Yamaha was the most widespread personal computer in the USSR in the mid-80s. It was everywhere: in the schools, in youth creativity centers, in young pioneer houses etc. I think you could not buy it for home at the time or it was too expensive (like a half of a car). There were less expensive Soviet-made computers for home use which were more affordable.
I started programming at 8 years (2nd grade) at a programming group at creativity center. The group was free as anything in the USSR. Besides Yamahas there were also Toshibas there, and other groups were equipped with Soviet computers. My school was also equipped with Yamahas but the informatics subject only started from 6th grade (although as the teacher was informed that I attended a group, she allowed me to program and play games after lessons).
It was a Soviet standard. All computers, both imported and domestically produced were required to follow it.
The IBM PC-compatible computers that emerged in mid-1990s did not respect any Soviet standards.