Apple was aware of these problems, and starting with the Macintosh IIfx they attempted to address them by including two dedicated "Input/Output Processors" (IOP), customized MOS 6502s running at 10 MHz. The IOPs offloaded the low-level driver code from the host CPU, running the floppy disks and serial ports, only interrupting the CPU when there was data to transfer. The IIfx also introduced a DMA mode for the SCSI-bus, but this was not available for use by the IOPs. The IOPs also appeared on the Quadra 900 and Quadra 950.