Data Organization and Addressing Capabilities
2-28 MC68030 USER’S MANUAL MOTOROLA
The memory indirect addressing modes use a long-word pointer in memory to access an
operand. Any of the modes previously described can be used to address the memory
pointer. Because the base and index registers can both be suppressed, the displacement
acts as an absolute address, providing indirect absolute memory addressing (refer to Figure
2–8).
The outer displacement (od) available in the memory indirect modes is added to the pointer
in memory. The syntax for these modes is ([bd,An],Xn,od) and ([bd,An,Xn],od). When the
pointer is the address of a structure in memory and the outer displacement is the offset of
an item in the structure, the memory indirect modes can access the item efficiently (refer to
Figure 2–9).
Memory indirect addressing modes are used with a base displacement in five basic forms:
1. [bd,An] — Indirect, suppressed index register
2. ([bd,An,Xn]) — Preindexed indirect
3. ([bd,An],Xn) — Postindexed indirect
4. ([bd,An,Xn],od) — Preindexed indirect with outer displacement
5. ([bd,An],Xn,od) — Postindexed indirect with outer displacement
The indirect, suppressed index register mode (see Figure 2–10) uses the contents of
register An as an index to the pointer located at the address specified by the displacement.
The actual data item is at the address in the selected pointer.
The preindexed indirect mode (see Figure 2–11) uses the contents of An as an index to the
pointer list structure at the displacement. Register Xn is the index to the pointer, which
contains the address of the data item.
Figure 2-8. Using Indirect Absolute Memory Addressing
POINTER DATA ITEM
bd
SYNTAX: ([bd])
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