5- 32
MC68349 USER’S MANUAL
MOTOROLA
Table 5-6. Exception Priority Groups
Group
.Priority
Exception and
Relative Priority
Characteristics
0
Reset
Aborts all processing (instruction or
exception); does not save old context.
1.1
1.2
Address Error
Bus Error
Suspends processing (instruction or
exception); saves internal context.
2
BKPT#n, CHK, CHK2,
Division by Zero, RTE,
TRAP#n, TRAPcc, TRAPV
Exception processing is a part of
instruction execution.
3
Illegal Instruction, Line A,
Unimplemented Line F,
Privilege Violation
Exception processing begins before
instruction execution.
4.1
4.2
4.3
Trace
Hardware Breakpoint
Interrupt
Exception processing begins when current
instruction or previous exception
processing is complete.
There are special cases to which the general rule does not apply. The reset exception will
always be the first exception handled since reset clears all other exceptions. It is also
possible for high-priority exception processing to begin before low-priority exception
processing is complete. For example, if a bus error occurs during trace exception
processing, the bus error will be processed and handled before trace exception
processing has completed.
5.6.2 Processing of Specific Exceptions
The following paragraphs provide details concerning sources of specific exceptions, how
each arises, and how each is processed.
5.6.2.1 RESET. Assertion of RESET by external hardware or assertion of the internal
RESET signal by an internal module causes a reset exception. The reset exception has
the highest priority of any exception. Reset is used for system initialization and for
recovery from catastrophic failure. When the reset exception is recognized, it aborts any
processing in progress, and that processing cannot be recovered. Reset performs the
following operations:
1. Clears T0 and T1 in the SR to disable tracing
2. Sets the S-bit in the SR to establish supervisor privilege
3. Sets the interrupt priority mask to the highest priority level ($7)
4. Initializes the VBR to zero ($00000000)
5. Generates a vector number to reference the reset exception vector
6. Loads the first long word of the vector into the interrupt SP
7. Loads the second long word of the vector into the PC
8. Fetches and initiates decode of the first instruction to be executed
Figure 5-15 is a flowchart of the reset exception