;No, return soon
LD      (HL),3          ;Time delay of first repeat

Therefore it scans when SCNCNT is equal to zero.
Is MSX2 BIOS different ?