Solaris 7 is 64 bit, does this mean I can no longer use my old SPARCs?

No, a 32 bit kernel is still supported on sun4c, sun4m, sun4d and even
sun4u hardware using UltraSPARC I, II or IIi processors.

The 32 bit kernel is not supported on systems with UltraSPARC IIe and
UltraSPARC III processors or newer (basically, all SPARC hardware that
is only supported by Solaris 8 or later)

Support for the 32 bit SPARC kernels is removed from Solaris 10;
Solaris 10 only supports systems with UltraSPARC II or later.

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