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X Index Register: Difference between revisions
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The '''X Index Register''' on 65x processors often holds the current index when iterating over things. It can be incremented or decremented by one with [[INX]] or [[DEX]], but there is no instruction to add or subtract more than one. | The '''X Index Register''' on 65x processors often holds the current index when iterating over things. It can be incremented or decremented by one with [[INX]] or [[DEX]], but there is no instruction to add or subtract more than one. Although INX and DEX can affect the [[negative flag]], the indexed [[addressing mode]]s always interpret the bit pattern in the X index register as a non-negative integer. | ||
On the [[S-SMP]] it is always 8 bits wide. It is the divisor for division commands.<sup>[2]</sup> | On the [[S-SMP]] it is always 8 bits wide. It is the divisor for division commands.<sup>[2]</sup> | ||
Revision as of 19:52, 12 August 2024
The X Index Register on 65x processors often holds the current index when iterating over things. It can be incremented or decremented by one with INX or DEX, but there is no instruction to add or subtract more than one. Although INX and DEX can affect the negative flag, the indexed addressing modes always interpret the bit pattern in the X index register as a non-negative integer.
On the S-SMP it is always 8 bits wide. It is the divisor for division commands.[2]
On the 65c816, it may be 8 or 16 bits wide. Indexing may cross bank boundaries.[1]
Unlike the Y index register, the value of the stack pointer can be transferred to/from X with TXS and TSX.
It is not specified to have any particular value after reset.
See Also
References
- https://wilsonminesco.com/816myths
- 8.1.2 of page 3-8-4 of Book I of the official Super Nintendo development manual