That type of code system is normal for civics and crxs from 89-91 in the U.S. My old d16a6 ecu blinks between 1-20 times indicating either a faulty o2(1 blink), to electrical load problem(20 blinks).
I believe more complicated ecus that have more sensors and whatnot to control use the short blink and long blink code system(1 long blink, 2 short blinks-code 12), including ecus that run vtec engines.
Besides that, a code 12 does indicate EGR valve problem. I don't have alot of experience with 90 civic lx engines, but if it does have an EGR valve, it may be unplugged, or may just need to be cleaned. Worst case scenario, you'd have to replace it.