ECU Inputs: ECU Self-Test Mode Selector
It turns out that the ECU can test itself. That is not mentioned anywhere that I can find. The tests are pretty rudimentary though, and that might be why.
The PH7 signal is an input that puts the ECU into self-test mode. There is no input circuitry for this pin. It is just a test point on the circuit board used during manufacturing test. The ECU test fixture will simply drive this pin low to put the ECU into self-test mode.
Once in self-test mode, the CPU performs an EPROM checksum. This makes sure that the EPROM data is exactly what is supposed to be in present. If this is the case, the processor will drive Port H1 to '1'. If the test fails, PH1 will be driven to '0'.
When the EPROM test is completed, the processor checks its RAM memory by writing it with two different patterns, then verifying that the RAM contains the appropriate patterns. It is not a very thorough method of testing RAM, but it will find gross failures. If the RAM test passes, PH3 is driven to a '1'. RAM failures drive PH3 to '0'.
These RAM and EPROM tests are only performed in self-test mode. You may find it interesting that under normal operation, the ECU does not bother to check its memory. Skipping these tests would allow the ECU to boot up (i.e. start operating) much faster, so it probably makes sense. But the more important consideration would be this: what would you want the ECU to do if it actually did find a RAM failure? Should it flash the EFI light & refuse to run leaving you stranded at the side of the road somewhere? That would suck, especially if it was a RAM bit that was not even used by the program, or if it was only used during some really weird operational state. It is in your best interest of getting home again if the ECU just crosses its fingers and assumes that things are good.