Page 184 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

6.2 Passing Parameters 161

Before we look at other argument passing techniques using our running inner
product subroutine example, we illustrate a common subroutine used in C and C++
compilers to implement the case statement by means of an in-line argument list of
addresses. Figure 6.31 illustrates how, when a variable between 0 and 3 is in accumulator
B, the program can jump to label LO if the variable is 0, to LI if the variable is 1, to L2
if the variable is 2, and to L3 if the variable is 3. For each case, an address is put in the
in-line argument list; the SWITCH subroutine reads one of these arguments into the PC,
as selected by the value in accumulator B. Note that this technique is more efficient than
a decision tree (Figure 2.14) when the same variable is tested for numbers, which happen
to be consecutive, going to a program segment that the number indicates. However, this
example is neither position independent nor does it use 8-bit in-line arguments to further
improve efficiency. The reader is encouraged to improve this technique to do this.

TABLE: DS 6
PARV: EQU 0
PARW: EQU 2
PARDP: EQU 4

b. Assembler directives

Figure 632. Parameters in a Table

We now consider the technique of passing parameters via a table. The argument list,
which is in-line when parameters are passed after the call, can be a table stored anywhere
in memory. This technique is quite similar to passing parameters after the call. For our
example, suppose that one uses a table whose address is passed in X and that looks like
Figure 6.32, where, as before, the suffixes H and L stand for the high and low bytes of
the 2-byte parameters PARV, PARW, and PARDP. The subroutine shown in Figure 6.33a
is called by a sequence shown in Figure 6.33b.
Passing parameters by a table is often used to control a floppy disk in a way that is
transparent to the user. The number of parameters needed to control a disk can be very
large; therefore the table can serve as a place to keep all the parameters, so only the
address of the table is sent to each subroutine that deals with the floppy disk.
In this section, we considered ways to pass arguments to and from subroutines. The
register technique is best for small subroutines that have just a few arguments. The stack
technique is best for larger subroutines because it is the most general. The in-line
argument list that passes parameters after the call is used in FORTRAN subroutines, and
the table technique is commonly used in operating system subroutines. The technique
that passes parameters in global variables was covered for completeness and is useful in
very simple micrcontrollers such as the 6805, but it is discouraged in the 6812.