STX (short for "STore X") is the mnemonic for a machine language instruction which stores a copy of the byte held in the X index register at the RAM or I/O address specified. The contents of the X index register itself remains unchanged through the operation.
STX supports 3 addressing modes, as shown in the table at right. In the assembler formats listed, nn represents a single-byte (8-bit) figure, and nnnn is a two-byte (16-bit) address.
STX does not affect any of the CPU's status flags.