*SUBROUTINE DOTPRD

*PARAMETERS

*

a. Thesubroutine

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.

The extended local access of stacked local variables can be used with this technique. Consider the body of the subroutine after the PSHC instruction and before the PULC instruction as a program segment. The local variables are allocated just after the PSHC instruction that saves registers and are deallocated just before the PULC instruction that returns to the calling routine. You now look at the saved register values as stacked local variables of an outer program segment that includes the PSHD, PSHC, PULC, and PULD instruction. Using extended local access, you can read these registers and write into them, too. This allows you to output data in a register, even if the data are saved.

Figure 637. A Subroutine with Multiple Entry Points

As an example of this, look at the preceding example again, now using saved registers as stacked local variables of an outer program segment. See Figure 6.36. This idea was expanded earlier to cover the passing of arguments on the stack. The basic idea is that you just have to know where the data are, relative to the current stack pointer SP, in order to access the data. Thus, access to the saved registers, the caller's stacked local