A: Yes and no. An emulator emulating only the Motorola
680x0 CPU wouldn't be completely compatible with a
"real" Amiga. To some degree, directly or indirectly,
many Amiga programs, beginning from
the Amiga operating system, and even some file formats (e.g. because they reference some
Display ID, or like IFF-ANIM which makes assumptions about pixel
arrangements in double-buffering screen modes), rely on some
custom chip functionality. While it would be possible to re-write
the software, this is not in the spirit of emulation, and it would
be almost impossible to re-write all older software.
Also, it must be considered that when a program does not make specific use
of a custom chip's functionality, this does of course not need to be emulated,
so there is little or no performance penalty (although there is a certain
complexity involved in maintaining the proper timing and synchronization of the
various custom chip components, even if they are idle). Setting the Workbench
screen or any other frontmost screen to an RTG mode (which
does not use the custom chips) instead of an Amiga PAL or NTSC
mode (which use the custom chips) greatly improves the emulation
speed. Disabling audio in the emulation settings can also slightly
improve the speed of emulation.
Specifically emulating certain aspects of the Amiga custom chips also allows
the emulation software to recognize certain specialized tasks, and better
emulate these by using similar and specific capabilities on the host
environment, if possible (e.g. for blitting, which for example is very fast
using an emulated RTG display card).
Please refer to the chapter on Amiga emulators
for additional information.