HP -28S Manual page 301

Registers Versus Variables
Fixed-stack calculators can deal efficiently only with real, floating-
point numbers for which the fixed, seven-byte register structure of the
stack and numbered data register memory is suitable (the HP-41 in-
troduced a primitive alpha data object constrained to the seven-byte
format). The HP-28S replaces numbered data registers with named
variables. Variables, in addition to having a flexible structure so that
they can accomodate different object types, have names that can help
you remember their contents more readily than can register numbers.
If you want to duplicate numbered registers on the HP-28S, you can
use a vector:
- : : 50 }
£1
COt·l 'I;;:EG' STO
creates a vector with 50 elements initialized to 0;
« 'REG' SWAP GET» 'NRCL' STO
creates a program NRCL that recalls the nth element from the vector,
where n is a number in level 1;
« 'REG' SWAP ROT PUT» 'NSTO' STO
creates the analogous store program NSTO.
300 B: Notes for HP RPN Calculator Users