Size for immediate operands depends on the value of the M flag
(for register A) or the X flag (for registers X and Y). A register is
8-bit when its respective flag is set and 16-bit when clear.
These flags can be set or unset independently, so this still isn't quite right
for when one flag is set and the other isn't, but it's an improvement. It's
possible to force a particular instruction to decode correctly by using the
`afh` command.
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