Page 180 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

6,2 Passing Parameters 157

* SUBROUTINE DOTPRD - LOCAL VARIABLES
TERM: EQU 0 ; First term of the dot product
NBYTES: EQU 2
* PARAMETERS
PARV: EQU 0 ; Copy of vector V
PARW: EQU 2 ; Copy of vector W
PARDP: EQU 4 ; Dot product of V and W
PSIZE: EQU 6
*
DOTPRD: PULX ; Return address into X
LEAS -NBYTES, SP ; Allocation for local variables
LDAA PARV, X
LDAB PARW,X
MUL
STD TERM, SP ; Copy first term into local variable
LDAA PARV+1,X
LDAB PARW+1,X
MUL
ADDD TERM,SP ; Dot product into D
STD PARDP, X ; Place dot product in out parameter
LEAS NBYTES, SP ; Deallocate local variables
JMP PSIZE,X

Figure 625. A Subroutine with Parameters after the Call, which Pulls the Return

PARV: EQU 0
PARW: EQU 2
PARDP: EQU 4
*
MOVW V, PARV+L, PCR ; Copy of V into parameter list
MOVW W,PARW+L,PCR ; Copy of W into parameter list
BSR DOTPRD
L: DS 6
MOVW PARDP+L, PCR, DTPD ; Copy result into DTPD

Figure 626. A Subroutine Calling Sequence for Figure 6.25

rather than a kludge of special methods that are restricted to limited sizes or applications.
The compiler has less to worry about and is smaller because less code in it is needed to
handle the different cases. This means that many subroutines that you write for high-
level languages such as C may require you to pass arguments by the conventions that it
uses. Moreover, if you want to use a subroutine already written for such a language, it
will pass arguments that way. It is a good idea to understand thoroughly the stack mode
of passing parameters.