Freescale Semiconductor MC68881 - Coprocessor Instruction General Format; FPCP Instruction Operation Word

to the effective addressing mode specified in the operation word or immediate operands
that are part of the instruction. The general format of an FPCP instruction is illustrated in
Figure 7-5.
OPERATION WORD
COPROCESSOR COMMAND WORD IIF ANY)
EFFECTIVE AOORESS EXTENSION WORDS (1 TO 6, IF ANY)
Figure 7-5. Coprocessor Instruction General Format
All coprocessor operations are based on the F-line operation codes (i.e., operation words
with bits [15:12]=$F) which instruct the MPU to call upon a coprocessor for execution of
the instruction. Figure 7-6 illustrates the format of this word.
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
i,,,,i i I ,',.O,P,NO,,T I
Figure 7-6. FPCP Instruction Operation Word
The Cp-ID field indicates which coprocessor is to be selected. Cp-IDs of 000-101 are reserved
by Freescale, and Cp-IDs 110 and 111 are reserved for user definition. The Freescale MPU
and FPCP assembler supplies 001 as the Cp-ID for FPCP instructions by default. The type
field indicates to the MPU which type of coprocessor operation is selected: general, branch,
conditional, save, or restore. The type and type-dependent fields and the coprocessor
command word for all FPCP instructions are described in 4.7 INSTRUCTION ENCODING
DETAILS.
7.4.1 Instruction Protocol
All FPCP instructions have a typical protocol which the MPU and FPCP use. This com-
munication protocol is as follows:
1. When the MPU detects an F-line operation word, communication is initiated by writing
information (a command, condition selector, or restore format word) to the appro-
priate FPCP coprocessor interface register location. (The FPCP save instruction is
initiated by a read operation.)
2. The MPU then reads the coprocessor response to the previous write operation. The
response may indicate any of the following:
a. The FPCP is busy. MPU checks for interrupts, processes them if any are pending,
and then queries the coprocessor again. This allows synchronizing the main pro-
cessor and coprocessor.
b. An exception condition exists, and the FPCP instructs the MPU to take an exception
using a specific exception vector, The MPU acknowledges the exception and ini-
tiates exception processing.
MC68881/MC68882 USER'S MANUAL FREESCALE
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