Motorola MC68030 User Manual page 72

III
2-26
For both the MC68020 and the MC68030, the register indirect modes can be
extended further. Since displacements can be 32 bits wide, they can represent
absolute addresses or the results of expressions that contain absolute ad-
dresses. This allows the general register indirect form to be (bd,Rn) or
(bd,An,Rn) when the base register is not suppressed. Thus, an absolute ad-
dress can be directly indexed by one or two registers (refer to Figure 2-6).
SYNTAX: (bd,An,Rn)
bd
An
Rn
Figure 2-6. Using Absolute Address with Indexes
Scaling provides an optional shifting of the value in an index register to the
left by zero, one, two, or three bits before using it in the effective address
calculation (the actual value in the index register remains unchanged). This
is equivalent to multiplying the register by one, two, four, or eight for direct
subscripting into an array of elements of corresponding size using an arith-
metic value residing in any of the 16 general registers. Scaling does not add
to the effective address calculation time. However, when combined with the
appropriate derived modes, it produces additional capabilities. Arrayed struc-
tures can be addressed absolutely and then subscripted, (bd,Rn*scale), for
example. Optionally, an address register that contains a dynamic displace-
ment can be included in the address calculation (bd,An,Rn*scale). Another
variation that can be derived is (An,Rn*scale). In the first case, the array
address is the sum of the contents of a register and a displacement, as shown
in Figure 2-7. In the second example, An contains the address of an array
and Rn contains a subscript.
MC68030 USER'S MANUAL
MOTOROLA