Store Index Register X to Memory

Stores the index register X into memory.

On all processors, the data written to memory is 8-bit. However, for 16-bit processors with the m flag is clear then the data written is 16-bit with the low-order 8-bits at the effective address and the high-order 8-bits at the effective address plus one.

Flags Affected
SyntaxOpcode Available on: # of # of Addressing Mode
(hex) 6502 65C02 65816 bytes cycles
STX addr8E x x x 3 41 Absolute
STX dp86 x x x 2 31, 2 Direct Page
STX dp,Y96 x x x 2 41, 2 Direct Page Indexed Y


  1. 65816: Add 1 cycle if m=0 (16-bit memory/accumulator)
  2. 65816: Add 1 cycle if low byte of Direct Page register is not 0

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