Motorola MC68030 User Manual page 456

-
10.5.2.2 F-lINE EMULATOR EXCEPTIONS. The F-line emulator exceptions de-
tected by the MC68030 are either explicitly or implicitly related to the en-
codings of F-line operation words in the instruction stream. If the main
processor determines that an F-line operation word is not valid, it initiates
F-Iine emulator exception processing. Any F-line operation word with bits
[8:6]
=
110 or 111 causes the MC68030 to initiate exception processing with-
out initiating any communication with the coprocessor for that instruction.
Also, an operation word with bits [8:6]
=
000-101 that does not map to one
of the valid coprocessor instructions in the instruction set causes the MC68030
to initiate F-Iine emulator exception processing. If the F-Iine emulator excep-
tion is either of these two situations, the main processor does not write to
the control CIR prior to initiating exception processing.
10-68
F-line exceptions can also occur if the operations requested by a coprocessor
response primitive are not compatible with the effective address type in bits
[0-5] of the coprocessor instruction operation word. The F-line emulator
exceptions that can result from the use of the M68000 coprocessor response
primitives are summarized in Table 10-6. If the exception is caused by re-
ceiving an invalid primitive, the main processor aborts the coprocessor in-
struction in progress by writing an abort mask (refer to 10.3.2 Control CIR)
to the control CIR prior to F-Iine emulator exception processing.
Another type of F-Iine emulator exception occurs when a bus error occurs
during the coprocessor interface register access that initiates a coprocessor
instruction. The main processor assumes that the coprocessor is not present
and takes the exception.
When the main processor initiates F-line emulator exception processing, it
uses the four-word pre-instruction exception stack frame (refer to Figure
1 0-41) and the F-line emulator exception vector number 11. Thus, if the ex-
ception handler does not modify the stack frame, the main processor attempts
to restart the instruction that caused the exception after it executes an RTE
instruction to return from the exception handler.
If the cause of the F-line exception can be emulated in software, the handler
stores the results of the emulation in the appropriate registers of the pro-
grammer's model and in the status register field of the saved stack frame.
The exception handler adjusts the program counter field of the saved stack
frame to point to the next instruction operation word and executes the RTE
instruction. The MC68030 then executes the instruction following the instruc-
tion that was emulated.
MC68030 USER'S MANUAL
MOTOROLA