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680: z80 emulator for 68k calculators
=====================================
Duncan Smith
Project started: 2010-06-06
GPL
The intent of this project is to provide a fast and correct TI-83+
emulator to run on the TI-89. Once that is done, perhaps I will
extend it to other models in the TI-83 family.
This project has a long and barren history, beginning with my first
contemplation of an emulator similar in interface to Macsbug -- in
September 2002. That foray fizzled after a long email thread with
Michael Vincent. The current iteration was sparked by a comment on
IRC by Brandon Wilson, on June 6 2010:
<BrandonW> chronomex, you should create a z80 emulator for the 68k calculators.
<chronomex> that sounds like a capital idea
<chronomex> I started and abandoned such a project in *2002*
<chronomex> http://students.washington.edu/f/projects/ti/ti83pemu.shtml
<BrandonW> I think we desperately need it.
<chronomex> yeah?
<chronomex> why is MulTI inadequate?
<BrandonW> My understanding is that it just runs select programs.
<BrandonW> Right?
<chronomex> I have not looked into it at all
<chronomex> well other than finding the webpage
<BrandonW> We need to be able to run the TI-OS.
The most difficult challenge in writing a 68k-hosted emulator
targetting the z80 is making it _fast_. TI-83+ calculators have a
clock rate in the neighborhood of 12MHz, as do TI-89s. z80
instructions take from 4 to 17 cycles to execute. I can dispatch an
instruction with a fixed 42 cycle overhead:
emu_fetch:
eor.w d0,d0 ; 4 cycles
move.b (a4)+,d0 ; 8 cycles
rol.w #5,d0 ;16 cycles adjust to actual alignment
jmp 0(a3,d0) ;14 cycles
;; overhead: 42 cycles
From there, an instruction will take anywhere from 0 to, well, lots of
additional cycles. Generally, however, it will take under 50, for 92
total. In the worst reasonable case, a 4 cycle instruction emlulated
in 92 cycles, that's a 23:1 ratio. In the best possible case, a
17-cycle instruction emulated in 42 cycles, is more nearly a 1:2
ratio.
I am not aiming for exactly correct relative timing of instructions,
choosing instead to maintain the highest possible speed. As a result,
programs that depend on cycle counts to function will not work as
expected.
## Useful resources:
* [68k timings](http://www.ticalc.org/pub/text/68k/timing.txt)
* [z80 instruction set in numerical order](http://z80.info/z80oplist.txt)
* [More z80 instruction set reference](http://nemesis.lonestar.org/computers/tandy/software/apps/m4/qd/opcodes.html)
* [Details on flags and other side effects](http://www.gaby.de/z80/z80code.htm)
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