A direct memory access (DMA) circuit which is physically positioned with an
input/output device, the DMA circuit storing a first reference value
pointing to a data structure which describes a buffer portion of system
memory in which data is stored for transfer to the I/O device, a value
determining a position within the buffer portion of system memory
beginning at which a next sequence of data is to be placed, and a value
determining a position within the buffer portion of system memory from
which a next sequences of data is to be copied to the I/O device, the DMA
circuit including circuitry for reading data from the buffer portion of
system memory beginning at the position from which a next sequences of
data is to be copied and for writing the data read to the I/O device.
