of Microprocessors

MICROPROCESSORS 167
registers can be used as a stack pointer with equal ease, a valuable feature for
many high-level languages.
Data registers act essentially like accumulators and all are equivalent;
none have dedicated functions. All of the standard arithmetic operations
including shifts and multiply/divide operate on data registers. Some of the
addressing modes can use values in the data registers as offsets and indices.
Since data registers are 32 bits long, they are often called upon to hold
shorter 8- and 16-bit values. In general, when a shorter value is loaded into
or created in a data register, it is put at the lower (rightmost) end and the
remainder of the register is not changed. This action can lead to program bugs
but is very useful when needed.
Address registers most closely resemble the index registers of classic
computers. Limited arithmetic such as add/subtract, compare, and increment/decrement
can be performed but not shifts or logical operations.
Address registers always contain 32-bit values; if a word value is loaded or
generated, it is sign extended to 32 bits. Byte-sized operations are not allowed
on address registers.
Addressing Modes
Historically, as small computers became more sophisticated, the
greatest improvement was in the variety of memory-addressing modes
available. For example, the Digital Equipment PDP-ll became the most
popular minicomputer largely because of its many and varied addressing
modes. Likewise, the 6502, Motorola 6809, and 68000 have reached the
heads of their respective classes because of flexible memory addressing.
Addressing modes are important because, when manipulating common
data structures, the availability of a single instruction with an appropriate
addressing mode can replace several time- and space-consuming address
calculation instructions. This is doubly important in music synthesis
applications, which typically manipulate large, moderately complex data
structures. A particularly strong point of the 68000 is that these addressing
modes work for data arrays as large as the addressing range, i.e., up to 16M
bytes. Other processors using segmented memory, such as the 8086 series,
may have nearly as many addressing modes as the 68000, but they are useless
when array sizes exceed 64K bytes. In fact, a considerable amount of segment
register swapping is usually necessary when the total size of all arrays exceeds
128K bytes.
Table 5-3 is a summary description of the addressing modes offered by
the 68000. Keep in mind that, whenever an address register or a data register
participates in an address mode calculation, the full 32 bits (or optionally 16
bits) of the register is used. Also, the result of an addresss calculation is
always a 32-bit integer of which the lower 24 are sent out to the address bus
on the 68000. The 68020, with its 4G-byte addressing range, merely sends
out all 32-bits of the result; the addressing modes still work the same way.