It seems it's an invalid encoding if I understand the article correctly. There seems to be some debugging utility in such a mechanism, e.g. you could use it to

It seems it's an invalid encoding if I understand the article correctly. There seems to be some debugging utility in such a mechanism, e.g. you could use it to run CPU tests, benchmarks or debug code in userspace of a stock OS and then directly communicate with serial port MMIO without needing to pollute the CPU state with a system call or change the kernel to directly map the MMIO into userspace.