TAX                TAX Transfer accumulator to index X                TAX

  Operation:  A -> X                                    N V - B D I Z C
                                                        / . . . . . / .

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Implied       |   TAX                 |   $AA   |    1    |    2     |
  +----------------+-----------------------+---------+---------+----------+

What it does: Transfers the byte in the Accumulator to the X Register.

Major uses: Sometimes you can copy the byte in the Accumulator into the X Register as a way of briefly storing the byte until it's needed again by the Accumulator. If X is currently unused, TAX is a convenient alternative to PHA (another temporary storage method).

However, since X is often employed as a loop counter, TAX is a relatively rarely used instruction.