Page 175 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

152 Chapter 6 Assembly Language Subroutines

Figure 6.16. Change a Global Parameter before Its Subroutine Has Used It

* SUBROUTINE DOTPD - LOCAL VARIABLES
TERM: EQU 0 ; First term of the dot product
MBYTES: EQU 2
*
DOTPRD: LEAS -MBYTES, SP ; Allocate local variables
LDAA V1 ; First component of V into A
LDAB W1 ; First component of W into B
MUL ; First term of dot product into D
STD TERM, SP ; Save first term
LDAA V2 ; Second component of V into A
LDAB W2 ; Second component of W into B
MUL ; Second term of dot product into D
ADDD 2,SP+ ; Dot product into D, Deallocate loc var
STD DTPD ; Place dot product
RTS

Figure 6.17. A Subroutine with Parameters in Global Variables

In assembly language, global variables are defined through a DS directive that is
usually written at the beginning of the program. These variables are often stored on page
zero on smaller microcontrollers so that direct page addressing may be used to access
them. However in the 6812, page zero is used for I/O ports. Assuming that the directives
are written somewhere in the program, the subroutine in Figure 6.17 does the previous
calculation, passing the parameters through these locations. Note that we use local
variables in this subroutine, as discussed in Section 6.1.
The subroutine in Figure 6.17 uses global variables VI, V2, Wl, W2, and DTPD to
pass parameters to the subroutine and from it. If the calling routine wants to compute the
dot product of its local variables LV and LW, which each store a pair of 2-element 1-byte
vectors, putting the result in LDP, the calling sequence in Figure 6.18 could be used.
Notice that the calling routine's local variables are copied into global variables VI, V2,
Wl, and W2 before execution and copied out of the global variable DTPD after execution.
Any other calling sequence for this version of DOTPRD must also copy the vectors of