Page 171 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

148 Chapter 6 Assembly Language Subroutines

was used for the local variables of the outer program segment. It is always in a known
position on the stack (in this case, on the very top of the stack), so it is easy to find. See
Figure 6.14, where the inner program segment can access the local variables of the outer
segment by loading the stack marker into any index register and using index addressing to
get the variable. Note that the stack marker is deallocated together with the other stacked
local variables at the end of the program segment.

MARK A: EQU 0 ; Stack mark for segment A
W: EQU 2 ; Input vector
WW: EQU 4 ; Input vector
SlZEAi EQU 6
*
START A: TFR SP,X ; Start for segment A
LEAS -SIZEA,SP
STX MARKA,SP
MOVW #$102,W,SP ; Initialize both bytes of VV
MOW #$304,WW,SP ; Initialize both by tes of WW
*
MARKB : EQU 0 ; Stack mark for segment B
TERM: EQU 2
SIZEB: EQU 4
*
STARTS: TFR SP,X
LEAS -SIZEB,SP
STX MARKB,SP
LDAA VV,X ;V(l)intoA
LDAB WW,X ;W(l)intoB
MUL ; First term is now in D
STD TERM, SP ; Store first term in TERM
LDAA W+1,X ;V(2)intoA
LDAB WW+1,X ;W(2)intoB
MUL ; Calculate second term
ADDD TERM,SP ; Add in TERM; dot product in D
ENDB: LEAS SIZEB ,SP ; End of segment B
*
ENDA: LEAS SIZEA,SP ; End of segment A
Figure 6.14. Accessing Stacked Local Variables Using a Stack Marker

Either the extended local access or the stack marker access mechanisms can be used
in cases where program segments are further nested. Consider program segment C, with
SIZEC stacked local variables, which is nested in segment B and needs to load
accumulator A with the value of SA, a stacked local variable of segment A. Using
extended local access, as in the first example, the following instruction will accomplish
the access.

LDAA SIZEC+SIZEB+SA,SP