Comment by FarmerPotato

Essentially, the B00 register is a Top Of (Return) Stack or TOS register. It’s great for leaf routines.

You have to push it to your preferred stack before the next operation. You do the cycle-counting to decide if it’s a good ISA for your implementation, or not.

Obviously, ISAs with a JSR that pushes to stack are always using an extra ALU cycle for the SP math, then a memory write.

Doing it with a (maybe costless) register transfer followed by (only sometimes) a stack PUSH can work out to the same number of cycles.

With improvements in memory speed or CPU speed, that decision can flip.

Consider that in this era, your ALU also had the job of incrementing the PC during an idle pipeline stage (maybe the instruction decode). Doing a SP increment for a PUSH might compete with that, so separating the two might make the pipeline more uniform. I don’t know any of the Cray ISAs so this is just a guess.