Intro    Settings    Syscalls    IDE    Debugging    Command    Tools    History    Limitations    Exception Handlers    Macros    Acknowledgements        MARS home

Writing and Using MIPS exception handlers in MARS


Exception handlers, also known as trap handlers or interrupt handlers, can easily be incorporated into a MIPS program. This guide is not intended to be comprehensive but provides the essential information for writing and using exception handlers.

Although the same mechanism services all three, exceptions, traps and interrupts are all distinct from each other. Exceptions are caused by exceptional conditions that occur at runtime such as invalid memory address references. Traps are caused by instructions constructed especially for this purpose, listed below. Interrupts are caused by external devices.

MARS partially but not completely implements the exception and interrupt mechanism of SPIM.

Essential Facts

Some essential facts about writing and using exception handlers include:

Example of Trap Handler

The sample MIPS program below will immediately generate a trap exception because the trap condition evaluates true, control jumps to the exception handler, the exception handler returns control to the instruction following the one that triggered the exception, then the program terminates normally.

   teqi $t0,0     # immediately trap because $t0 contains 0
   li   $v0, 10   # After return from exception handler, specify exit service
   syscall        # terminate normally

# Trap handler in the standard MIPS32 kernel text segment

   .ktext 0x80000180
   move $k0,$v0   # Save $v0 value
   move $k1,$a0   # Save $a0 value
   la   $a0, msg  # address of string to print
   li   $v0, 4    # Print String service
   move $v0,$k0   # Restore $v0
   move $a0,$k1   # Restore $a0
   mfc0 $k0,$14   # Coprocessor 0 register $14 has address of trapping instruction
   addi $k0,$k0,4 # Add 4 to point to next instruction
   mtc0 $k0,$14   # Store new address back into $14
   eret           # Error return; set PC to value in $14
   .asciiz "Trap generated"

Widely Used Exception Handler

The exception handler exceptions.s provided with the SPIM simulator will assemble and run under MARS. The MARS assembler will generate warnings because this program contains directives that it does not recognize, but as long as the setting "Assembler warnings are considered errors" is not set this will not cause any problems.