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CHAPTER 2 V
R
4120A
Preliminary User’s Manual S14767EJ1V0UM00
163
2.6.1.3 Exception/error levels
Returning from an exception resets the exception level to normal (0) (for details, see
APPENDIX A MIPS III
INSTRUCTION SET DETAILS
).
The registers that retain address, cause, and status information during exception processing are described in
Section 2.6.3 Exception processing registers
. For a description of the exception process, see
Section 2.6.4
Details of exceptions
.
2.6.2 Precision of exceptions
V
R
4120A CPU exceptions are logically precise; the instruction that causes an exception and all those that follow it
are aborted and can be re-executed after servicing the exception. When succeeding instructions are discarded,
exceptions associated with those instructions are also discarded. Exceptions are not taken in the order detected, but
in instruction fetch order.
The exception handler can determine the cause of an exception and the address. The program can be restarted
by rewriting the destination register - not automatically, however, as in the case of all the other precise exceptions
where no status change occurs.
2.6.3 Exception processing registers
This section describes the CP0 registers that are used in exception processing. Table 2-56 lists these registers,
along with their number-each register has a unique identification number that is referred to as its register number.
The CP0 registers not listed in the table are used in memory management (for details, see
Section 2.5 Memory
Management System
).
The exception handler examines the CP0 registers during exception processing to determine the cause of the
exception and the state of the CPU at the time the exception occurred.
The registers in Table 2-56 are used in exception processing, and are described in the sections that follow.
Table 2-56. CP0 Exception Processing Registers
Register Name
Register Number
Context register
4
BadVAddr register
8
Count register
9
Compare register
11
Status register
12
Cause register
13
EPC register
14
WatchLo register
18
WatchHi register
19
XContext register
20
Parity Error register
Note
26
Cache Error register
Note
27
Error EPC register
30
Note
This
compatibility with the V
R
4100. This register is
not used in the
μ
PD98501 hardware.
register
is
prepared
to
maintain