7-10
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part II. PowerPC Microprocessor Module
7.1.2.8 Decrementer Exception (0x00900)
A decrementer exception occurs when no higher priority exception exists, a decrementer
exception condition occurs (for example, the decrementer register has completed
decremented), and MSR[EE] = 1. The decrementer register counts down, causing an
exception request when it passes through zero. A decrementer exception request remains
pending until the decrementer exception is taken and then it is cancelled. The decrementer
implementation meets the following requirements:
¥
The counters for the decrementer and the time-base counter are driven by the same
fundamental time base.
¥
Loading a GPR from the decrementer does not affect the decrementer.
¥
Storing a GPR value to DEC replaces the DEC contents with the value in the GPR.
¥
Whenever bit 0 of the decrementer changes from 0 to 1, a decrementer exception
request is signaled. If multiple decrementer exception requests are received before
the Þrst can be reported, only one exception is reported.
¥
If the decrementer is altered by software and if bit 0 is changed from 0 to 1, an
exception request is signaled.
The register settings for the decrementer exception are shown in Table 7-9.
When a decrementer exception is taken, instruction execution resumes at offset 0x00900
from the physical base address indicated by MSR[IP].
7.1.2.9 System Call Exception (0x00C00)
A system call exception occurs when a System Call (sc) instruction is executed. The
effective address of the instruction following the sc instruction is placed into SRR0. MSR
bits are saved in SRR1, as shown in Table 7-10. Then a system call exception is generated.
Table 7-9. Register Settings after a Decrementer Exception
Register
Setting Description
SRR0
Set to the effective address of the instruction that the processor would have attempted to execute next if
no exception conditions were present.
SRR1
0
Loaded with equivalent bits from the MSR
1Ð4
Cleared
5Ð9
Loaded with equivalent bits from the MSR
10Ð15
Cleared
16Ð31
Loaded with equivalent bits from the MSR
Note that depending on the implementation, reserved bits in the MSR may not be copied to SRR1.
MSR
POW 0
ILE
Ñ
EE
0
PR
0
FP
0
ME Ñ
SE
0
BE
0
IP
Ñ
IR
0
DR
0
RI
0
LE
Set to value of ILE
Содержание MPC860 PowerQUICC
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