Signature byte

In 65x parlance, a signature byte is the second byte that follows certain instructions, including:

• BRK
• COP
• WDM

Each of these instructions will normally advance the PC by two bytes, even though the hardware does not make any direct use of the second "operand" byte.

For this reason, these instructions have been treated both as one-byte and two-byte instructions in various reference documents and assemblers.

A software response to BRK or COP may use the return address on the stack to deduce the location of the operand byte and inspect it.

• BRK with an explicit operand can be used for error codes, or as a compact system call dispatch, handled by the BRK vector at $FFF6.
• COP can be used in the same way as BRK, but it is handled by the COP vector at $FFF4 instead.

Alternatively, if the signature byte is not needed, a BRK or COP handler may wish to decrement the return address on the stack before RTI, returning as if it were a one-byte instruction.

The COP instruction was intended for use with a co-processor, for which the signature byte could indicate a command to send to the co-processor. However, there is no hardware to support this usage on the SNES, and it is simply a second software interrupt, equivalent to BRK.

The WDM instruction was reserved for future use, but was never actually used. It is simply a 2-byte alternative to NOP. Mesen's debugger provides a break-on-WDM instruction which can make it convenient as an emulator-only breakpoint.

There is no standard for how assemblers treat BRK or COP. If BRK emits only 1 byte, a signature byte can be added manually with a data byte following.

• ca65 in .p816 mode has an optional signature for BRK, allowing either 1 or 2 bytes. COP and WDM always require the signature byte.

Though the 65C816 has no unused opcodes, on the 6502 many were left open with unspecified behaviour. This allowed the use of "unofficial" illegal opcodes, including several NOP variants with an unused signature byte. See: NESDev: CPU unofficial opcodes