User`s manual

Dynamic C User’s Manual digi.com 179

11.4.1.1 Stack Frame Diagram

Figure 11.1 shows the general appearance of a stack frame.

Figure 11.1 Assembly Code Stack Frame

The return address is always necessary. The presence of auto variables depends on the function definition.

The presence of arguments and structure return space depends on the function call. (The stack pointer may

actually point lower than the indicated mark temporarily because of temporary information pushed on the

stack.)

The shaded area in the stack frame is the stack storage allocated for auto variables. The assembler sym-

bol @SP represents the size of this area.

11.4.1.2 The Frame Reference Point

The frame reference point is a location in the stack frame that immediately follows the function’s return

address. The IX register may be used as a pointer to this location by putting the keyword useix before

the function, or the request can be specified globally by the compiler directive #useix. The default is

#nouseix. If the IX register is used as a frame reference pointer, its previous value is pushed on the

stack after the function’s return address. The frame reference point moves to encompass the saved IX

value.

Lower Addresses

Higher Addresses

(stack grows down)

Optional

Stack Frame

Frame Reference

Point

Last Auto Variable

First Auto Variable

IX Register

Return Address

First Parameter

(pushed last)

Last Parameter

(pushed first)

Structure Return

Space