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Index Register Select

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Index Register Select (X) is a flag in the processor status register (bit 4) of the 65c816. It indicates how wide the index registers are:

  • When clear, both index registers are 16 bits wide. It can only be clear in native mode.
  • When set or in emulation mode, both index registers are 8 bits wide.

It is not possible to control the width of the two index registers individually.

It can be affected by:

  • REP (clears it if bit 4 of operand is set)
  • SEP (sets it if bit 4 of operand is set)
  • PLP (pops it off the stack)
  • RTI

It affects the behavior of (possibly incomplete list):

In emulation mode, the x flag becomes the break flag.

There are no BXS or BXC instructions that examine this flag.

See Also

Reference