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IDT MIPS32 4Kc Processor Core
Exceptions
79RC32438 User Reference Manual
2 - 46
November 4, 2002
Notes
Entry Vector Used:
General exception vector (offset 0x180)
Bus Error Exception — Instruction Fetch or Data Access
A bus error exception occurs when an instruction or data access makes a bus request (due to a cache
miss or an uncacheable reference) and that request terminates in an error. The bus error exception can
occur on either an instruction fetch or a data access. Bus error exceptions that occur on an instruction fetch
have a higher priority than bus error exceptions that occur on a data access.
Bus errors taken on the requested (critical) word of an instruction fetch or data load are precise. Other
bus errors, such as stores or non-critical words of a burst read, can be imprecise. These errors are taken
when the EB_RBErr or EB_WBErr signals are asserted and may occur on an instruction that was not the
source of the offending bus cycle.
Cause Register ExcCode Value:
IBE:Error on an instruction reference
DBE:Error on a data reference
Additional State Saved:
None
Entry Vector Used:
General exception vector (offset 0x180)
Debug Software Breakpoint Exception
A debug software breakpoint exception occurs when an SDBBP instruction is executed. The DEPC
register and DBD bit in the Debug register will indicate the SDBBP instruction that caused the debug excep-
tion.
Debug Register Debug Status Bit Set:
DBp
Additional State Saved:
None
Entry Vector Used:
Debug exception vector
Execution Exception — System Call
The system call exception is one of the six execution exceptions. All of these exceptions have the same
priority. A system call exception occurs when a SYSCALL instruction is executed.
Register State
Value
BadVAddr
Failing address
Context
The BadVPN2 field contains VA
31:13
of the failing address.
EntryHi
The VPN2 field contains VA
31:13
of the failing address; the
ASID field contains the ASID of the reference that missed.
EntryLo0
UNPREDICTABLE
EntryLo1
UNPREDICTABLE
Table 2.23 CP0 Register States on a TLB Invalid Exception