[stella] Question about NOP abs...byte $0C

[stella at casperkitty.com]

>>
Has anyone used this instruction on the VCS? It's marked as undocumeneted
on http://www.oxyron.de/html/opcodes02.html but it's not flagged as
unpredictable. I think Thomas used it in Thrust (correct me if I'm wrong).
Has anyone else used this or noticed any side effects?
<<

The only side-effect is that it will perform a memory access to the
specified address.  In cases where this is deliberate it will, of course,
be desirable.  In most cases where it's not deliberate it will be harmless.
There are some important exceptions, though.

For example, on the SuperCharger:

Entrypoint1:
  LDA #$00
  byte $0C
Entrypoint2:
  lda #$10

may have some very nasty side-effects if run from Entrypoint1 (the same
side effects would occur if using the BIT instruction as is common).  Using
the BIT-skip (or NOP-skip) approach is safe on F8/F6/F4 bankswitch carts,
since there are no two-byte opcodes in the range $F4-$FB.  A Parker
Brothers cart could have trouble skipping over something like "SBC #$1F"
but that's probably not a terribly common case.  A 3F cart could get
tripped up by a few more things (e.g. "AND #$20") but they're probably not
too common.

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