Файл: Introduction to Microcontrollers. Architecture, Programming, and Interfacing of the Motorola 68HC12 (G.J. Lipovski, 1999).pdf

which it wants to compute the dot product, call the subroutine,and get the dot product result from DTPD. Note also that

ADDD 2 ,SP+ ; Dot product into D, also deallocate local variable

rendered the last LEAS instruction of the subroutine unnecessary.

Figure 6.19. Calling a Subroutine with Parameters on the Stack for Figure 6.2]

We now consider a very general and powerful method of passing parameters on the stack. We illustrate the main idea, interpreting it as another use of local variables, as well as the technique that makes and erases "holes" in the stack, and we consider variations of this technique that are useful for very small computers and for larger microcontrollers like the 68332.

Input and output parameters can be passed as if they were local variables of the program segment that consists of the calling sequence that allocates and initializes. The local variables are allocated and initialized around the subroutine call. In this mode the parameters are put on the stack before the BSR or JSR. For our particular dot product example, the calling sequence might look like Figure 6.19.

For simplicity, we have assumed that input parameter values come from global variables V and W, and the output parameter is placed in the global variable DTPD.All of these global variables could, however, just as well have been local variables of the calling routine. The idea is exactly the same. The stack is as shown in Figure 6.20 as execution progresses. The dot product subroutine is now as shown in Figure 6.21.

*SUBROUTINE DOTPRD

*LOCAL VARIABLES

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

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.