Addressing Capabilities; Format - Motorola MC68030 User Manual

Several of the addressing techniques described in this section use data registers and
address registers interchangeably. While the MC68030 provides this capability, its
performance has been optimized for addressing with address registers. The performance of
a program that uses address registers in address calculations is superior to that of a
program that similarly uses data registers.The performance has been optimized for
addressing registers in address calculations is superior to that of a program that similarly
uses data registers. The specification of addresses with data registers should be used
sparingly (if at all), particularly in programs that require maximum performance.

2.6.1 Addressing Capabilities

In both the MC68020 and the MC68030, setting the base register suppress (BS) bit in the
full format extension word (see Figure 2–4) suppresses use of the base address register in
calculating the effective address. This allows any index register to be used in place of the
base register. Since any of the data registers can be index registers, this provides a data
register indirect form (Dn). The mode could be called register indirect (Rn) since either a
data register or an address register can be used. This addressing mode is an extension to
the M68000 Family because the MC68030 and MC68020 can use both the data registers
and the address registers to address memory. The capabilities of specifying the size and
scale of an index register (Xn.SIZE*SCALE) in these modes provides additional addressing
flexibility. Using the SIZE parameter, either the entire contents of the index register can be
used, or the least significant word can be sign-extended to provide a 32-bit index value (refer
to Figure 2–5).
31
D1.L
31
D1.W
USED IN ADDRESS CALCULATION
MOTOROLA
Figure 2-5. Using SIZE in the Index Selection
MC68030 USER'S MANUAL
Data Organization and Addressing Capabilities
16 15
0
D1
0
D1
2-25