HP HP-32S - Program Control with (I)

Program Control With (i)
Since the contents of i can change each time a program runs—or even
in different parts ofthe same program—a program instruction such as
GTO <i > can branch to a different label at different times. This main
tains flexibility by leaving open (until the program runs) exactly
which variable or program label willbe needed. (See the first example
below.)
Indirect addressing is very useful for counting and controlling loops.
The variable i serves as an index, holding the address of the variable
that contains the loop-control numberfor the functions DSE and ISG.
(See the second example below.)
Example: Choosing Subroutines With (f). The 'Curve Fitting' pro
gram on page 204 in part 4 uses indirect addressing to determine
which model to use to compute estimated values for x and y. (Differ
ent subroutines compute xand y for the different models.) Notice that
i is stored and then indirectly addressed in
widely separated parts of
the program.
The first four routines (S, L, E, P) of the program specify the curve-
fittingmodelthat willbe usedand assign a number (1, 2,3, 4)to each
of these models. This number is then stored during routine Z, the
common entry point for all models:
Z83 STO i
Routine Y uses i to call the appropriate subroutine (by model) to cal
culate the x- and y-estimates. line Y03 calls the subroutine to
compute y:
Y03 XEGKi)
and line Y08 calls a different subroutine to compute x after i has been
increased by 6:
Y86 6
Y07 ST0+ i
Y08 XEGKi)
6: Programming Techniques 105