How do I interrupt the system boot sequence on SPARC?

As opposed to Solaris Intel system which always prompt during boot-up,
Solaris SPARC systems will boot without asking.

To interrupt booting, use the key sequence "Stop-A" (or L1-A) on
a Sun keyboard, using "Stop" as if it's a modifier (shift/control) key.
On systems with a serial console, send a BREAK; most terminals
and emulators support this.