[p_l]

The historical point was that for Spring OS "fast path" calls, if you kept register stack small enough, you could avoid spilling at all.

Switching from task A to task B to service a "fast path" call AFAIK (have no access to code) involved using WIM register to set windows used by task A to be invalid (so their use would trigger a trap), and changing the ASID value - so if task B was already in TLB you'd avoid flushes, or reduce them only to flushing when running out of TLB slots.

The "golden" target for fast-path calls was calls that would require as little stack as possible, and for common services they might be even kept hot so they would be already in TLB.

[__d]

Is the Spring source available anywhere?

[p_l]

Unfortunately, as far as I know, not. Only published articles and technical reports :(

[__d]

I once had the university CD-ROM release, but I've lost it, sadly.