7.5.4 Exception Processing
This group of dialogs is actually a set of special cases of the dialogs described previously;
they are grouped here for quick reference, and to simplify the preceding discussions. For
each of the exception processing dialogs, only the differences from the normal instruction
dialogs shown previously are discussed here. Also, it should be noted that these dialogs
do not include all exception processing sequences that involve the FPCP; they only include
those exceptions that are directly related to the coprocessor interface operation. For ex-
ample, main processor detected F-line exceptions are not included since no coprocessor
interface dialog occurs as part of the exception processing for this type of an exception.
The dialog for the coprocessor protocol violation exception is also omitted from the fol-
lowing diagrams. This is because these exceptions are not expected to occur during the
normal operation of a fully debugged system, and the dialog is not readily predictable
(either before or after the protocol violation occurs). By definition, the cause of the exception
for main processor detected protocol violations is a hardware failure (since the FPCP cannot
return an illegal response primitive).
For FPCP-detected protocol violations, the cause is most likely a software failure that causes
a new instruction to be initiated before the previous instruction dialog is completed. In this
case, the new instruction dialog is aborted immediately, but the previous instruction dialog
may not terminate for some time. (The previous dialog may be completed incorrectly, since
the protocol violation is never reported to the previous instruction.)
7.5.4.1 TAKE PRE-INSTRUCTION EXCEPTION. This dialog is utilized by the MC68881 when
an arithmetic (OPCLASS 000, 010, or 011) or conditional instruction is initiated and an
arithmetic exception is pending from a previous instruction; or when the main processor
writes an undefined, reserved command word to the command CIR. In either case, this
dialog consists of two write cycles and one read cycle, as shown in Figure 7-27. First, the
main processor attempts to initiate a new instruction by writing to the command CIR; it
then reads the response CIR to determine the next required action. The MC68881 returns
the take pre-instruction exception in this case, indicating the appropriate vector number.
~ == =! -
-J .u
Figure 7-27. Take Pre-lnstruction Exception Dialog m MC68881
MC68881/MC68882 USER'S MANUAL FREESCALE
7-31
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