7
new FSAVE instruction (by reading from the save CIR). In this case, the FPCP returns the
invalid format word to signal the illegal nesting of the FSAVE instruction. The main pro-
cessor services this format word by writing an abort to the control CIR and initiating
exception processing. The dialog for this operation is shown in Figure 7-39.
MC68020/MC68030
MC68881/MC68882
Figure 7-39. FSAVE Format Exception Dialog
Since the MPU writes an abort to the FPCP in response to the illegal format word, the
F~AVE or FRESTORE that was interrupted by the nested FSAVE is destructively aborted
wtth no indication to the suspended instruction of this occurrence. Thus, a suspended save
operation continues to read the "frame" from the operand CIR if it is resumed, even though
the data in the operand CIR is not valid. Likewise, a suspended restore operation writes
the remainder of the frame to the operand CIR if it is resumed, even though the data written
is ignored and the restore operation is not performed. Due to the destructive behavior of
a nested FSAVE instruction, programmers must be certain that the FPCP is not executing
an FSAVE or FRESTORE instruction prior to an attempt to execute a new FSAVE instruction.
If there is a possibility that a nested FSAVE might occur, the MPU MOVES instruction might
be used to read the save CIR before the FSAVE is executed. If the value returned from the
save CIR is the illegal format word, then the new FSAVE should be postponed. Reading
the save CIR in this manner is not destructive.
7.5.4.7 FORMAT EXCEPTION, FRESTORE INSTRUCTION. This dialog is utilized by the FPCP
when an FRESTORE instruction is initiated by writing an invalid format word value to the
restore CIR. (In this context, the term invalid format value refers to any value that is not a
null, idle, or busy format word value recognized by the FPCP.) In this case, the FPCP returns
the explicit invalid format word ($02xx) to signal the unrecognized format word value. The
main processor services this format word by writing an abort to the control CIR and initiating
exception processing. The dialog for this operation is shown in Figure 7-40. Note that this
is a destructive exception since any instruction that was executing is aborted when the
FRESTORE instruction is initiated. However, this should not be detrimental since a suc-
cessful restore operation also aborts any previously executing instruction.
FREESCALE
7-40
MC68881/MC68882 USER'S MANUAL
To Next Page
Loading...