Page 182 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

6.2 Passing Parameters 159

* 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
*
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
LDY PARV,X
LDAA 1, Y
LDY PARW,X
LDAB 1, Y
MUL
ADDD TERM, SP ; Dot product into D
STD [ PARDP, X ] ; Place dot product in out parameter
LEAS NBYTES, SP ; Deallocate local variables
JMP 0, X
Figure 6.29. A Subroutine With In-Line Parameters that Are Addresses
BSR DOTPRD
DC.W ADDRV ; Address for V
DC.W ADDRW ; Address for W
DC.W ADDRDP ; Address for dot product
Figure 630. An In-Line Argument List of Addresses for Figure 6.29
In the first alternative, the parameters are placed after the BSR or JSR instructions
in what is called an in-line argument list. Looking at Figure 6.24, we see that the return
address, which is pushed onto the hardware stack, points to where the parameters are
located. When parameters are passed this way, sometimes referred to as after the call, the
subroutine has to increment the return address appropriately to jump over the parameter
list. If this is not done, the MPU would, after returning from the subroutine, try to
execute the parameters as though they were instructions. For our dot product example,
assume that the parameter list appears as shown in Figure 6.24. Notice that the
subroutine must skip over the six bytes in the parameter list when it returns to avoid
"executing the parameters." The subroutine shown in Figure 6.25 does this.