WDM Reserved for future expansion

Do not use this instruction. It will break if/when a future processor is released with additional instructions.

The 65802 & 654816 processors use 255 out of the possible 256 8-bit opcodes. The remaining opcode is this one, labeled WDM which happens to be the initials of William D. Mensch who designed the processors.

To allow additional instructions to be added later this instruction act's as a prefix allowing an additonal 256 opcodes. This is a similar technique to the Z80 & 8080 processors which have 2-byte extension opcodes.

The actual number of bytes and cycles involved will be depended on those extensions, however the byte size will be a minimum of 2 bytes.

On the 65802 & 65816 this instruction will execute as a 2-byte NOP.

SyntaxOpcode Available on: # of # of Addressing Mode
(hex) 6502 65C02 65816 bytes cycles
WDM 42 x 21 ?1 Implied


  1. byte & cycle count subject to change in future processors