Hello Compgeek,
COM1 is a standard communications port that is usually associate with external posts such as a the serial port. Most computers, these days, do not ship with the external port. However, the system's BIOS may contain the settings for the COM port. This would cause the failure you are seeing when running the motherboard test.
More about the registers test:
Description: Verifies the serial port can accurately send and receive data internally
Detailed Description" Performs an internal loop back test on the serial port register and verifies that all registers contain valid values. The first port found determines the base address, the serial port registers are initialized, and the port is
reset. The test then reads the registers and determines if they are set to invalid values. If they are, it records an error message listing the port number in the test log and the final test result is Failed.