Abstract: A nonclock-synchronous interface between a microprocessor and a coprocessor. A request line (404) from the coprocessor and an acknowledgment line (402) from the microprocessor provide for operand transfer from the coprocessor to the microprocessor. A busy line (410) and an error line (408) from the coprocessor allow the microprocessor to monitor the condition of the coprocessor. Data (406) are transferred through a data channel in the microprocessor using the full memory management and protection mechanism of the microprocessor so that the protection mechanism is not circumvented. A memory-read cycle is generated using the address taken from the memory-address register (401). The data is buffered inside the microprocessor and the coprocessor's request is acknowledged. The memory-address register is then incremented by a predetermined amount and an I/O write cycle is generated using a prewired address into the coprocessor.