Page 178 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

6.2 Passing Parameters 355

* SUBROUTINE DOTPRD - LOCAL VARIABLES
TERM: EQU 0 ; First term of the dot product
MBYTES: EQU 2
* PARAMETERS
LOCV: EQU 0
LOCW: EQU 2
LOCDP: EQU 4
PSIZE: EQU 6
*
DOTPRD: LEAS -MBYTES ,SP ; Allocation for local variables
LDAA LOCV+NBTTTES+2, SP
LDAB LOCW+NBYTES+2,SP
MUL
STD TERM, SP ; First term to local variables
LDAA LOCV+NBYTES+2+1,SP
LDAB LOCW+NBYTES+2+l,SP
MUL
ADDD TERM, SP ; Dot product into D
STD LOCDP+NBYTES+2, SP ; Dot product to output parameter
LEAS MBYTES, SP ; Deallocate local variables
RTS
Figure 6,22. Revised Subroutine with Local Variables and Parameters on the Stack
LEAS -PSIZE, SP ; Allocate space for parameters
MOVW V, LOCV, SP ; Put copy in parameter location
MOVW W, LOCW, SP ; Put copy in parameter location
BSR DOTPRD
MOVW LOCDP,SP,DTPD ; Put in global location
LEAS PSIZE, SP ; Deallocate space for parameters
Figure 6.23. Calling a Subroutine with Parameters on the Stack for Figure 6.22
Notice several things about the way this version of the subroutine is written. We do
not need the local variables to hold copies of vectors V and W as we did in the earlier
versions because the copies are already on the stack as parameters where we can access
them using the extended local access technique described in Section 6.1. Because the
number of parameters and local variables is small and because each is equal to two bytes,
we can easily calculate the stack offsets ourselves, particularly if we use a dummy
parameter RETN for the return address. Notice particularly how we have redefined the
labels LOCV, LOCW, and LOCDP in the subroutine with the EQU directive to avoid
adding an additional offset of 4 to each parameter to account for the number of bytes in
the return address and the local variables. Suppose now that we write the subroutine as
shown in Figure 6.22. When EQU is used in this way, the additional offset of
MBYTES-1-2 is needed to access the parameters to account for the local variables and the
return address. No EQU directives are needed in the calling sequence, however, because
EQU is a global definition; that is, the labels LOCV, LOCW, LOCDP, and PSIZE are
fixed, respectively, at 0, 2, 4, and 6 throughout the program. The calling sequence for
this case is shown in Figure 6.23.