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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
gx_01.fm.(1.2)
March 27,2006
PowerPC 750GX Overview
Page 49 of 377
The PowerPC Architecture supports four types of exceptions:
1.7.2 750GX Microprocessor Exception Implementation
The 750GX exception classes described above are shown in the Table 1-4. Although exceptions have other
characteristics, such as priority and recoverability, Table 1-4 describes the precise or imprecise characteris-
tics of exceptions the 750GX uniquely handles. Table 1-4 includes no synchronous imprecise exceptions;
although the PowerPC Architecture supports imprecise handling of floating-point exceptions, the 750GX
implements these exception modes precisely.
Table 1-5 on page 50 lists the 750GX exceptions and conditions that cause them. Exceptions specific to the
750GX are indicated.
Synchronous,
precise
These are caused by instructions. All instruction-caused exceptions are handled
precisely. That is, the machine state at the time the exception occurs is known and
can be completely restored. This means that (excluding the trap and system call
exceptions) the address of the faulting instruction is provided to the exception
handler and that neither the faulting instruction nor subsequent instructions in the
code stream will complete execution before the exception is taken. Once the
exception is processed, execution resumes at the address of the faulting instruc-
tion (or at an alternate address provided by the exception handler). When an
exception is taken due to a trap or system call instruction, execution resumes at an
address provided by the handler.
Synchronous,
imprecise
The PowerPC Architecture defines two imprecise floating-point exception modes,
recoverable and nonrecoverable. Even though the 750GX provides a means to
enable the imprecise modes, it implements these modes identically to the precise
mode (that is, enabled floating-point exceptions are always precise).
Asynchronous,
maskable
The PowerPC Architecture defines external and decrementer interrupts as
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 no instructions are in the execution units, the
exception is taken immediately upon determination of the correct restart address
(for loading SRR0). As shown in the Table 1-4, 750GX Microprocessor Exception
Classifications, the 750GX implements additional asynchronous, maskable excep-
tions.
Asynchronous,
nonmaskable
There are two nonmaskable asynchronous exceptions: system reset and the
machine-check exception. These exceptions might not be recoverable, or might
provide a limited degree of recoverability. Exceptions report recoverability through
the MSR[RI] bit.
Table 1-4. 750GX Microprocessor Exception Classifications
Synchronous/Asynchronous Precise/Imprecise Exception Type
Asynchronous, nonmaskable Imprecise Machine check, system reset
Asynchronous, maskable Precise
External, decrementer, system-management, performance-monitor,
and thermal-management interrupts
Synchronous Precise Instruction-caused exceptions