Page 164 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

6.1 Local Variables 141

Figure 63. Changing a Global Variable before It Has Been Completely Used

illustrates a program segment using TEMP to store a variable to be recalled later. Before
that value is recalled, however, TEMP has been changed by subroutine B, which is called
by subroutine A, which itself is called by the program segment. This case is difficult to
debug because each subroutine will work correctly when tested individually but will not
work when one is called, either directly or indirectly through other subroutines, from
within the other. This technique also confuses documentation, specifically the meaning
of the local variable TEMP, generally making the program less clear.
With the other technique, the local variables will be put in different memory
locations, having different symbolic names. See Figure 6.5. This approach is superior to
the last approach, because differently named local variables, stored in different locations,
will not interfere with the data stored in other locations. The names can be chosen to
denote their meaning, reducing the need for comments. However, memory is taken up by
these local variables of various program segments, even though they are hardly ever used.
In a single-chip 'A4 or 'B32, only IK bytes of SRAM are available. Using all these
bytes for rarely used local variables leaves less room for the program's truly global data.

TEMP: D S 6 ; Allocate 6 bytes of memory for temporary variables
enter: MOVB#1, TEMP ; Allocate and initialize V( 1)
MOVE #2,TEMP+1 ; Allocate and initialize V(2)
MOVE #3,TEMP+2 ; Allocate and initialize W(l)
MOVE #4,TEMP+3 ; Allocate and initialize W(2)
LDAA TEMP ; V(l) into A
LDAB TEMP+2 ; W(l) into B
MUL ; The value of first term is now in D
STD TEMP+4 ; Store first term in TERM
LDAA TEMP+1 ; V(2) into A
LDAB TEMP+3 ; W(2) into B
MUL ; Calculate second term
ADDD TEMP+4 ; Add in TERM; dot product is now in D

Figure 6.4. Inner Product Utilizing a Global Variable such as TEMP (a Bad Example)