
4-14
MPC603e & EC603e RISC Microprocessors User's Manual
MOTOROLA
The IEEE floating-point exception mode bits (FE0 and FE1) together define whether
floating-point exceptions are handled precisely, imprecisely, or whether they are taken at
all. (Note that FE0 and FE1 are not supported on the EC603e microprocessor.) The possible
settings and default conditions for the 603e are shown in Table 4-6. For further details, see
Chapter 6, “Exceptions,” of
The
Programming Environments Manual
.
MSR bits are guaranteed to be written to SRR1 when the first instruction of the exception
handler is encountered.
4.2.1 Enabling and Disabling Exceptions
When a condition exists that may cause an exception to be generated, it must be determined
whether the exception is enabled for that condition.
IEEE floating-point enabled exceptions (a type of program exception) are ignored
when both MSR[FE0] and MSR[FE1] are cleared. If either of these bits are set, all
IEEE enabled floating-point exceptions are taken and cause a program exception.
(Not supported on the EC603e microprocessor.)
Asynchronous, maskable exceptions (that is, the external, system management, and
decrementer interrupts) are enabled by setting the MSR[EE] bit. When MSR[EE] =
0, recognition of these exception conditions is delayed. MSR[EE] is cleared
automatically when an exception is taken, to delay recognition of conditions causing
those exceptions.
A machine check exception can occur only if the machine check enable bit,
MSR[ME], is set. If MSR[ME] is cleared, the processor goes directly into checkstop
state when a machine check exception condition occurs. Individual machine check
exceptions can be enabled and disabled through bits in the HID0 register, which is
described in Table 2-2.
System reset exceptions cannot be masked.
Table 4-6. IEEE Floating-Point Exception Mode Bits
FE0
FE1
Mode
0
0
Floating-point exceptions disabled
0
1
Floating-point imprecise nonrecoverable*
1
0
Floating-point imprecise recoverable*
1
1
Floating-point precise mode
* Not implemented in the 603e