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+z680k: The Tricky Bits
+======================
+
+For those following along at home, here's some explanation of the
+tricky bits of z680k.
+
+Flags
+-----
+
+Computing flags is hard, and can take a long time.  So I avoid doing
+it whenever possible.  Flag computation is usually done only when an
+instruction asks for it, and then generally a minimum amount of work
+is done in order to maintain performance.
+
+After every instruction that can influence flags, z680k notes down
+what has changed.  It records this information in one of several ways:
+
+1. Simulated F register
+
+   The simplest to understand is the simulated F register, which is
+   composed of `flag_storage` and `flag_valid`.  `flag_valid` is a
+   mask indicating what bits of `flag_storage` are part of the F
+   register.  This storage space may be fully, partially, or not at
+   all valid.  It is considered most authoritative.
+
+2. Saved 68k Condition Code Register
+
+   After all operations that affect the Sign, Zero, Parity/oVerflow
+   (in the oVerflow mode), or Carry flags, the 68k condition code
+   register is saved in `f_host_ccr`.  As necessary, this is looked up
+   in `lut_ccr` for a mapping to Z80 flags.  The validity mask of this
+   table is at most `11000101`.
+
+3. Saved Operands
+
+   After arithmetic operations that affect the half-carry (H) flag,
+   the operands are saved in `f_tmp_src_b` and `f_tmp_dst_b`.
+   (Instructions that affect the half-carry flag and operate on words
+   use `f_tmp_???_w` instead.)
+
+4. Miscellany
+
+   Parity may be recorded immediately?  I haven't written it yet.
+   `f_tmp_p_type` records whether it is parity or overflow, and
+   whether it's been calculated or not.  Parity is looked up in
+   `lut_parity`, a table that was stolen^Wborrowed from some other
+   emulator.  No malice intended; I simply forget which it was.
+