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@@ -5,6 +5,37 @@ Astrid 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: