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CHAPTER 5 EXCEPTION PROCESSING
90
5.3.4 Compare Register (11)
The Compare register causes a timer interrupt; it maintains a stable value that does not change on its
own.
When the value of the Count register (see Section 5.3.3) equals the value of the Compare register, the
IP(7) bit in the Cause register is set. This causes an interrupt as soon as the interrupt is enabled.
Writing a value to the Compare register, as a side effect, clears the timer interrupt request.
For diagnostic purposes, the Compare register is a read/write register. Normally, this register is only
used for a write. Figure 5-4 shows the format of the Compare register.
Figure 5-4. Compare Register Format
32
0
31
Compare
Compare: Value that is compared with the count value of the Count register
5.3.5 Status Register (12)
The Status register is a read/write register that contains the operating mode, interrupt enabling, and the
diagnostic states of the processor. Figure 5-5 shows the format of the Status register. Figure 5-6
shows the details of the Diagnostic Status (DS) field. All DS field bits other than the TS bit are writeable.
Major Status register fields are detailed below.
Figure 5-5. Status Register Format
29 28 27 26 25 24
16 15
8
7
6
5
4
3
2
1
0
31
0
CU0
0
RE
DS
IM
KX SX UX KSU
ERL
1
2
1
3
9
EXL
IE
8
1
1
1
2
1
1
1
CU0:
Enables/disables the use of the coprocessor (1
→
Enabled, 0
→
Disabled).
CP0 can be used by the kernel at all times.
Reserved. To be set to 0.
Enables/disables reversing of the endian setting in User mode (0
→
Disabled, 1
→
Enabled).
This bit must be set to 0 since the V
R
4101 supports the little-endian order only.
Diagnostic Status field (see Figure 5-6).
Interrupt Mask field used to enable/disable interrupts (0
→
Disabled, 1
→
Enabled). This
field consists of 8 bits that are used to control eight interrupts. The bits are assigned to
interrupts as follows:
IM7
:
Masks a timer interrupt.
IM(6:2) :
Mask ordinary interrupts (Int(4:0)
never occur in the V
R
4101.
IM(1:0) :
Mask software interrupts.
0:
RE:
DS:
IM:
Note
and write requests). However, Int(4:2)
Note