Editing TOPS-20 (section)

==PA1050==
Some of what came with TOPS-20 was merely an emulation of the [[TOPS-10]] Operating System's calls. These were known as UUO's, standing for Unimplemented User Operation,<ref>{{cite web |url=http://www.abbreviations.com/term/223192 |title=What does UUO mean in Software?
 |website=abbreviations.com}}</ref> and were needed both for compilers, which were not 20-specific, to run, as well as user-programs written in these languages. The package that was mapped into a user's address space was named PA1050: PA as in PAT as in compatibility; 10 as in DEC or PDP 10; 50 as in a PDP 10 Model 50, 10/50, 1050.<ref name=JSYS104>The 10/50 was the top-of-the-line KA machine at that time. {{cite web |url=http://tenex.opost.com/hbook.html
|title=Origins and Development of TOPS-20 |author=Dan Murphy  |date=1989}} The family continued with another KA, the 10/55, and then came KI, KL & KS.</ref>

Sometimes PA1050 was referred to as PAT, a name that was a good fit to the fact that PA1050, "was simply unprivileged user-mode code" that "performed the requested action, using JSYS calls where necessary."<ref name=JSYS104 />