M68060 USER’S MANUAL
8.3 EXCEPTION PRIORITIES
Exceptions can be divided into the five basic groups identified in Table 8-3. These groups
are defined by specific characteristics and the order in which they are handled. Table 8-3
represents the priority used for simultaneous faults, as viewed by the MC68060 hardware.
In Table 8-3, 0.0 represents the highest priority, while 4.1 is the lowest. Note that there are
shared priorities for exceptions within Group 3, since these types are mutually exclusive.
Within an MC68060 system, more than one exception can occur at the same time. The reset
exception is unique; central processing unit (CPU) reset overrides and clears all other
exceptions which may have occurred at the same time. All other exceptions are handled
according to the priority relationship defined in Table 8-3.
The method used to process exceptions in the MC68060 is similar to that found on the
MC68040 due to the restart exception model. In general, when multiple exceptions are
pending, the exception with the highest priority is processed first, and the remaining excep-
tions are regenerated when the original faulting instruction is restarted.
To clarify the exception priority within group 1, it is important to note that instruction fetch
pipeline (IFP) access errors are not recognized until the faulted portion of the instruction is
actually used (or attempted to be used). As an example, consider a “move.l (An), xxx.l”
instruction. If the source operand defined at the address contained in An is faulted, the oper-
and access error will occur. If the extension words defining the destination address are also
faulted, the IFP access error would be processed after the source operand fault. Thus, in
Section 6 Floating-Point Unit (MC68060 Only)
for details concerning floating-point instructions. For
the case of an emulated FTRAPcc instruction and a floating-point BSUN exception, the BSUN is considered
Table 8-3. Exception Priority Groups
Group.Priority Exception and Relative Priority Characteristics
0.0 Reset The processor aborts all processing (instruction
or exception) and does not save old context.
Instruction Access Error
Data Access Error
The processor suspends processing and saves
the processor context.
Exception processing begins before the
instruction is executed.
2.6 Floating-Point Unimplemented Instruction,
Floating-Point Unsupported Data Type
Exception processing begins after the initial
memory operand address is calculated, but
before instruction is executed.
, CHK, CHK2, Divide-by-
Zero, TRAPV, TRAPcc, TRAP #n, RTE Format
Exception processing is part of instruction
execution and begins after instruction
Exception processing begins when the current
instruction is completed.