Hello there, I was wondering how I could declare a bit in FASM.

You can't. But you can declare a byte with "db" or reserve a byte with "rb". See the manual for exact usage.

Are you looking to declare a bitfield or just a single bit?

Just a single bit.

Since memory is only byte addressable, a single bit would have to be stored in a byte somewhere. Ergo you have to declare at least a byte. If you have multiple bits, you can use the same byte for at most 8 of them, but you must have a byte.

Or just keep it in a register or flag, but that gets a bit more complicated. If you keep it in a flag, you have to avoid intrusions which change that flag (hard to do). If you keep it in a register, you have to essentially dedicate that register to only holding a bit (or be really careful moving things into and out of the register, assuming those things have a predictable bit pattern that allows this to work), which would probably be a net negative to performance.