can be used by a frontend to implement breakpoints or tracelogging
currently hooked: M68K executing/reading/writing, VDP reading/writing (only for MegaDrive)
Some games can benefit from a little overclocking but start behaving
strangely at 2x. Make the internal overclock ratio a fixed point
number and add 3 fractional settings.
This reduces all cycle costs by half for the primary CPU. There
is a delay upon loading before overclocking is applied as a
workaround for some games that detect PAL/NTSC systems by counting
cycles in a frame at startup.
