The programmable logic array (PLA) in the Commodore 64 (C64) is used to create chip select signals from various other signals, e.g., from the current address. These signals control which chip is to be connected to the data bus. Therefore the PLA is responsible to implement the memory map of the C64.
If the PLA is broken, the CPU and the VIC-II1 direct memory access (DMA) can not access the right memory and I/O devices anymore. In this case some chips can not be selected or more than one chip is active at the same time. A total or partial malfunction of the computer is the result. If a PLA is replaced with a part which does not meet certain timing or electrical constraints of the chips connected to it, the computer becomes unstable, possibly depending on temperature or hardware extensions used or may refuse to work at all.