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IDT MIPS32 4Kc Processor Core
Exceptions
79RC32438 User Reference Manual
2 - 41
November 4, 2002
Notes
The ErrorEPC register is loaded with PC-4 if the state of the processor indicates that it was execut-
ing an instruction in the delay slot of a branch. Otherwise, the ErrorEPC register is loaded with PC.
Note that this value may or may not be predictable.
PC is loaded with 0xBFC0_0000.
Cause
Register ExcCode Value:
None
Additional State Saved:
None
Entry Vector Used:
Reset (0xBFC0_0000)
Operation:
StatusBEV << 1
StatusTS << 0
StatusSR << 1
StatusNMI << 0
StatusERL << 1
if InstructionInBranchDelaySlot then
ErrorEPC << PC - 4
else
ErrorEPC << PC
endif
PC << 0xBFC0_0000
Debug Single Step Exception
A debug single step exception occurs after the CPU has executed one/two instructions in non-debug
mode, when returning to non-debug mode after debug mode. One instruction is allowed to execute when
returning to a non jump/branch instruction, otherwise two instructions are allowed to execute since the
jump/branch and the instruction in the delay slot are executed as one step. Debug single step exceptions
are enabled by the SSt bit in the Debug register, and are always disabled for the first one/two instructions
after a DERET.
The DEPC register points to the instruction on which the debug single step exception occurred, which is
also the next instruction to single step or execute when returning from debug mode. So the DEPC will not
point to the instruction which has just been single stepped, but rather the following instruction. The DBD bit
in the Debug register is never set for a debug single step exception, since the jump/branch and the instruc-
tion in the delay slot is executed in one step.
Exceptions occurring on the instruction(s) executed with debug single step exception enabled are taken
even though debug single step was enabled. For a normal exception (other than reset), a debug single step
exception is then taken on the first instruction in the normal exception handler. Debug exceptions are unaf-
fected by single step mode, e.g. returning to a SDBBP instruction with debug single step exceptions
enabled causes a debug software breakpoint exception, and the DEPC will point to the SDBBP instruction.
However, returning to an instruction (not jump/branch) just before the SDBBP instruction, causes a debug
single step exception with the DEPC pointing to the SDBBP instruction.
To ensure proper functionality of single step, the debug single step exception has priority over all other
exceptions, except reset and soft reset.
Debug Register Debug Status Bit Set
DSS
Additional State Saved
None
Entry Vector Used