Central Processing Unit
MPC561/MPC563 Reference Manual, Rev. 1.2
Freescale Semiconductor
3-53
3.15.4.9
Decrementer Exception (0x0900)
A decrementer exception occurs when no higher priority exception exists, the decrementer register has
completed decrementing, and MSR[EE] = 1. The decrementer exception request is canceled when the
exception is handled. The decrementer register counts down, causing an exception (unless masked) when
passing through zero. The decrementer implementation meets the following requirements:
•
Loading a GPR from the decrementer does not affect the decrementer.
•
Storing a GPR value to the decrementer replaces the value in the decrementer with the value in the
GPR.
•
Whenever bit 0 of the decrementer changes from zero to one, an exception request is signaled. If
multiple decrementer exception requests are received before the first can be reported, only one
exception is reported. The occurrence of a decrementer exception cancels the request.
•
If the decrementer is altered by software and if bit 0 is changed from zero to one, an interrupt
request is signaled.
The register settings for the decrementer exception are shown in
Machine State Register (MSR)
IP
No change
ME
No change
LE
Set to value of ILE bit prior to the exception
DCMPEN
This bit is set according to (BBCMCR[EN_COMP] AND
BBCMCR[EXC_COMP])
Other
Cleared to 0
1
If the exception occurs during an instruction fetch in Decompression On mode, the SRR0 register will contain a
compressed address.
Table 3-30. Register Settings Following a Decrementer Exception
Register
Bits
Setting Description
Save/Restore Register 0 (SRR0)
1
All
Set to the effective address of the instruction that the processor
would have attempted to execute next if no exception
conditions were present.
Save/Restore Register 1 (SRR1)
[0:15]
Cleared to 0
[16:31]
Loaded from MSR[16:31]
Machine State Register (MSR)
IP
No change
ME
No change
LE
Set to value of ILE bit prior to the exception
DCMPEN
This bit is set according to (BBCMCR[EN_COMP] AND
BBCMCR[EXC_COMP])
Other
Cleared to 0
Table 3-29. Register Settings following a Floating-Point Unavailable Exception (continued)
Register
Bits
Setting Description
Summary of Contents for MPC561
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