This is a conditional GoTo instruction. It compares the contents of X with the first argument nn and does a GoTo program line LLLL if X=nn. X and nn are in the number range 0-255.
Also pops the stack, so the original content of X is lost.
Dialect restriction: This instruction is not implemented on boards prior to dialect D=7
See also GoIfXNE, GoIfXGT, GoIfXGE, GoIfXLT, GoIfXLE, GoIfZ, GoIfNZ