
MOTOROLA
Chapter 5. Exceptions
5-3
Exception Classes
identically to the precise mode (that is, all enabled floating-point exceptions are
always precise on the G2 core).
Asynchronous, maskable—The external (core_int), system management interrupt
(core_smi), and decrementer exceptions are maskable asynchronous exceptions.
The critical interrupt (core_cint) exception of the G2_LE core is also a maskable
asynchronous exception. When these exceptions occur, their handling is postponed
until the next instruction completes execution and until any exceptions associated
with that instruction complete execution. If there are no instructions in the execution
units, the exception is taken immediately upon determination of the correct restart
address (for loading SRR0).
Asynchronous, nonmaskable—There are two nonmaskable asynchronous
exceptions: system reset and the machine check exception. These exceptions may
not be recoverable, or may provide a limited degree of recoverability. All exceptions
report recoverability through the MSR[RI] bit.
The G2 core exception classes are shown in Table 5-1.
Table 5-1 defines exception categories that are handled uniquely by the G2 core. Note that
Table 5-1 includes no synchronous imprecise exceptions. While the PowerPC architecture
supports imprecise handling of floating-point exceptions, the G2 core implements
floating-point exception modes as precise exceptions. Although the PowerPC architecture
specifies that the recognition of the machine check exception is nonmaskable, on the G2
core the stimuli that cause this exception are maskable. For example, the machine check
exception is caused by the assertion of core_tea, core_ape, core_dpe, or core_mcp.
However, core_mcp, core_ape, and core_dpe can be disabled by bits 0, 2, and 3,
respectively, in HID0. Therefore, the machine check caused by asserting core_tea is the
only truly nonmaskable machine check exception.
The G2 core exceptions, and conditions that cause them, are listed in Table 5-2.
Table 5-1. Exception Classifications
Synchronous/Asynchronous
Precise/Imprecise
Exception Type
Asynchronous, nonmaskable
Imprecise
Machine check
System reset
Asynchronous, maskable
Precise
External interrupt
Decrementer
System management interrupt
Critical interrupt (G2_LE core only)
Synchronous
Precise
Instruction-caused exceptions
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