I wrote <snip> For z/OS, this question relates to each work unit and the question is "does the work unit use FPRs 8-15 or VRs?". If the answer is no, then that work unit does not save/restore those regs upon undispatch/redispatch and thus saves some cycles. </snip>
"FPRs 8-15" is incomplete/inaccurate within that. It should be "FPRs 1,3,5,7-15 or the floating point control register (FPCR)". Historically, only FPRs 0,2,4,6 existed and were saved/restored upon undispatch/redispatch. Maybe Shmuel would know even the "pre-history" as to whether FPRs 1,3,5,7 ever existed. The addition of the "additional floating point registers" (1,3,5,7-15 and the FPCR) came with enablement by control register bit, with a program interrupt upon usage without that control register bit (upon which program interrupt, in many cases, the system would automatically activate saving/restoring of the additional FPRs). Peter Relson z/OS Core Technology Design
