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680: z80 emulator for 68k calculators
=====================================
Duncan Smith
Project started: 2010-06-06
GPL
The aim of this project is to provide a fast and correct TI-83+
emulator to run on the TI-89.
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.
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 23 cycles to execute. I can dispatch an instruction
with a fixed 30 cycle overhead:
emu_fetch:
eor.w d0,d0 ; 4 cycles
move.b (a4)+,d0 ; 8 cycles
rol.w #5,d0 ; 4 cycles adjust to actual alignment
jmp 0(a3,d0) ;14 cycles
;; overhead: 30 cycles
From there, an instruction will take anywhere from 0 to lots of
additional cycles, but generally under 50.
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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