Transfer

Transfer data between registers

Tags:

6502 instruction

The transfer register set of instructions allows for data to be passed between different registers.

In all of these transfer instructions, the source register is unchanged.

For example, on the 6502 to save the X register on the stack you would need to use the following:

1TXA      ; Transfer X into A
2PHA      ; Push A to the stack

Note: On the 65C02 and later this is replaced by the PHX instruction which doesn't touch the accumulator.

TAX - Transfer Accumulator to Index Register X

Transfers the Accumulator to X. On the 8-bit processors the registers are all the same size, however on the 16-bit processors the registers can be of different sizes. The following table describes how the data is transferred when a size mismatch occurs:

Source Size	Dest Size	m	x	Action performed
8	8	All types		Value transferred is 8-bit
8	16	1	0	Value transferred is 16-bit. The 8-bit A becomes the low byte of the index register. The 8-bit hidden B register becomes the high byte of the index register.
16	8	0	1	The low byte of A is transferred to the index register
16	16	0	0	The full 16-bit A is transferred to the index register

TAY - Transfer Accumulator to Index Register Y

Transfers the Accumulator to Y. It follows the same rules as TAX.

TCD - Transfer 16-bit accumulator to Direct Page Register

TCD transfers the 16-bit accumulator C to the direct page register D, regardless of the accumulator/memory flag.

The C accumulator is used to indicate that 16-bits are transferred regardless of the m flag. If the A accumulator is 8-bit due to m=1 or in 6502 emulation mode then C = A as the low 8-bits and the hidden B accumulator as the high 8-bits.

TCS - Transfer Accumulator to Stack Pointer

TCS transfers the 16-bit accumulator C to the stack pointer S, regardless of the accumulator/memory flag. The C register is defined above for TCD. An alternate mnemonic for TCS is TAS.

Note: Unlike most transfer instructions, TCS does not affect any flags.

TDC - Transfer Direct Page Register tp 16-bit Accumulator

TDC transfers the Direct Page Register to the 16-bit accumulator C. The C register is defined above for TCD. An alternate mnemonic for TDC is TDA.

TSC - Transfer Stack Pointer to Accumulator

TSC transfers the stack pointer S to the 16-bit accumulator C, regardless of the accumulator/memory flag. The C register is defined above for TCD. An alternate mnemonic for TCS is TSA.

TSX - Transfer Stack Pointer to Index Register X

TSX transfers the stack pointer to X. The stack pointer is unchanged. On 8-bit processors only the low byte is transferred to X. On 16-bit processors (x=0) the full 16-bit value is tranferred to X.

TXA - Transfer Index Register X to Accumulator

TXA transfers X into the accumulator. On the 8-bit processors the registers are all the same size, however on the 16-bit processors the registers can be of different sizes. The following table describes how the data is transferred when a size mismatch occurs:

Source Size	Dest Size	m	x	Action performed
8	8	All types		Value transferred is 8-bit
8	16	1	0	Value transferred is 16-bit. The 8-bit index register becomes the low byte of the accumulator. The high-byte of the accumulator is set to 0.
16	8	0	1	Value transferred is 8-bit. The low 8-bits of the index register becomes the low byte of the accumulator. The high-byte of the hidden accumulator B is not affected by the transfer.
16	16	0	0	The full 16-bit index register is transferred to the accumulator

TXS - Transfer Index Register X to the Stack Pointer

TXS transfers X to the stack pointer to. The X is unchanged. On 8-bit processors only the low byte is transferred to S. On 16-bit processors (x=1) the low 8-bits of X is transferred to S. The high 8-bits of S are zeroed. On 16-bit processors (x=0) the full 16-bit value of X is transferred to S.

Note: Unlike most transfer instructions, TXS does not affect any flags.

TXY - Transfer index register X to Y

TXY transfers X to Y. X is unchanged. The registers are always the same size, so when 8-bit then that's what's transferred. When 16-bit (x=0) then 16-bits are transferred.

TYA - Transfer Index Register Y to Accumulator

TYA transfers Y into the accumulator. It follows the same rules as TXA above.

TYX - Transfer index register Y to X

TYX transfers Y to X. Y is unchanged. The registers are always the same size, so when 8-bit then that's what's transferred. When 16-bit (x=0) then 16-bits are transferred.

XBA - Exchange the B & A accumulators

On the 16-bit processors the 16-bit C accumulator is formed of two 8-bit accumulators, A for the low 8-bits and B for the upper 8-bits. XBA swaps the contents of the A & B registers.

In 8-bit memory mode, B is usually referred to as the hidden B accumulator, so the XBA instruction can be used to swap the accessible A with B, providing an in-processor scratch accumulator rather than pushing a value to the stack.

The flags are based on the value of the 8-bit A accumulator

Flags Affected

Flags

Set if most significant bit of the transferred value is set

Set if value transferred is zero

Instructions

Src	Dest	Syntax	Opcode	Available on:			# of	# of	Addressing Mode	Notes
Src	Dest	Syntax	(hex)	6502	65C02	65816	bytes	cycles	Addressing Mode	Notes
A	X	TAX	AA	x	x	x	1	2	Implied
A	Y	TAY	A8	x	x	x	1	2	Implied
C	D	TCD	5B			x	1	2	Implied
C	S	TCS	1B			x	1	2	Implied	Flags are unaffected
D	C	TDC	7B			x	1	2	Implied
S	C	TSC	3B			x	1	2	Implied
S	X	TSX	BA	x	x	x	1	2	Implied
X	A	TXA	8A	x	x	x	1	2	Implied
X	S	TXS	9A	x	x	x	1	2	Implied	Flags are unaffected
X	Y	TXY	9B			x	1	2	Implied
Y	A	TYA	98	x	x	x	1	2	Implied
Y	X	TYX	BB			x	1	2	Implied
B	A	XBA	EB			x	1	2	Implied	Exchanges both registers, flags based on A post exchange

Last modified November 5, 2021: Add instruction categories (6b74ff9)