A bit of history here: When PERFORM PSEUDORANDOM NUMBER OPERATION was first being developed circa 2012, (a) pseudorandom was hyphenated (improperly ... it's really a single word), and (b) the proposed mnemonic was initially PPRNO, but shortened to PRNO. One of the firmware developers was concerned that — when pronounced aloud — PRNO sounded too suggestive for a conservative mainframe company, so we relented and called it PPNO. (Similar sensitivity training applied to the mnemonic for COMPARE LOGICAL IMMEDIATE AND TRAP and others.)
With the addition of the message-security-assist extension 7, where the instruction can now produce either deterministic or true random numbers, we decided that the risqué mnemonic was more appropriate (and the firmware developer had overcome his initial concerns). So now, we're back to PRNO. Note Well: As suggested by programming note 4.a (p. 7-358) , PRNO-TRNG is not the swiftest operation in the instruction set. Programming notes 4.b. and 4.c suggest periodically using PRNO-TRNG to re-seed a deterministic random number generator (the frequency of such re-seeding based on the durability of the DRNG function).
