Compaq licensed the BIOS from IBM. IBM thought there was no market for portables so they licensed this one time only. Compaq and IBM alone had the Basic A interpreter in BIOS. Since a lot of early PC developers wrote their programs in Basic A, they would run only on IBMs and Compaqs.

I believe it was Phoenix that did the first "clean room" BIOS, but nobody ever produced a "clean room" Basic A.