Coprocessor Interface Description; Introduction - Motorola MC68020 User Manual

SECTION 7

COPROCESSOR INTERFACE DESCRIPTION

The M68000 family of general-purpose microprocessors provides a level of performance
that satisfies a wide range of computer applications. Special-purpose hardware, however,
can often provide a higher level of performance for a specific application. The coprocessor
concept allows the capabilities and performance of a general-purpose processor to be
enhanced for a particular application without encumbering the main processor
architecture. A coprocessor can efficiently meet specific capability requirements that must
typically be implemented in software by a general-purpose processor. With a general-
purpose main processor and the appropriate coprocessor(s), the processing capabilities of
a system can be tailored to a specific application.
The MC68020/EC020 supports the M68000 coprocessor interface described in this
section. This section is intended for designers who are implementing coprocessors to
interface with the MC68020/EC020.
The designer of a system that uses one or more Motorola coprocessors (the MC68881 or
MC68882 floating-point coprocessor, for example) does not require a detailed knowledge
of the M68000 coprocessor interface. Motorola coprocessors conform to the interface
described in this section. Typically, they implement a subset of the interface, and that
subset is described in the coprocessor user's manual. These coprocessors execute
Motorola-defined instructions that are described in the user's manual for each
coprocessor.

7.1 INTRODUCTION

The distinction between standard peripheral hardware and an M68000 coprocessor is
important from a programming model perspective. The programming model of the main
processor consists of the instruction set, register set, and memory map. An M68000
coprocessor is a device or set of devices that communicates with the main processor
through the protocol defined as the M68000 coprocessor interface. The programming
model for a coprocessor is different than that for a peripheral device. A coprocessor adds
additional instructions and generally additional registers and data types to the
programming model that are not directly supported by the main processor architecture.
The additional instructions are dedicated coprocessor instructions that utilize the
coprocessor capabilities. The necessary interactions between the main processor and the
coprocessor that provide a given service are transparent to the programmer. That is, the
programmer does not need to know the specific communication protocol between the
main processor and the coprocessor because this protocol is implemented in hardware.
Thus, the coprocessor can provide capabilities to the user without appearing separate
from the main processor.
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