When the main processor reads the response CIR to receive the true/false result of the
conditional evaluation, the FPCP returns the take pre-instruction exception primitive instead
of the null primitive. In order to update the FPIAR, the PC bit of this primitive is also set.
The main processor services this primitive by transferring the program counter, writing
an exception acknowledge to the control CIR, and then initiating exception processing.
Although the MC68020 or MC68030 always performs the program counter transfer when
it is requested, other main processors may choose to ignore this request from an MC68881
without incurring a protocol violation. The MC68882 returns a protocol violation whenever
the main processor ignores a request for transfer of the program counter.
Note that the write of the exception acknowledge causes the response CIR encoding to be
changed to the null primitive, thus assuring that the take exception primitive is received
by the main processor while avoiding spurious request primitives in non-MC68020 or non-
MC68030 based systems.
7.5.4.5 F-LINE EMULATOR EXCEPTION. This dialog is utilized by the FPCP when a general
instruction is initiated, and the value written to the command CIR is not a legal FPCP
command word encoding. In this case, the dialog consists of two write cycles and one
read cycle, as shown in Figure 7-38. 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 appropriate action to be taken. In this case, the first read of the response CIR returns
a take exception primitive with the F-line emulator vector number. The main processor
services this primitive by writing an exception acknowledge to the control CIR and initiating
exception processing.
Note that the write of the exception acknowledge causes the response CIR encoding to be
changed to the null primitive while avoiding spurious request primitives in non-MC68020
or non-MC68030 based systems.
7.5.4.6 FORMAT EXCEPTION, FSAVE INSTRUCTION.
This dialog is utilized by the FPCP
when an FSAVE or FRESTORE instruction dialog is interrupted by an attempt to initiate a
MC6802O/MC68030
MC68881/MC68882
I:::::
Figure 7-38. Take F, Line Emulator Exception Dialog
MC68881/MC68882 USER'S MANUAL FREESCALE
7-39
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