Quiz submission record for quiz6-2-2 at Thu Jul 29 04:48:46 2004:

Your Answer for Question 1:
The EPC (exception program counter) register holds the address of the instruction that caused an exception.  The Cause register holds information about the cause of the exception; its lower order bit indicates whether the exception was caused by an undefined instruction (0) or arithmetic overflow (1).  The EPC and Cause registers are located in coprocessor 0, so they can be accessed using mfc0 (move from coprocessor 0).

Your Answer for Question 2:
Imprecise exceptions are exceptions in which the correct exception is not necessarily associated with the instruction that caused it.  A machine with imprecise exceptions leaves it up to the operating system to determine which instruction caused the problem.

Your Answer for Question 3:
Polling requires too much processor power.  In the example in the book, polling overhead for transfering data from floppy or hard disk requires 2-20% of the processor's clock cycles, which is not acceptable.  The solutions to the problems caused by polling are using interrupt-driven I/O devices which use interrupts to tell the processor that an I/O device needs attention, and a DMA (direct memory access) mechanism, which transfers data independent of the processor.

Your Answer for Question 4:
"Memory-mapped io" refers to the use of registers that are mapped in memory, ie. using registers that appear as a special memory location to handle I/O.
li $t0, 0xffff0004 # load address of receiver
lbu $v0, 0($t0)    # deference receiver and store


Your unique submission ID is quiz6-2-2-cs61c-ck-1091101726-2451.