gives "Missing number" and "Illegal unit" errors, while
\dimen0\tenrm
gives only the first error (assuming \font\tenrm=cmr10\relax). Similarly, \muskip0\relax gives "Missing number" and "Illegal unit", while \musip0\tenrm gives "Missing number" and "Incompatible glue units" errors.
This is due to the fact that \tenrm is "something internal" hence is scanned by the TeX procedure scan_something_internal, which throws a "Missing number" error then inserts the equivalent of \c_zero_dim.
gives "Missing number" and "Illegal unit" errors, while
gives only the first error (assuming
\font\tenrm=cmr10\relax). Similarly,\muskip0\relaxgives "Missing number" and "Illegal unit", while\musip0\tenrmgives "Missing number" and "Incompatible glue units" errors.This is due to the fact that
\tenrmis "something internal" hence is scanned by the TeX procedurescan_something_internal, which throws a "Missing number" error then inserts the equivalent of\c_zero_dim.