Does the old ROM have no limit? seems strange. I also remember the limit
being 2G and 4G depending on the system.
The limit varies depending on the machine. From the sparc/boot man page:
On sun4 and early PROM version sun4c machines, the PROM can only boot
from the first 1Gb of the disk.
On later PROM version sun4c and early PROM version sun4m machines, the
PROM can only boot from the first 2Gb of the disk.
On later PROM version sun4m machines, the PROM can only boot from the
first 4Gb of the disk.