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caused the fault. All write faults have the RR bit set, to indicate that the write
should be rerun upon return from the exception handler.
The remainder of the stack frame contains sufficient information to continue
MOVEM with operand transfer following a faulted transfer. The address of the
next operand to be transferred, incremented or decremented by operand size,
is stored in the faulted address location ($08). The stacked transfer counter is
set to 16 minus the number of transfers attempted (including the faulted cycle).
Refer to Figure 6-3 for the stacking format.
6.3.1.4 Type IV: Faults During Exception Processing
The fourth type of fault occurs during exception processing. If this exception is a
second address or bus error, the machine halts in the "double bus fault"
condition. However, if the exception is one that causes a four- or six-word stack
frame to be written, a bus cycle fault frame is written below the faulted exception
stack frame.
The SSW for a fault within an exception contains the following bit pattern:
15
14
13
12
11
10
9
8
7
6
5
4
3
2
0
I
1
I
0
I
0
I
1R
I
B1
I
BO
I
0
I
0
I
0
I
1
I
LG
I
SIZ
FUNC
TR, B1, and BO are set if a corresponding exception is pending when the BERR
exception is taken.
The contents of the faulted exception stack frame are included in the bus fault
stack frame. The pre-exception status register and the format/vector word of the
faulted frame are stacked. The type of exception can be determined from the
format/vector word. If the faulted exception stack frame contains six words, the
program counter of the instruction that caused the initial exception is also
stacked. This data is placed on the stack in the format shown in Figure 6-4. The
return address from the initial exception is stacked for RTE .
6.3.2 Correcting a Fault
Fault correction methods are discussed in the following paragraphs.
There are two ways to complete a faulted released write bus cycle. The first is
to use a software handler. The second is to rerun the bus cycle via RTE.
Type II fault handlers must terminate with RTE, but specific requirements must
also be met before an instruction is restarted.
MOTOROLA
6-22
EXCEPTION
PROCESSING
CPU32 REFERENCE MANUAL
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