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diff --git a/technical.markdown b/technical.markdown new file mode 100644 index 0000000..819e2be --- /dev/null +++ b/technical.markdown @@ -0,0 +1,48 @@ +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. + |