Page 181 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

158 Chapter 6 Assembly Language Subroutines

* SUBROUTINE DOTPRD
* LOCAL VARIABLES
TERM: EQU 0 ; First term of the dot product
MBYTES: 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
*
DOTPRD: LDX 0, SP ; Return address into X
LEAS -NBYTES, SP ; Allocation for local variables
LDAA PARV+2,X
LDAB PARW+2,X
MUL
STD TERM, SP ; Copy first terra into local variable
LDAA PARV+1+2,X
LDAB PARW+1+2,X
MUL
ADDD TERM,SP ; Dot product into D
STD PARDP+2, X ; Place dot product in out parameter
LEAS NBYTES, SP ; Deallocate local variables
RTS

Figure 6.27. A Subroutine with Parameters after the Call, which Uses RTS

entry: MOVW V, PARV+L, PCR ; Copy of V into parameter list
MOW W,PARW+L,PCR ; Copy of W into parameter list
BSR DOTPRD
BRA LI
*
L: DS 6
*
L1: MOVW PARDP+L, PCR, DTPD ; Copy parameter list into DTPD

Figure 6.28. A Subroutine Call with Parameters after the Call for Figure 6.27

We now consider another common method of passing arguments in which they are
put after the BSR or equivalent instruction. This way, they look rather like addresses that
are put in the instruction just after the op code. Two variations of this technique are
discussed below.