When silicon chips are fabricated, defects in materials (e.g. silicon) and manufacturing errors can result in defective circuits. A very common defect is for one wire to affect the signal in another. This is called a cross-talk fault. A special class of cross-talk fault is when a signal is connected to a wire that has a constant logical value (e.g. a power supply wire.) In this case, we have a stuck-at-0 or a stuck-at-1 fault and the affected wire always has a logical value of 0 or 1 respectively.

Registers, Source 2, bit 3

ALU, zero

Instruction memory, Address, bit 1

Design a test (by filling the PC, registers, data memory and instruction memory with some values) to detect a stuck-at-0 fault on this wire. Design a test to detect a stuck-at-1 fault on this wire. If we know the processor has a stuck-at-1 fault on this wire, is the processor still usable? To be usable, we must be able to convert any program that executes on a normal MIPS processor into a program that works on this processor. The processor is usable if every instruction broken by this fault can be replace with a sequence of working instructions that achieve the same effect.