Short answers, pulled from the story.
COMPASS stands for COMPrehensive ASSembler and served the 3000 series mainframes as well as the massive 60-bit CDC 6000 series machines. It also supported the 7600 and Cyber 70 and 170 series computers that defined the company's output in the 1970s.
The Central Processor hardware maintained twenty-four operational registers named A0 to A7, X0 to X7, and B0 to B7. Registers X0 through X7 measured sixty bits long while registers B0 to B7 measured eighteen bits.
Ralph Grishman published Assembly Language Programming for the Control Data 6000 Series through Algorithmics Press in 1972. This text documented the specific techniques used by engineers working with these complex systems.
Specific documentation exists for the CDC3100, 3200, 3300, and 3500 machines that utilized this language. A dedicated section covers COMPASS for the CDC3600 48-bit system which required different handling.