Some programs overwrite the ROM, either due to an error, because they just don't care, or by pure intention, probably as part of their copy protection. These programs don't run reliably on 68k Mac Spectacle using the "Fastest emulation" mode, because in this mode the ROM is not write protected. You must use "Better emulation" or "best emulation" mode instead.
Some programs rely on an exact interrupt timing. Somehow and for some reason they expect this-or-that amount of instructions to be performed between two interrupts. These programs run only in "Exact speed" or "Highres" mode.
The implementation of the DAA instruction in the 68k assembler coded Z80 engine is optimized for speed. In some rare cases it does not work correctly. Then the "Best emulation" mode is the only solution.
Some undocumented side effects of Z80 instructions are handled more or less carefully in different the modes. Most notably the undocumented setting of flags after bit testing is ignored in the "Fastest emulation" mode.
Some programs synchronize writing to screen with the interrupt which is synchronized with the cathode ray frame flyback by the ULA. This way they avoid flickering of sprites on a real ZX Spectrum. On the emulator you must use "Highres" mode to get the same results.
Conclusion:
• Use "Fastest emulation" mode only, if your Mac is too slow (LC II or LC III)
• Prefer "Exact speed" for arcade style games
• Prefer "Unlimited speed" for other games and programs
The Basic interpreter
The floating point unit sometimes uses address $0000 as dummy target for calculations. Therefore triggering an NMI to perform a reset doesn't work reliable in "Fastest emulation" mode. Sometimes the character set is hit. Besides of this, it seems to be safe to use "Fastest emulation" mode for basic programs.
FRED
Sometimes overwrites parts of the ROM and crashes if Fred is in the top level of the pyramid. Bonus calculation after leaving a pyramid implies a DAA correction which is not handled correctly by the 68k assembler coded Z80 engine. Switch to "Best emulation" mode before leaving the pyramid.
WIZARDS LAIR
Overwrites the ROM and relys on exact interrupt timing.
STARQUAKE
Overwrites the ROM and relys on exact interrupt timing. (Same programmer as Wizards Lair)
SABRE WULF
The little rhino gets stuck in corners if the undocumented setting of flags after bit testing isn't emulated right. (The so-called "little rhino error") Use "Better" or "Best" emulation mode.
FOX FIGHTS BACK
Tiny little effect when selecting an option in the start-up menu only visible in highres mode.
