
CHAPTER 5 EXCEPTION PROCESSING
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5.4.11 System Call Exception
Cause
A System Call exception occurs during an attempt to execute the SYSCALL instruction. This exception
is not maskable.
Processing
The common exception vector is used for this exception, and the Sys code in the ExcCode field of the
Cause register is set.
The EPC register contains the address of the SYSCALL instruction unless it is in a branch delay slot, in
which case the EPC register contains the address of the preceding branch instruction.
If the SYSCALL instruction is in a branch delay slot, the BD bit of the Status register is set to 1;
otherwise this bit is cleared.
Servicing
When this exception occurs, control is transferred to the applicable system routine.
To resume execution, the EPC register must be altered so that the SYSCALL instruction does not re-
execute; this is accomplished by adding a value of 4 to the EPC register before returning.
If a SYSCALL instruction is in a branch delay slot, interpretation of the branch instruction is required to
resume execution.
5.4.12 Breakpoint Exception
Cause
A Breakpoint exception occurs when an attempt is made to execute the BREAK instruction. This
exception is not maskable.
Processing
The common exception vector is used for this exception, and the BP code in the ExcCode field of the
Cause register is set.
The EPC register contains the address of the BREAK instruction unless it is in a branch delay slot, in
which case the EPC register contains the address of the preceding branch instruction.
If the BREAK instruction is in a branch delay slot, the BD bit of the Status register is set to 1.
Servicing
When the Breakpoint exception occurs, control is transferred to the applicable system routine.
Additional distinctions can be made by analyzing the unused bits of the BREAK instruction (bits 25 to 6),
and loading the contents of the instruction whose address the EPC register contains. A value of 4 must
be added to the contents of the EPC register to locate the instruction if it resides in a branch delay slot.
To resume execution, the EPC register must be altered so that the BREAK instruction does not re-
execute; this is accomplished by adding a value of 4 to the EPC register before returning.
If a BREAK instruction is in a branch delay slot, interpretation of the branch instruction is required to
resume execution.