There were four bugs in these instructions. First, the loaded value was being
stored into a floating point register as floating point, changing the value as
it was transfered. Second, the meaning of the "up" bit had been reversed.
Third, the statically sized microop array wasn't bit enough for all possible
inputs. It's now dynamically sized and should always be big enough. Fourth,
the offset was stored as an unsigned 8 bit value. Negative offsets would look
like moderately large positive offsets.
This change moves the writeback of load multiple instructions to the beginning
of the macroop. That way, the MicroLdrRetUop that changes the mode will
necessarily happen later, ensuring the writeback happens in the original mode.
The actual value in the base register if it also shows up in the register list
is undefined, so it's fine if it gets clobbered by one of the loads. For
stores where the base register is the lowest numbered in the register list,
the original value should be written back. That means stores can't write back
at the beginning, but the mode changing problem doesn't affect them so they
can continue to write back at the end.
Instead of panic immediately when these instructions are executed, an
UndefinedInstruction fault is returned. In FS mode (not currently
implemented), this is the fault that should, to my knowledge, be triggered in
these situations and should be handled using the normal architected
mechanisms. In SE mode, the fault causes a panic when it's invoked that gives
the same information as the instruction did. When/if support for speculative
execution of ARM is supported, this will allow a mispeculated and unrecognized
and/or unimplemented instruction from causing a panic. Only once the
instruction is going to be committed will the fault be invoked, triggering the
panic.
Shifting to the right of a signed value when the MSB is one is technically
undefined behavior, even though in my experience it's done the "right thing"
and sign extended the value. This replaces the arithmetic right shift code in
ARM that uses that coincidence with some code that relies on bit math.
