Question

The simple exception handler always jumps back to the instruction following the exception. This works fine unless the instruction that causes the exception is in the delay slot of a branch. In that case, the next instruction is the target of the branch. Write a better handler that uses the EPC register to determine which instruction should be executed after the exception.

Short answer

If there is any exception occurred, then this exception handler returns the instructions and not re-execute the debug instructions, if the same exception is occurring again.

Step-by-step solution

Exception Handler and EPC.

Exception Handler:It is aninterrupt that occur during the execution of the code, that can be handled by the exception. In MIPS instruction an exception caused to a MIPS processor to jump back to a piece of code at address 80000180_hex in the kernel but not the user address space.

There are some instructions to be used to avoid the delay slot of branch in exception handling.

EPC:EPC has the register number 14 which is defined as an address of instruction that can cause an exception. A mask bit 1 can enable an interrupt and mask bit 0 refers to disable an interrupt. If the exception is delayed, then it refers to the delay bit 1 which executed in the delay slot of a branch.

Exception without delay

This code raises exception without delay:

ktext 0x80000180

mov $k1,$at #save $at register

sw $a0, save0 #Handler is not re-entrant and can’t use

sw $a1, save1

mfc 0$k0, $13 #move cause into $k0

srl $a0, $k0, 2 #Extract branch delay bit

andi $a0, $a0, 0xf #Get branch offset

bgtz $a0, done1

Exception with delay. 

This code raises exception with delay:

mfc 0 $k0, $13 #Move cause into $k0

srl $a0, $k0, 31 #Extract branch delay bit

beq $a0, $0, done1 #Not in the delay slot

mfc0 $k0, $14 # $14 is EPC

lw $a0, 0($k0) #Get the branch instruction

andi $a0, $a0, 0xffff #Get branch offset

sll $a0, $a0, 16 #shift left by 16

sra $a0, $a0, 16 #Sign extension

sll $a0, $a0, 2 #Shift left by 2

mtc0 $a0, $14 #Write branch target address

#back to EPC

j done2 #jump to done2

done1:

If there is any exception occurred, then this exception handler returns the instructions and not re-execute the above debug instructions, if the same exception is occurring again.