This also adds tests for those functions. We have, furthermore, removed
some redundant (and non-externed!) functions from apple2.mem.c which was
the old bank switching code that got moved to apple2.bank.c.
Previously, we just used whatever the segment was that got passed in.
But aside from switching the stack and zero page, ALTZP also changes
where bank-switchable memory is read from and written to; it's either
always aux memory (ALTZP is on) or always main memory (ALTZP is off).
There is one failing test, which I expect to fail at this point; that's
because we need to write a read/write map function for the stack and
zero page so that they use aux memory when BANK_ALTZP is on, and main
memory when not.
To allow this to work, we had to allow the CPU struct to record what the
last opcode/operand/address were, although in truth we only needed the
last address.
Both main and auxiliary memory need to keep an extra 4k of memory that
is bank-switchable, so we have changed to store that memory literally
within the main and aux segments.
The execute function should just work from the PC register. It might
seem to be easier to test by passing an arbitrary opcode into the
function, but because so much of the chip's execution is
context-sensitive (that is, it expects PC to be pointing at the opcode,
to have its operand in front of it, etc.), passing an arbitrary opcode
is not really reflective of what needs to be in place for the function
to work correctly.
It's really wrong, because we _should_ be storing PC + 2 in the stack.
And software's definitely gonna bork when we try emulating them, because
they will invariably inspect the stack and assume that's what we have in
there.
But the proper fix is to not actually do next_byte(), and to never
advance PC outside of the execute function. But that's a bigger change
than I want to do at this specific moment.
So, in the meantime, you have me prattling on in a commit message. Lucky
you!
