Is the Genesis always running at 7.67MHz?

[Kirk_Johnston]

Or are there times and/or modes where it's actually running slower, like maybe H32 mode for example?

[+selgus]

Not exactly what you are asking, but when we did the XBAND Modem and needed to keep two different consoles in sync with one another, we would have a process that would constantly be hitting the hardware register to trigger interlace video vs. non-interlace, as a way of speeding up and slowing down the SEGA. This was because no two consoles were exactly in sync, and turning on and off interlacing would allow us to synchronize the two consoles while playing a game.

[Chilly Willy]

As far as I understand it, the 68K always runs at the master clock / 7. The master clock is 53.693175 MHz for NTSC, and 53.203424 MHz for PAL. The Z80 runs at the master clock / 15. The VDP runs at the master clock / 10 for H32 mode. When Sega added H40 mode, they left the timing at H32 speeds, but changed the VDP clock during the active line to the master clock / 8. During the sync, it switches back to / 10.

Edited November 5, 2022 by Chilly Willy
speeling

[Kirk_Johnston]

On 11/5/2022 at 11:20 PM, Chilly Willy said:

As far as I understand it, the 68K always runs at the master clock / 7. The master clock is 53.693175 MHz for NTSC, and 53.203424 MHz for PAL. The Z80 runs at the master clock / 15. The VDP runs at the master clock / 10 for H32 mode. When Sega added H40 mode, they left the timing at H32 speeds, but changed the VDP clock during the active line to the master clock / 8. During the sync, it switches back to / 10.

So, the VDP runs a little slower overall in H32 mode, but the rest is basically the same as in H40 mode?

[Chilly Willy]

23 hours ago, Kirk_Johnston said:

So, the VDP runs a little slower overall in H32 mode, but the rest is basically the same as in H40 mode?

Yeah, that's a good way to think of it. Because it runs slower in H32 mode, there's fewer cycles for fetching data, which is why there are more limits on things like the number of sprites per line (16 instead of 20).

[Kirk_Johnston]

15 hours ago, Chilly Willy said:

Yeah, that's a good way to think of it. Because it runs slower in H32 mode, there's fewer cycles for fetching data, which is why there are more limits on things like the number of sprites per line (16 instead of 20).

I still find it bizarre that the Genesis seems to reduce most of its specs in the lower resolution mode rather than the other way around where it has to show more screen view in H40 mode, so you'd think it would have to reduce some of the other specs to compensate for that or be able to do/show even more in H32 mode (other than resolution, obviously).

 

Is there some logical reason for that being the case? I mean, why not just run it all at the same speed in H32 mode and then allow for even more sprites on-screen and the like than in H40 mode, since it would have little spare power to work with as it doesn't have to fill up as large a screen space every frame? 

 

Because, in standard H40 mode, the Genesis actually has some notable advantages over its biggest competitor of the time, the SNES, such as the obvious higher horizontal resolution and more sprites [above 8x8] per scanline before flicker, but it appears to actually be less capable than the SNES in most ways when both are running in 256x224 mode, such as even less sprites on-screen total and actually a little bit less sprites per scanline before flicker and such, along with all the normal stuff like less colours and fewer background layers and no proper transparency, etc, which is bonkers.

 

To me, it would be like if the SNES was running in 512x224 mode, which clearly requires more of the system's resources to draw that much higher overall amount of on-screen visuals, yet somehow it could show more sprites total and more sprites per scanline and other stuff than in 256x224 mode. It just wouldn't make logical sense to me. Yet that seems to be exactly what happens on Genesis. It sort of makes H32 mode on the Genesis a bit pointless, other than purely for ports of games that already run in that mode, since everything about H40 appears to be better there. Maybe that's the only point of the H32 mode though, those 256x224 ports. But I still don't get why the system doesn't actually do more of everything in H32 mode to be honest.

 

Thankfully, there aren't that many Genesis games that run in 256x224 mode that are also on the SNES for direct comparison, or else it seems the Genesis versions of those games really wouldn't fare well at all if it were stuck at the same resolution with even less total sprites on-screen than normal (down from 80 to 64 vs SNES 128) a little bit less sprites per scanline before flicker (down from 20 to 16 vs SNES 32), the same smaller max built-in sprite size (32x32 on Genesis vs 64x64 on SNES), way less colours, less background layers, no proper transparency, no built-in Mode 7, etc. But that extra horizontal resolution and more sprites [above 8x8] per scanline before flicker in H40 mode really is one of the Genesis' best advantages, even more so than the likes of the CPU speed imo. A well designed SNES game with properly optimized code can basically nullify the claimed issues with its generally slower CPU, but you really can't do a lot on SNES to make up for the smaller standard horizontal screen view or to match the Genesis full potential when it comes to displaying more sprites [above 8x8] in a row before flicker occurs, which really is useful in say putting a bunch of enemies on-screen in a beat 'em up [without flicker] and the like.

 

Long story short: H32 mode on Genesis seems kinda rubbish imo, especially when compared to the SNES in standard 256x224 mode, but H40 mode really does it very proud.

Edited December 16, 2022 by Kirk_Johnston

[Chilly Willy]

H32 mode is for backwards compatibility. It's basically the same as the Master System video. They started with the SMS VDP and added to it to get the MD VDP. MD games are supposed to run in H40 mode for better resolution, more sprites, etc., but MD games CAN use the 256 wide mode when porting a game that is only 256 wide and they don't want to spend the money changing the graphical assets for the new resolution. If a game NEEDED more sprites, you moved to H40 mode and paid an artist to update the graphics.

[Kirk_Johnston]

13 hours ago, Chilly Willy said:

H32 mode is for backwards compatibility. It's basically the same as the Master System video. They started with the SMS VDP and added to it to get the MD VDP. MD games are supposed to run in H40 mode for better resolution, more sprites, etc., but MD games CAN use the 256 wide mode when porting a game that is only 256 wide and they don't want to spend the money changing the graphical assets for the new resolution. If a game NEEDED more sprites, you moved to H40 mode and paid an artist to update the graphics.

OK, that makes sense.