TXA Instruction (Transfer Index Register X to Accumulator)

Transfers the value of index register X to the accumulator.

TXA Overload

Opcode	Syntax	Addressing mode	Byte count	Execution cycles	Note
8A	TXA	Imp.	1	2

Status flag changes

n-----z-

n: Receives the MSB of the transferred value.
z: Set if the transferred value is 0; cleared otherwise.

Explanation

The TXA instruction transfers the value of index register X to the accumulator.
Index register X is not cleared, and the value that was in the accumulator before TXA executed is discarded.

For register-to-register transfer instructions, the number of bytes transferred changes according to the status flags as follows.

Source	Destination	Status flags	Transfer width	Note
8-bit accumulator	8-bit index register	m = 1, x = 1	8-bit
8-bit accumulator	16-bit index register	m = 1, x = 0	16-bit	1
16-bit accumulator	8-bit index register	m = 0, x = 1	8-bit	2
16-bit accumulator	16-bit index register	m = 0, x = 0	16-bit
8-bit index register	16-bit accumulator	m = 0, x = 1	16-bit	3
16-bit index register	8-bit accumulator	m = 1, x = 0	8-bit	4
16-bit stack pointer	8-bit index register	x = 1	8-bit	5
8-bit index register	16-bit stack pointer	x = 1	16-bit	6

Note

The unused high byte of the accumulator is transferred to the high byte of the index register.
Only the low byte of the accumulator is transferred to the index register.
16 bits are transferred, but the high byte of the accumulator is always set to 0.
Only the low byte of the index register is transferred; the unused high byte of the accumulator is not changed.
Only the low byte of the stack pointer is transferred.
16 bits are transferred, but the high byte of the stack pointer is always set to 0.

Examples

Doing arithmetic on an index

    TXA             ; move the index into A to do arithmetic
    CLC
    ADC.w #$0010
    TAX

When X is 16-bit and A is 8-bit, only the low byte is transferred and the high byte of A is left as is.