How does this "CPU Turbo mode" (5.37 MHz) work?

Thanks! Great info!

Can this be emulated in openmsx? For both turbo and this 7MHz, it could be nice to see how the game performs

The Panasonic MSX2+ 5.37 MHz turbo is emulated in openMSX. Though not sure if the effect it has on the PSG pitch is, too; the PSG is pitched up by 50%, so the program that enables the turbo needs to compensate for that (SMW does).

However 7 MHz mods are not really emulated. You can set a custom CPU speed in openMSX, however the mods have circuitry to slow the Z80 down back to on I/O, which is not emulated. They do this to improve compatibility (especially for the VDP), but there is no unified standard way, some mods only slow down on I/O to specific devices, others slow down on all I/O.

Really these mods are hacks, so the user knows that they don’t always work. For example, enabling 7 MHz messes up music because most sound expansions use the system clock which is directly fed from the CPU. It’s a simple mod without motherboard modification to prevent this. And due to the constant clock speed switching (for I/O) it doesn’t just pitch up an octave, but sounds garbled.

If you have it installed there are situations where it’s useful and where you would enable it, e.g. to speed up DOS2 file operations, compression or compiling code. However you would typically keep it disabled while playing games. Though I personally remember enabling it sometimes if I had to walk long distances in Metal Gear 2, it would mess up the music but I could get from A to B quite a bit more quickly.

You can more or less test in openMSX by setting a custom CPU speed and disabling the VDP speed limit, however it’s not a completely accurate emulation due to the absence of the I/O slowdown.