Page 185 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

162 Chapter 6 Assembly Language Subroutines

* SUBROUTINE DOTPRD
* PARAMETERS
*
PARV: EQU 0 ; Copy of vector V
PARW: EQU 2 ; Copy of vector W
PARDP: EQU 4 ; Dot product of V and W
*
* LOCAL VARIABLES
*
TERM: EQU 0
MBYTES: EQU 2
DOTPRD: LEAS -NBYTES, SP ; Allocation for local variables
LDAA PARV,X
LDAB PARW,X
MUL ; First term of DP into D
STD TERM, SP ; Store in local variable
LDAA PARV+1,X
LDAB PARW+1,X
MUL ; Second term into D
ADDD TERM,SP
STD PARDP, X ; Place dot product
LEAS NBYTES, SP ; Deallocate local variables
RTS
a. The subroutine

LDX fTABLE
MOVW V, PARV, X ; Place copy of V into parameter
MOVW W, PARW, X ; Place copy of W into parameter
BSR DOTPRD ; Call Subroutine
MOVW PARDP, X, DTPD ; Copy result into global variable
b. Calling sequence

Figure 633. Calling Sequence for Passing Arguments in a Table

6.3 Passing Arguments by Value, Reference, and Name

Computer science students, as opposed to electrical engineering students, study the
passing of parameters in high-level language subroutines on a different level than that
used in the preceding section. We include this section to explain that level to you. On
the one hand, this level is very important if, say, you are writing or using a subroutine
that is used by a high-level language program and that subroutine has to conform to the
properties discussed below. On the other hand, the differentiation between some of the
characteristics discussed below is rather blurry in assembly language programs.