MC68020 Datasheet, PDF (202/306 Pages) Motorola, Inc – MICROPROCESSORS USERS MANUAL

The PC value saved in this stack frame is the operation word address of the coprocessor
instruction during which the primitive is received. The scanPC field contains the value of
the MC68020/EC020 scanPC when the primitive is received. If the current instruction does
not evaluate an effective address prior to the exception request primitive, the value of the
effective address field in the stack frame is undefined.
The coprocessor uses this primitive to request exception processing for an exception
during the instruction dialog with the main processor. If the exception handler does not
modify the stack frame, the MC68020/EC020 returns from the exception handler and
reads the response CIR. Thus, the main processor attempts to continue executing the
suspended instruction by reading the response CIR and processing the primitive it
receives.
7.4.20 Take Postinstruction Exception Primitive
The take postinstruction exception primitive initiates exception processing using a
coprocessor-supplied exception vector number and the postinstruction exception stack
frame format. This primitive applies to general and conditional category instructions.
Figure 7-44 shows the format of the take postinstruction exception primitive.
15 14 13 12 11 10
9
8
7
0
0 PC 0
1
1
1
1
0
VECTOR NUMBER
Figure 7-44. Take Postinstruction Exception Primitive Format
The take postinstruction exception primitive uses the PC bit as described in 7.4.2
Coprocessor Response Primitive General Format. The vector number field contains
the exception vector number used by the main processor to initiate exception processing.
When the main processor receives this primitive, it acknowledges the coprocessor
exception request by writing an exception acknowledge mask to the control CIR (refer to
7.3.2 Control CIR). The MC68020/EC020 then performs exception processing as
described in Section 6 Exception Processing. The vector number for the exception is
taken from the vector number field of the primitive, and the MC68020/EC020 uses the six-
word stack frame format shown in Figure 7-45.
15
12 11
0
SP
STATUS REGISTER
+02
SCAN PC
+06 0 0 1 0
+08
VECTOR NUMBER
PROGRAM COUNTER
Figure 7-45. MC68020/EC020 Postinstruction Stack Frame
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