
MOTOROLA
Chapter 4. Exceptions
4-3
Synchronous, imprecise—The PowerPC architecture defines two imprecise
floating-point exception modes, recoverable and nonrecoverable. Even though the
PowerPC 603e provides a means to enable the imprecise modes, it implements these
modes identically to the precise mode (that is, all enabled floating-point enabled
exceptions are always precise on the 603e). (The EC603e microprocessor does not
support floating-point operations.)
Asynchronous, maskable—The external, system management interrupt (SMI), and
decrementer exceptions are maskable asynchronous exceptions. When these
exceptions occur, their handling is postponed until the next instruction, and any
exceptions associated with that instruction, completes 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 603e exception classes are shown in Table 4-1.
Table 4-1. Exception Classifications
Although exceptions have other characteristics as well, such as whether they are maskable
or nonmaskable, the distinctions shown in Table 4-1 define categories of exceptions that the
603e handles uniquely. Note that Table 4-1 includes no synchronous imprecise exceptions.
While the PowerPC architecture supports imprecise handling of floating-point exceptions,
the 603e, with the exception of the EC603e microprocessor, implements floating-point
exception modes as precise exceptions. (The EC603e microprocessor does not support
floating-point operations.)
Although the PowerPC architecture specifies that the recognition of the machine check
exception is nonmaskable, on the 603e the stimuli that cause this exception are maskable.
For example, the machine check exception is caused by the assertion of TEA, APE, DPE,
or MCP. However, the MCP, APE, and DPE signals can be disabled by bits 0, 2, and 3
respectively in HID0. Therefore, the machine check caused by TEA is the only truly
nonmaskable machine check exception.
Synchronous/Asynchronous
Precise/Imprecise
Exception Type
Asynchronous, nonmaskable
Imprecise
Machine check
System reset
Asynchronous, maskable
Precise
External interrupt
Decrementer
System management interrupt
Synchronous
Precise
Instruction-caused exceptions