byuu says:
I split the Register class and read/write handlers into DataRegister and
AddressRegister, given that they have different behaviors on byte/word
accesses (data tends to preserve the upper bits; address tends to
sign-extend things.)
I expanded EA to EffectiveAddress. No sense in abbreviating things
to death.
I've now implemented 26 instructions. But the new ones are just all the
stupid from/to ccr/sr instructions.
Ryphecha confirmed that you can't set the undefined bits, so I don't
think the BitField concept is appropriate for the CCR/SR. Instead, I'm
just storing direct flags and have (read,write)(CCR,SR) instead. This
isn't like the 65816 where you have subroutines that push and pop the
flag register. It's much more common to access individual flags. Doesn't
match the consistency angle of the other CPU cores, but ... I think this
is the right thing to for the 68K specifically.
byuu says:
Redesigned the handling of reading/writing registers to be about eight
times faster than the old system. More work may be needed ... it seems
data registers tend to preserve their upper bits upon assignment; whereas
address registers tend to sign-extend values into them. It may make
sense to have DataRegister and AddressRegister classes with separate
read/write handlers. I'd have to hold two Register objects inside the
EffectiveAddress (EA) class if we do that.
Implemented 19 opcodes now (out of somewhere between 60 and 90.) That gets
the first ~530,000 instructions in Sonic the Hedgehog running (though
probably wrong. But we can run a lot thanks to large initialization
loops.)
If I force the core to loop back to the reset vector on an invalid opcode,
I'm getting about 1500fps with a dumb 320x240 blit 60 times a second and
just the 68K running alone (no Z80, PSG, VDP, YM2612.) I don't know if
that's good or not. I guess we'll find out.
I had to stop tonight because the final opcode I execute is an RTS
(return from subroutine) that's branching back to address 0; which is
invalid ... meaning something went terribly wrong and the system crashed.
byuu says:
Another six hours in ...
I have all of the opcodes, memory access functions, disassembler mnemonics
and table building converted over to the new template<uint Size> format.
Certainly, it would be quite easy for this nightmare chip to throw me
another curveball, but so far I can handle:
- MOVE (EA to, EA from) case
- read(from) has to update register index for +/-(aN) mode
- MOVEM (EA from) case
- when using +/-(aN), RA can't actually be updated until the transfer
is completed
- LEA (EA from) case
- doesn't actually perform the final read; just returns the address
to be read from
- ANDI (EA from-and-to) case
- same EA has to be read from and written to
- for -(aN), the read has to come from aN-2, but can't update aN yet;
so that the write also goes to aN-2
- no opcode can ever fetch the extension words more than once
- manually control the order of extension word fetching order for proper
opcode decoding
To do all of that without a whole lot of duplicated code (or really
bloating out every single instruction with red tape), I had to bring
back the "bool valid / uint32 address" variables inside the EA struct =(
If weird exceptions creep in like timing constraints only on certain
opcodes, I can use template flags to the EA read/write functions to
handle that.
byuu says:
Up to ten 68K instructions out of somewhere between 61 and 88, depending
upon which PDF you look at. Of course, some of them aren't 100% completed
yet, either. Lots of craziness with MOVEM, and BCC has a BSR variant
that needs stack push/pop functions.
This WIP actually took over eight hours to make, going through every
possible permutation on how to design the core itself. The updated design
now builds both the instruction decoder+dispatcher and the disassembler
decoder into the same main loop during M68K's constructor.
The special cases are also really psychotic on this processor, and
I'm afraid of missing something via the fallthrough cases. So instead,
I'm ordering the instructions alphabetically, and including exclusion
cases to ignore binding invalid cases. If I end up remapping an existing
register, then it'll throw a run-time assertion at program startup.
I wanted very much to get rid of struct EA (EffectiveAddress), but
it's too difficult to keep track of the internal effective address
without it. So I split out the size to a separate parameter, since
every opcode only has one size parameter, and otherwise it was getting
duplicated in opcodes that take two EAs, and was also awkward with the
flag testing. It's a bit more typing, but I feel it's more clean this way.
Overall, I'm really worried this is going to be too slow. I don't want
to turn the EA stuff into templates, because that will massively bloat
out compilation times and object sizes, and will also need a special DSL
preprocessor since C++ doesn't have a static for loop. I can definitely
optimize a lot of EA's address/read/write functions away once the core
is completed, but it's never going to hold a candle to a templatized
68K core.
----
Forgot to include the SA-1 regression fix. I always remember immediately
after I upload and archive the WIP. Will try to get that in next time,
I guess.
Tim Allen