Motorola M68000 User's Guide download pdf (Page 199)

CHAPTER 1 4

THE 68020

The MC68020 is a dramatically improved member of the 68000 family

with a plethora of new features. Not only is the basic core performance

of the processor much improved, but the new features expand the func

tionality of this chip so much that it brings it into serious competition

with mini- and super minicomputers. The major enhancements can be

summarized as:

1. A full 32-bit address bus

2. An instruction cache for faster execution

3. Built-in coprocessor support

4. New addressing modes

5. New and enhanced instructions

6. 8-, 16-, and 32-bit.data bus interface

The 68020 is a true 32-bit architecture. With the 32-bit data bus capa

bility and internal 32-bit operations, the 68020 can operate at its maximum

potential with 32-bit operands. Even though the 68000 performs 32-bit op

erations, the data bus path is restricted to 16 bits at a time. For this reason

the 68000 is sometimes classed as a 16-bit micro. The 68020 is clearly a

32-bit architecture in every way. In order to allow the 68020 to be used in

systems that are restricted to an 8- or 16-bit data bus, the 68020 provides

the option of using 8-, 16-, or 32-bit data bus widths. Unfortunately, you

can’t just plug in a 68020 where a 68000 was. The physical construction

is very different. The 68020 uses a 114-pin grid array, whereas the 68000

uses the more conventional dual in-line package (DIP).

Along with the expanded addressing capability comes an added

bonus: the restriction that word or longword data must be aligned on

even-byte boundaries is relaxed. Words and longwords can start on any

byte address. However, the restriction that instructions start on even bytes

has not been removed.

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Motorola M68000 User's Guide Page 199