STX & STY – Store the X and Y Registers

After learning about STA, the instructions STX and STY follow naturally. They allow your program to store the contents of the X and Y index registers into memory.

If you’re managing data structures, looping through arrays, or setting up pointers, you’ll frequently want to move index values into memory — and that’s where these instructions come in.

What Do STX and STY Do?

Both instructions work exactly like STA, but they store the contents of the X or Y register instead of the accumulator.

They are purely output instructions — they move a value from a register into a given memory location.

Supported Addressing Modes

Instruction	Addressing Mode	Example	Opcode	Bytes	Cycles
STX	Zero Page	`STX $00`	`$86`	2	3
	Zero Page, Y	`STX $10,Y`	`$96`	2	4
	Absolute	`STX $1234`	`$8E`	3	4
STY	Zero Page	`STY $00`	`$84`	2	3
	Zero Page, X	`STY $10,X`	`$94`	2	4
	Absolute	`STY $1234`	`$8C`	3	4

💡 Notice:

STX supports Zero Page, Y

STY supports Zero Page, X

Neither supports absolute,X/Y or indirect modes

And like STA, no immediate mode

Flags Affected

None.
Just like STA, these instructions do not modify any status flags.

Example Usage

LDX #$FF
STX $D020      ; Set C64 border color using X register

LDY #$12
STY $C0        ; Store Y into zero page memory

LDX #$33
STX $0200,Y    ; Not valid! STX does not support (abs),Y

Indexed zero page example:

LDY #$02
LDX #$A0
STX $10,Y      ; Store X at address $12

Common Pitfalls

Misunderstanding addressing support.
STX and STY are more limited than STA. If you need flexible addressing, move the value into A and use STA.
Trying to use STX/STY with absolute,X/Y.
These modes are not valid with STX or STY.

Summary

STX and STY are crucial for saving index values to memory:

Store the contents of the X or Y register
Support zero page, indexed zero page, and absolute addressing
Don’t affect CPU flags
Great for setting up buffers, counters, and memory-mapped I/O

Combined with STA, these instructions give you full control over writing all main registers to memory.

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