Motorola MC68030 User Manual page 393

The encoding of bits 0-8 of the coprocessor instruction operation word is
dependent on the particular instruction being implemented (see 10.2 CO-
PROCESSOR INSTRUCTION TYPES).
10.1.4 Coprocessor System Interface
The communication protocol between the main processor and coprocessor
necessary to execute a coprocessor instruction uses a group of interface
registers, called coprocessor interface registers, resident within the copro-
cessor. By accessing one of these interface registers, the MC68030 hardware
initiates coprocessor instructions. The coprocessor uses a set of response
primitive codes and format codes defined for the M68000 coprocessor in-
terface to communicate status and service requests to the main processor
through these registers. The coprocessor interface registers (CIRs) are also
used to pass operands between the main processor and the coprocessor.
The CIR set, response primitives, and format codes are discussed in 10.3
COPROCESSOR INTERFACE REGISTER SET and 10.4 COPROCESSOR RE-
SPONSE PRIMITIVES.
10.1.4.1 COPROCESSOR CLASSIFICATION. M68000 coprocessors can be classi-
fied into two categories depending on their bus interface capabilities. The
first category, non-DMA coprocessors, consists of coprocessors that always
operate as bus slaves. The second category, DMA coprocessors, consists of
coprocessors that operate as bus slaves while communicating with the main
processor across the coprocessor interface, but also have the ability to op-
erate as bus masters, directly controlling the system bus.
If the operation of a coprocessor does not require a large portion of the
available bus bandwidth or has special requirements not directly satisfied
by the main processor, that coprocessor can be efficiently implemented as
a non-DMA coprocessor. Since non-DMA coprocessors always operate as
bus slaves, all external bus-related functions that the coprocessor requires
are performed by the main processor. The main processor transfers operands
from the coprocessor by reading the operand from the appropriate CIR and
then writing the operand to a specified effective address with the appropriate
address space specified on the function code lines. Likewise, the main pro-
cessor transfers operands to the coprocessor by reading the operand from
a specified effective address (and' address space) and then writing that op-
erand to the appropriate CIR using the coprocessor interface. The bus inter-
face circuitry of a coprocessor operating as a bus slave is not as complex as
that of a device operating as a bus master.
MOTOROLA
MC68030 USER'S MANUAL
10-5