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STY (short for "STore Y") is the mnemonic for a machine language instruction which stores a copy of the byte held in the Y index register at the RAM or I/O address specified. The contents of the Y index register itself remains unchanged through the operation.

Addressing modes[edit | edit source]

Opcode Addressing
in bytes
Number of
Dec Hex
140 8C Absolute STY nnnn 3 4
132 84 Zeropage STY nn 2 3
148 94 Zeropage,X STY nn,X 2 4

STY 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.

CPU flags[edit | edit source]

STY does not affect any of the CPU's status flags.