Re: Cycle by Cycle Emulator

[mjmahon@aol.com (Michael J. Mahon)]

Bryan Parkoff wrote:

>It is
>designed to emulate at cycle by cycle unlike instruction by
>instruction that most emulators have instruction by instruction.

If it is not possible to tell by its behavior whether it is cycle-
by-cycle or instruction-by-instruction, then there is no _functional_
difference.

In creating a completely faithful emulation of the 6502, one can
either emulate cycle-by-cycle, with few special cases, or
instruction-by-instruction, with several special cases (like
page wrap and page-crossing behaviors).

In the cycle-by-cycle case, I would estimate a factor of two
slowdown because of extra emulation loop overhead, while
with instruction-by-instruction emulation, handling the special
cases might amount to a few percent additional overhead.

It is a matter of the required efficiency and the designer's
taste that determines the approach taken.

For, example, a purely functional emulation might be
better done with instructions as the unit of emulation.
But if the object was bus emulation, as well, then
a cycle-level emulation would be more natural.

-michael

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