Page 173 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

150 Chapter 6 Assembly Language Subroutines

Before we begin, however, we reiterate that these techniques are quite similar to
those used in Section 6.1 to store local variables. However, these techniques are used
between subroutines, while the latter were used entirely within a subroutine.

* SUBROUTINE DOT PRODUCT
DOTPRD: MUL ; First term is now in D
EXG D, Y ; Store first term in Y, get W(2) in B
EXG A,X ;V(2)intoA
MUL ; Calculate second term
LEAY D, Y ; Add terms, to get result in Y
RTS ; Return to the calling program

a. A subroutine

LD AA # 2 ; Copy of V( 1) into A
LDX # 7 ; Copy of V(2) into low byte of X
LDAB # 6 ; Copy of W( 1) into B
LDY # 3 ; Copy of W(2) into low byte of Y
BSR DOTPRD ; Call the subroutine
STY DTPD ; Store dot product in DTPD

b. A calling sequence
LDAA LV, SP ; Copy of V(l) into A
LDX LW-1, S P ; Copy of V(2) into low byte of X
LDAB LV+1,SP ; Copy of W(l) into B
LDY LW, S P ; Copy of W(2) into low byte of Y
BSR DOTPRD ; Call the subroutine
STY DTPD ; Store dot product in DTPD

c. Another calling sequence

Figure 6.15. A Subroutine with Parameters in Registers

In this section we examine six methods used to pass parameters to a subroutine. We
illustrate each method with the dot product from Section 6.1. We first consider the
simplest method, which is to pass parameters in registers as we did in our earlier
examples. Then the passing of parameters by global variables is discussed and
discouraged. We then consider passing parameters on the stack and after the call, which
are the most common methods used by high-level languages. We then discuss the
technique of passing parameters using a table, which is widely used in operating system
subroutines.