Okay, I chased this all the way down and want to correct my earlier 102 Hz claim it's colour depth, and it splits into two regimes.
I confused myself at first by changing depth while also toggling power management and rebooting, so I was seeing 68, 87 and 102 Hz without tracking what caused what. Isolated properly now, measured directly on FLM.
During boot (before the Mac OS splash screen), the frame rate depends on the colour-depth setting:
1-bit (B&W): 68 Hz
2-bit (4 greys): 87.4 Hz
4-bit (16 greys): 102 Hz
Once the OS splash/loading screen appears, it switches to a fixed ~69.94 Hz and stays there regardless of depth.
So the depth-dependent rates I first reported and the constant ~70 Hz I measured in the loaded OS are both real they're just different phases. Looks like the ROM picks a boot frame rate based on the stored depth setting, then the OS graphics driver (the DB-Lite driver) takes over and runs its own fixed rate. That fits your note about ROM setting things up before the OS is discovered.
Comparing to your figures: 1-bit (68 Hz) and 2-bit (87 Hz) match exactly. The difference is that your notes group 2-bit and 4-bit together at 87 Hz on the 180 they're distinct, with 4-bit running a full step higher at 102 Hz. So the 180 gives each greyscale depth its own boot frame rate (68/87/102), presumably scaling the refresh up for the extra dithering.
None of this touches the boot screen content or power management I ruled both of those out. It's purely depth-driven during boot, fixed in the OS.
I confused myself at first by changing depth while also toggling power management and rebooting, so I was seeing 68, 87 and 102 Hz without tracking what caused what. Isolated properly now, measured directly on FLM.
During boot (before the Mac OS splash screen), the frame rate depends on the colour-depth setting:
1-bit (B&W): 68 Hz
2-bit (4 greys): 87.4 Hz
4-bit (16 greys): 102 Hz
Once the OS splash/loading screen appears, it switches to a fixed ~69.94 Hz and stays there regardless of depth.
So the depth-dependent rates I first reported and the constant ~70 Hz I measured in the loaded OS are both real they're just different phases. Looks like the ROM picks a boot frame rate based on the stored depth setting, then the OS graphics driver (the DB-Lite driver) takes over and runs its own fixed rate. That fits your note about ROM setting things up before the OS is discovered.
Comparing to your figures: 1-bit (68 Hz) and 2-bit (87 Hz) match exactly. The difference is that your notes group 2-bit and 4-bit together at 87 Hz on the 180 they're distinct, with 4-bit running a full step higher at 102 Hz. So the 180 gives each greyscale depth its own boot frame rate (68/87/102), presumably scaling the refresh up for the extra dithering.
None of this touches the boot screen content or power management I ruled both of those out. It's purely depth-driven during boot, fixed in the OS.
