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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
- Eyes & Lichty, page 422, Table 18.2. 65x Flags.