The toolchain's disassembler prints the segment-prefixed disp32 absolute as FS:0, but the bare-name branch read the segment register alone and dropped the offset, so MOVQ FS:0, DX silently encoded a register move. The colon-offset form now lowers to the same segment-absolute operand the 0(FS) spelling takes. Assisted-by: GLM 5.3 Flash