Index Register Select

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):

• LDX
• LDY
• STX
• STY
• CPX
• CPY
• PHX
• PHY
• PLX
• PLY
• INX
• INY
• DEX
• DEY
• TXY
• TYX
• TXS
• TSX

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

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

See Also

• Memory/Accumulator Select
• X Index Register
• Y Index Register

Reference

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